Remove Roof Shingles Before Installing a Metal Roof?
March 26, 2015 | Filed under: Articles, Roofing Installation
I frequently am asked whether homeowners and contractors should remove roof shingles from a home’s roof before a new metal roof is installed. My answer often surprises people. However, it is based upon experience with tens of thousands of installations over many years.
My Experience
The vast majority of installations of the metal roofing systems produced by my company are over existing shingles. The low weight of metal roofing is one thing that encourages this. Steel shingles weigh about 1/4 what asphalt shingles weigh and aluminum shingles weigh about 1/8 what standard shingles weigh. I would say that in most cases, less weight is being added to the home by the new metal roof than what the current shingles have lost due to granule wear and oil evaporation. In other words, the total roof weight was probably greater when the last layer of shingles was installed than it will be when the metal roof is completed.
Asphalt Over Old Asphalt? No.
One important thing for folks to keep in mind if they are trying to decide what type of roof to buy is that new asphalt shingles should not be placed on top of old asphalt shingles. With most asphalt shingle manufacturers, doing so will void the warranty on the new roof. This is partly because the old shingles will not allow the sun’s heat to pass so easily into the attic so the new shingles end up staying at a warmer temperature, shortening their life.
Why is Metal Different?
That is not the case with metal — temperature does not damage the metal or the paint finish. The warranty on your new metal roof, at least from my company, will not be impacted at all by going over the old shingles.
In over 30 years and tens of thousands of installations over old shingles, I have yet to go back on a job later and find myself saying “Hmm … maybe the old shingles should have been removed.” It just never becomes an issue. And many of those installations actually have been over wood shingles and thinner wood shakes! I have also re-roofed personal properties with our products five times now over the years — every time was over the old shingles and no regrets.
I like going over the old shingles for 3 reasons:
- It stops the need to fill up landfills with old shingles. It ultimately increases the thermal resistance (R-Value) of the roof assembly, actually increasing energy efficiency compared to tearing off the old shingles. The benefit of this is during the summer.
- It allows the property owner to spend discretionary dollars on a better roof rather than on removing the old roof and disposing of it.
- Going over the old shingles also avoids the potentially damaging issues that can occur, like unexpected rainstorms, when old roofs are torn off.
Still, ultimately, it is the property owner’s decision but I would have no qualms about going over a layer of old shingles in most cases. It’s not uncommon to go over multiple layers either actually. On older homes, of course, I do put the caveat out there that if there are signs of an existing weight issue or old leaks, those need to be addressed and that may require removing the old shingles. If there are concerns about the integrity of the roof decking, then pull-out resistance tests using the roof fasteners can be performed.
One other thing to keep in mind is that building codes, where active, usually require no more than two layers of roofing of any type on a structure. There have been instances of building inspectors waiving that requirement but that is not an easy thing to get accomplished.
Remove Roof Shingles Before Installing a Metal Roof?
In a typical year, approximately 90% of the roofs our company manufactures are installed over old shingles. In my opinion, going over the old shingles is worthy of consideration because of the benefits of doing so. Every roof needs to be evaluated individually. Please feel free to email me your questions or circumstances for my input. My email is todd@asktoddmiller.com
Todd Miller has spent his entire career in the metal building products manufacturing industry. He is president of Isaiah Industries, an organization recognized as one of the world’s leading metal roofing manufacturers. Todd is currently Vice President of the MRA (Metal Roofing Association) and a Past Chair of MCA (Metal Construction Association). Through his website, he strives to raise the bar on standards and practices to provide property owners with the best possible products for successful roofing projects.
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Great article. The most commonly asked question by our customers is whether or not their new metal roof can be installed over pre existing asphalt roofing shingles.
Thank you for your comment. I visited your website and you have some good information on it as well. If there is ever a way in which I or my company might be of service, please let us know. Information on our products is available at http://www.kasselandirons.com
I have a 70 year old house with flat ceramic tile roof. The ceramic tile is extremely high maintenance. Can I go over the tile with a metal roof?
Hi Janine,
Generally speaking, tile roofs will need to be removed and would not be suitable to be left in place with any no roof. Feel free though to send me pictures or more information and perhaps I can provide greater detail. My email is todd@asktoddmiller.com
Great site. Does this also apply to click lock standing seam which uses a solid substrate rather than strapping? I’ve heard you can put a membrane over the shingles, then the metal, providing the deck is in good condition and the usual caveats are met. What are your views on this? If it applies, would you install the new valley over the existing aluminium valley (and membrane)? How is the existing drip edge dealt with? It seems like if you cut away the shingles to remove it, it would leave too much of a bump down. Thanks.
Thanks Craig. Products that sit largely flat against the roof deck (standing seam primarily) are more prone to showing ripples from unevenness in the surface below them. Standing seam of course also doesn’t handle dips and swales in the roof as well as more heavily textured products will.
That said, many standing seam roofs are installed over old shingles along with appropriate underlayment. It can be done. You just have to look at each job individually and consider the condition of the roof deck and surface.
In most cases, existing metal valley pans are left in place and just covered over. If they have a pronounced V rib down the center, the new valley can be formed to accommodate that.
As far as existing drip edges … new drip edges can be formed to go over the existing or the existing can be cut back to be flush with the fascia. If you do remove the drip edge, you’re correct … you may have to build that area back up with lumber or perhaps roofing membranes. Another option is to cut back the eaves (lumber and all) but 3.5″ and then place a 1 x 4 in that area. Many times the decking at the eave is sort of spongy … so putting press lumber there is never a bad idea.
Thanks for your questions — they are great!
What are the pros and cons of installing metal rib panels onto asphalt shingles with furring strips vs without furring strips
Thanks Ruth. Not all metal panels can be installed on battens (furring strips) and only a select few products are designed so that they MUST be on battens. Always adhere to manufacturer recommendations.
Regardless of whether or not battens are used, underlayment should be installed over the roof deck and / or existing shingles before the roof is installed.
Generally speaking, the only benefit I see to battens is the thermal break they create to help stop heat transfer. However, to be really effective, you need vertical battens and then horizontal battens so that the resulting cavity can be actively vented.
Now, other than that … if the attic has little or no ventilation and insulation, there can be benefit to battens.
Each job must be analyzed individually. If you wish to share more details about your situation, I will be happy to weigh in with specifics related to your roof.
My email is todd@asktoddmiller.com
I have an old 20 year old asphalt roof that the shingles are in pretty bad shape. The roof itself it very strong. Can I put a metal roof on a roof that old?
Every situation will be different but generally speaking, installing one of the more heavily textured metal roofs such as a corrugated vertical seam or or a horizontal metal shingle over your existing shingles, should not be a problem. You will want to install layer of underlayment between the old and new roofs. A true standing seam vertical roof, however, will require that either the old shingles be very smooth, they be removed, or battens be installed over them. This will help avoid showing ripples in the metal roofing panels.
should standing seam metal roofing not be installed over asphalt shingles due to telegraphing of the shingles underneath the standing seam panels?
, when snow piles up on a standing seam roof that was installed over an asphalt roof, will the shingles likely make horizontal dents in the metal panels. Also, will expanding and contracting due to temperature changes cause rubbing against the stone coating on asphalt shingles. – could this eventually cause some corrosion on the underside of metal panels.
Underlayment should be installed beneath the new roof regardless. Yes, there is a risk of unevenness of the old shingles telegraphing through and causing ripples in the standing seam panels. The more textured the metal roofing is, the less chance of that happening. And, yes, walking on it or perhaps cascading or very heavy snow could cause damage to the metal roofing as well.
I have a metsl stanfing seem roof over wood shingles and an inspector just claimed thst the edges where wood shingkes shiw under driipedge should be covered. It is a bout 1 inch exposed between fascia and metal but not out in open. I was told by one roofer that it is good to breathe and cutious if snother drip strip needs to be added? The current system has been in place for 9 years with no visible deterioration.
It is my opinion that a drip edge should cover them but, in all honesty, I have no real reason for that. I do not think it is a code requirement. Not entirely sure I buy the “breathing” comment … doesn’t seem like much breathing will really happen. I’d probably just go on enjoying your roof as is unless this is an inspector that can require you to do something.
Please excuse typos…
I’m framing up over an existing shingle roof with 2″ by 4″ ‘s because the sheeting is sagging.I figured if I used 2by4 instead of 1 by 3″ it will help the metal lay even without taking shape of the sheeting that’s sagged.well after nailing one 2 by 4 up there I noticed there’s some spots where there’s gaps between the 2 by 4 and the existing roof in some spots from the sheeting sagging. Homeowner was addimant about laying over the existing roof even with the sagging in the sheeting!!! Is this OK or what? Also what’s the easiest solution to install drip edge over existing roof and still ensure the metal lays properly
Thanks Chuck. I assume that you’re installing a vertical seam metal roof and it is a vertical seam profile approved for installation over spaced boards. I assume, too, that your spacing of the boards is per the load table provided by the roofing manufacturer.
What you’re doing will help take up any dips between rafters on the roof. It is not uncommon for this to be done just for this purpose. It will not of course resolve any crown that is in the roof … and that can be a big problem for vertical seam metal roofs.
You will want to have one of your 2 x 4’s at the eave for the starter to be fastened to. That should create a pretty even surface there.
Feel free to email me direct or even send photos if you have more questions. todd@asktoddmiller.com
Thanks for your response.one last question for you. What steps should I take to take the crown out or is this even possible without tearing off the existing shingle roof?I was paid to frame it up and lay the metal on the existing roof. Should I inform the homeowner he should have a framer come in and fix this first or is there a way I can take the crown out as in doing it myself? Sorry more than one question. I’d email you these but I can’t find my password for my own email account!
Thanks Chuck. Removing the crown is difficult. Depending on the construction and exactly what is happening, he may be able to do some things from inside the attic — shore up rafters, etc.
If there is “belly” going up the roof rather than crown, you could put down 2 x 4s as sleepers vertically up the roof over the rafters — before your horizontal battens are put down. That could take out some of the “belly”
If there is “crown” going up the roof and it can;t be remedied from the inside, then it would require removing the decking and doing significant work.
The problem with crown is that it will likely force ripples into the metal roofing panels. I am not sure what type of metal roof panel you’re installing? More textured and exposed fastened products may handle this situation better but then you’re dealing with exposed fasteners.
Another option for crown would be to install a metal shingle roof as the shingle courses will usually accommodate the crown very well. Many of these products thought must be installed direct to the roof deck rather than over horizontal battens … so that could be an issue because you’d not be able to use the horizontal battens.
We r thinking about putting tin over shingles. There’s only one layer. But we have heard by doing this will cause mold. Is this true? What kinda maintenance is required w tin. Want to keep it looking good cuz we have oak trees around the house. We have other houses n our area w tin n their roofs have black on them. Wondering if this is mold or just dirty rain. These house do not have trees n their yard. Looking for someone w great prices material n to do the job right. We live n Clarksville Tn. I have two houses we r thinking about putting tin on. Please give me some advise n recommendations on material n mayb if u know of a company here. Thanks for help
Thanks Jackie. It is good to hear from you. Here is a link to a comprehensive article I have written on this subject of going over old shingles:
https://www.asktoddmiller.com/history/should-existing-asphalt-shingles-be-removed-before-a-metal-roof-is-installed/
In normal situations, doing so absolutely will not lead to mold.
Oak trees can drop sap and other stuff that can streak / stain any roof. On metal, provided the metal has a good coating, it is usually something that can be cleaned off. In some situations though there is no avoiding it because of the trees. A Kynar / Hylar paint finish on the metal roof will be most resistant to this dirt accumulation and streaking.
As far as the blackness showing up without trees … I’d say that unless there is something very odd in the atmosphere in your area, that is related to a lower quality coating being on the metal roof.
I will email you direct concerning a contractor in your area.
Thanks much.
All Best.
We live in Central Florida and are researching using stamped metal shingles over our asphalt shingle roof. Do roofers need any special expertise to do this installation? Are there quality differences we should be aware of between the various stamped shingles? I realize the finish coating on the shingle is a factor to consider, but is there anything else we should know?
Thanks Linda.
Well seeing how my company manufactures metal shingles, I probably have some preferences. 😉 But, I will be objective. Most of these products will have Kynar / Hylar finishes which are my preference by far. I would look for products that have interlocks on all four sides and do not have exposed fasteners. Beyond that, though, you’re right — it comes down to proper installation. Make sure you’re comfortable with the installer. Call the manufacturer and make sure they can say good things about the installer. Go see past jobs, talk to past customers … and make sure that the installers on your roof will be the same as on the roofs you went to see. I will take a properly and well installed mediocre product over a poorly installed “world’s finest” product any day. Please feel free to email me direct if you have additional questions. tmiller@classicroof.com
I have a pole barn with a shingled roof. The sheeting has a foil backed insulation attached directly to it on the underside inside the barn. This was done during construction because the insulation is a continuous length-wise roll sandwiched between the roof trusses and sheeting. Sort of creating a cheap insulated panel. The shingles on the south side of the building have cooked in the hot sun and are significantly curled and are now blowing off in high wind storms. What do you recommend for replacing this roof with steel?
I would remove the old shingles since they are in such bad shape. I would add a layer of synthetic underlayment, or more if required by code. Then install your metal panel. Make sure that the metal panel you install is approved for use on the pitch of this roof. If you wish, email me at todd@asktoddmiller.com and I will send you my ebook on How To Buy A Metal Roof.
I saw that some standing seem metal roofing companies remove the asphalt shingles and add ice guard before they install the roof felt. I was curious about any ice issues that could arise from doing new metal over old asphalt shingle. I would like to do standing seem over my old shingles or at least an option that conceals the fasteners from the elements. Is there any product that looks like standing seem with concealed fasteners but can be installed directly over existing shingles without shape issues?
Thank you. There should not be ice issues from installing over the old shingles. In fact, the R Value of the shingles will help keep interior heat from reaching the roof, and that can help prevent ice dams. You’re correct though on the possible shape issues. You could look for a structural standing seam that can be installed over battens. Apply underlayment over the old shingles, battens, and then the metal roof. The battens will even things out. You could “cross batten” by installing vertical battens and then horizontal battens. This allows for ventilation up the roof and can enhance energy efficiency. I hope this helps.
What size wood would you use to make the battens? And by cross do you mean a vertical run from drip edge to peak and then full length horizontal boards on top of them running side to side?
The manufacturer of the roofing should specify batten size but it’s not uncommon for horizontal battens to be 1 x 2 or 1 x 4. Yes, cross battens would start with vertical battens on top of the decking but running on top of the rafters as “sleepers”. Then horizontal boards run and spaced as required by the metal roofing panel. Metal roofs that can be installed this way will have “load tables” that specify batten spacing. Cross battening provides vertically oriented chambers. Air can be brought in at the bottom and exhausted out at the top, creating good energy efficiency. In cold climates, this can also help avoid ice dams on the roof.
Metal shingles also create a good option. These formed panels have an integral airspace between the metal and the roof deck. That airspace, while it may not be vented, does provide a thermal break which is also very effective for energy efficiency.
I am considering putting metal
Over my existing shingles. I have a place in my roof where the decking was sagging and I basically bandaged it for 2 years. Do I need to replace that piece of decking before adding metal and if so do I need to replace the old shingles in that area so that it will
Level out before adding metal to prevent wrinkling of metal?
I would never advise fastening into decking that has rotted or delaminated, jeopardizing the fastener pull out resistance. Therefore, it does sound like this needs to be addressed and, yes, you will have to use some material to build the roof up to the same level as the rest of the roof once that is done.
Ok thank you very much for the advise. Have a good day.
Honolulu, Hawaii. I am planning to install aluminum standing seam roofing (SHURLOCK 12″.032 ALUM LG MAUNAKEA WHITE-DURACOAT LOW GLOSS) after stripping off asphalt shingles applied to a 1×8 shiplap roof deck — boards are fairly tightly spaced. (I did the shingle install myself with 3 friends approx. 20 years ago.)
Additionally the metal roofing will be installed only covering an area of approximately 20′ by 20′ starting from a rear gable end and ending 20′ where meeting a straight vertical cut-off of existing asphalt shingles. (There is estimated to be an additional 5-year life in the existing asphalt shingles.)
This design is specifically to be a good, solid, long-term roof for under PV panels which will not, therefore, require any additional thru-roof protrusions. The seller, who forms the metal panels locally in his shop, recommends a synthetic underlayment (PALISADE SA UNDERLAYMENT 36″ X 66.7′-SELF-ADHERE 2SQ/ROLL).
Cost is the primary “driver” of the design. The PV system is relatively small requiring 13 solar panels. (Have considered simply a new asphalt shingle roof under the PV panels — cheaper — but that would require many thru-roof protrusions for the PV tracks and I certainly want to avoid the cost of raising the PV panels at some later date.)
The idea is to do, now, the minimum to get the PV up and running … face the question of how to complete the remaining 2,600 SF re-roof in 5 years.
My question … Because I have not seen any discussion of re-roofing over shiplap, I am curious as to whether you might have any cautions or concerns given the situation described above.
Thank you. Assuming the roof meets the minimum pitch required by the designed of the metal roof (probably 2:12), I do not see any issues with this plan. I assume you’re using clamps to attach the PV to the standing ribs. I am curious though … do you have a vented airspace beneath the ship lap?
Wow! That was quick! Thank you.
Pitch is 4:12. Yes, using clamps.
About 80% — or more — of the interior of the house is what we refer to in Hawaii as open beam (i.e. cathedral ceiling) — 4×6 rafters on 3′ centers, except for a 10′ addition (new rules) with 2′ centers. Attic spaces are over the kitchen/dining area, parts of two relatively small bathrooms, and three 2’+ deep linear bedroom closets. It has been my presumption that additional airspace would not be necessary.
We have jalousie windows in most rooms just above floor level which are open 99% of the year. The roof ridge has a “homemade” vent framed with 2x4s — therefore, raised the height of a 2×4 on edge (3-1/2″) — which we built when we re-roofed 20 year ago. The cut-out of the shiplap along the ridge has a span of about 24″ running the entire length of the house — one straight ridge for the entire house. (The ridge vent “deck” runs down about 3′ on each side of the ridge line at the same slope as the “field” deck. For now, the ridge vent will remain — with some “tricky” flashing to keep the inner vent dry!)
Any further comment you might have on the adequacy/(need) of (additional) ventilation would be most appreciated.
Thanks. I am not sure I am 100% following things but let me tell you my concern. When warm moist air hits a cold surface, of course, condensation occurs. A standing seam roof, in contact with the ship lap (even with underlayment between the two) can drop the temperature of the ship lap a few degrees compared to what it was with a different roof system. The metal just cools down more quickly than other materials. This couple of degree differential in rare cases has been the straw that broke the camel’s back and caused condensation that was not there before. Perhaps looking into a very thin insulation board between the metal standing seam and the underlayment / ship lap, would be helpful.
OK. Will do. Thanks again for your comments and advice. Really appreciate.
Hi , just wondering if we can put a metal roof over existing shingkes that show signs of buckling and have moss on them. Thank You
Can you send a photo to me along with details on the type of metal roof you plan to install? This can be done in some cases but may not always be the way to go. My email is todd@asktoddmiller.com
Hello!
Do you have experience in very cold climates? I am in Quebec, Canada and it’s been known to reach temps in the high -20s to -30s F in the winter. If I install a metal roof over existing shingles, won’t there be a risk of condensation between the two?
We are looking at two manufacturers; one is going to install a fabric between the two (Kasselwood), the other company (Decra) will install directly on the shingles saying they have ventilation channels in the product. I know you are biased to Kasselwood, but why are the techniques so different?
Thanks so much for your question. It’s important to think of condensation potential in terms of dew point. Condensation occurs when warm moist air hits a cool surface. Think of the glass of cold lemonade sitting outside on a hot day and the condensation on the outside of the glass. So, condensation requires a significant temperature differential and high moisture levels.
Almost never does that sort of situation occur in the outer layers of a roof assembly — in those layers, you do not have a temperature differential that can cause condensation to occur.
Condensation in a roof assembly is normally a risk in the attic space. Warm moist air originates inside the living space and raises to the attic. If the attic is properly ventilated, then it can not condense. However, if the warm moist air is not vented out and the roof deck cools down at night, it will condense. That condensation normally occurs on the under side of the roof deck.
So, the question here has much more to do with whether you have good attic ventilation than it does with layers of the roof assembly. If you do not have good attic ventilation, and good attic ventilation can’t be achieved, then we need to talk some more and discuss options.
The underlayment installed beneath is KasselWood roof is a moisture barrier from the outside which is a good thing. In essence should any water ever get beneath the metal roof or should the very rare chance occur of condensation, the underpayment prevents it from getting into the structure. But, again, that is pretty unusual to have happen.
On the Decra products … there is an air gap between the metal and the roof deck. That acts as a thermal break. KasselWood has the same thing though the air gap may be smaller. It all depends on what Decra product is being considered. Some Decra products have greater air space than others. Very rarely are they installed though in a way which actively ventilates that air gap. More often it acts as a thermal break and perhaps a tiny bit of passive ventilation. Give me a call if you’d like to talk further. 937 773 9840 ext 201.
All Best.
I had a steel roof put on over my shingles. They took my whirl bird venting off and now i have no venting.
they did not take off the shingle cap and cut the wood to allow the heat through the steel cap. Is this a problem
In normal construction, proper attic ventilation is critical. It sounds like your attic ventilation has been diminished. Generally speaking that puts you at risk for higher air conditioning costs in warm weather, ice dams on the roof in snow climates, and greater risk of condensation and potential mold issues in the attic, particularly during the Spring and Fall months.
sorry I may not have given you are the info and your answer may change.
a steel roof was installed over very bad curled shingles with horizontal strapping. no vents were reinstalled and I don’t believe the shingle caps were removed.and if they were would the wood need to be cut for a vent cap. the top cap just looks like a cap and I wanted a vent cap. do they look different?.i have a picture if I could send it and you could tell me if it is a vent cap
it is on a cottage roof. only used from May to October. the temperature would be 32F in the spring to 90F in the middle of summer. no air condition and no heaters. and it is on a big lake with very little trees.
Thanks. It is still attic ventilation that concerns me. A vented ridge cap on a metal roof may not look significantly different from a non vented metal roof. Yes, if a ridge vent was installed, its purpose is to vent the attic, not the space between the steel roof and the old shingles. Therefore, to vent the attic, the ridge would have needed to be cut open. What did your contract call for as far as ventilation? Feel free to send photos to todd@asktoddmiller.com
Hi, I was reading your info about covering asphalt shingles with metal. Would love to do this at our cabin. We have no leaks but do have moss. Should with remove it first with power washing. Worried that we’ll have to wait for it to dry out after the wash and it’ll slow us down. I think we’ll be using norclad. I sent photos via email.
Sent from my iPhone
Thanks Karen. I will reply to you via email but, right off hand, you should be able to install over the existing, along with a layer of underlayment. Once the fungus is isolated from sunlight and moisture, it dies.
Hi, I have a 1:12 pitch roof on my home with 2 layers of asphalt shingles. The home does not have an attic, as the interior ceiling matches the slope of the roof. With only a small insulated space between the ceiling and the roof I am not sure how to handle my metal roof install. I have found standing seam products rated for 1:12 pitch roofs but I’m not sure if I should leave old shingles on or not. I also am not sure about underlayment types or if I should add a space between the existing roof and the new metal roof. This home is in Boise, ID which is hot and dry in the summer and cold in the winter with low amounts of snowfall. Thanks for any help you can offer.
It sounds like you have done much of the right research. Here is my concern though — the metal roof will potentially make the roof deck colder than it’s been in the past. This could cause condensation on the inside of the decking that has not happened before. Condensation occurs when warm moist air hits a cool surface. This would occur especially during evening in the fall and spring when ambient humidity is high yet temperatures get cool at night.
So, to that end, leaving the old shingles is good. It also could be good to add insulation or a vented air space or even an air gap. These things will all help to create thermal breaks to keep the inside of the roof deck from getting too cold.
Please realize that if you go over the old shingles, it could create unevenness that might force ripples into the standing seam panels.
If you do anything in regards to insulation or ventilation, make sure that what you are doing is approved by the manufacturer of your roofing.
Good luck and feel free to contact me anytime.
All best.
We are getting ready to install a metal roof on our home. We had to remove the shingles and replace the wood on the roof due to multiple leaks. We have put 30# felt paper down on top of the new wood but need to know whether or not we need to put anything else down (such as foam board or shingles) before we put the metal roof on. Any advice is appreciated! Thanks
Thanks. It depends a bit upon what type of metal roof you’re installing and where you’re located. In warmer climates, if you’re installing a vertical seam metal roof that rests largely against the underlayment, there is a risk of oils leaching out of the felt paper and sticking to the back of the metal. Then, as the metal tries to expand and contract with temperature changes, it can “bunch up” or tear the underlayment. Not good. My general advice is a layer of synthetic underlayment over the felt. This will act as a slip sheet and work well under virtually any metal roof. All of my advice is assuming there is nothing unusual about your home in regards to ventilation, cathedral ceilings, etc. Feel free to email direct to me with details or photos if you have more questions. todd@asktoddmiller.com
Thank you for the quick reply! This is a double wide modular and we are in central Indiana. The metal is galvanized sheet metal that is roll formed and came in a steel coil.
Thank you. Please call me at my office when you can. 1-800-543-8938 ext 201. You have a situation that we need to talk about a bit … I want to make sure that you do not do something which results in condensation where it did not exist before. I will be in the office tomorrow and Thursday.
This is such a great site and useful thread! I have a small 10×13 garden shed w cedar shingles that is about ready for a new roof. I like the appearance of the shingles, and would like to replace the side of the roof that faces the house with the same cedar shingles. However, the back side of the shed isn’t visible, as it faces a bluff, and collects a lot of tree debris. For purposes of longevity, ease of installation, and weight, I’d like to put a metal roof on the back side. Do you see any problem with cedar shingles on one side and a metal roof on the other? Can I use cedar ridgecaps at the peak, instead of metal? Thanks!
Thanks for your kind words. I think that you could do this. However, the ridge cap will need to be metal but then you could put the cedar ridge caps on top of that for “decoration”. As another thought … have you considered a steel shake facsimile? Something along the lines of http://www.kasselshake.com
Hi. I’m clueless about roofs. I’m a lady (and that doesn’t mean we are all clueless) but while my husband is away for work 90% of the time I am left with the decisions about renovations etc. I have been burned in the past because of this, with a contractor for things we needed done on the inside of the house. Now it’s roof time. We have an older house, it’s been about 40 years since we got a new roof. It’s aluminum, but its already flying off so it has to be done. Now underneath are cedar shingles. I have had two very different quotes. One is for a quote that just includes the removal of old and putting new steel on. The other involves sheathing. The price difference is $1500. The company that wants to do sheathing swears up and down that new steel cannot be put over the old cedar, and the other company says it can. Not wanting to get ripped off, and not wanting a bad job done I am at a loss as to who to believe. My husband has nothing to offer for an opinion because this area is not his area of expertise. Who do I believe?
Thanks so much. First of all, if you wish, email photos of the roof to me at todd@asktoddmiller.com However, here’s my advice. Technically could it be done? Probably yes. Should it be done? Probably not. The wood shingles will be pretty broken up from the fasteners through them, etc. I would suggest tearing off the existing metal roof, the old cedar shingles … and then installing solid decking … unless sold decking is already in place beneath the cedar shingles. In any and all events, underlayment should be installed beneath the new metal roof.
Hi Todd, I am just having a metal roof installed as we have just bought an old house (with an old roof). I have a few leaks actually and had a contractor come out today to take a look. The contractor explained that because the roof was made with tongue and groove ship lap, as it ages and the wood hardens, the nails are being pushing up through the asphalt shingles. Now with the few leaks that I have, I was concerned with rotting wood. As the contractor walked on it he said it is in great confusion. He proposes taking a look at the insulation as it may have water damage but the wood is fine. Then he proposed adding the metal roof over the shingles. Now should I truly take his word for it that the wood is fine? Could I regret that decision if there is some water damage down the road? Maybe we should remove the shingles to check out the wood (not a substantial attic space to check from the underside)? Also what is to prevent more nails from being pushed out of the ship lap after we add a metal roof? I feel like my questions may be silly, but we are talking our first investment..
Congratulations on your new (to you) house. Your questions are not silly. Many many metal roof installations are over existing shingles. The low weight of metal encourages this. It is easier to do with more heavily textured metal roofs such as exposed fastened corrugated products and metal shingles. Standing seam roofs may have unevenness of the old shingles telegraphed through and cause ripples in the standing seam.
Underlayment should be installed before the metal roof, regardless of whether the old shingles are removed.
I can’t really say much on the old nails working their way up. Can’t say I have ever seen this before. So, I’d be very surprised if it is a real risk but ask your contractor what they think.
As far as the wood condition … if you are not aware of active leaks and the contractor says the roof seems solid, it probably is okay. If there is bad lumber, they will know it when they try to fasten into it.
I hope this helps. Please contact me anytime. My direct email is todd@asktoddmiller.com
I have a 12/12 pitch roof on an old 1930’s church, it has two layers of asphalt shingles on it, the top layer is really ragged on one side of the roof. Different roofers are telling me different things, …can I put metal over 2 payers of shingles even if one side of the roofs shingles has flaking off shingles. I know underlayment(synthetic is essential, and a roof vent), I’m concerned about the weight of new 2×4’s and the metal on my 2×6 old roof. I live in a cold and hot climate with major snows in the winter…but with my steep pitch is that a factor? Winter is coming, please help? Thank you
Thanks for your question. First of all, the International Building Code which is enforced in much of the USA but may or may not be enforced where this structure is located, requires no more than two layers of roofing. That is important to know and keep in mind.
That said, the weight of a metal roof is pretty low. Most steel roofs weigh around 80 pounds for 100 square feet and aluminum is even less than that. Asphalt shingles by contrast are 275 – 450. Chances are that the last layer of shingles on the roof has lost more weight in worn off granules and evaporated oils than a metal roof will add back.
The lumber weight is also not real great but it would require inspection and evaluation by a structural engineer to render an expert opinion on that. Undoubtedly this structure has been built to handle some pretty good weight from snow.
As far as going over old shingles that are uneven … it can be done … more heavily textured and formed metal roof products such as corrugated sheets and metal shingles though are far more forgiving than is standing seam … standing seam can have ripples forced into it if it’s installed over an uneven surface.
I hope this helps. Please contact me anytime. You can email photos of the roof and structure to me if you like at todd@asktoddmiller.com
We are currently looking at purchasing a home in the mountains of Colorado that can have high winds, hail, and temperature variances. The A-frame home was built in 1963 and has an original shake roof on the sloped portions and a flat roof everywhere else. We want to install a metal roof on the sloped portion but are unsure whether to install the roof over the existing shingles or have them removed first. There is a cathedral ceiling over the A frame portion but no attic in any other existing portion of the home. Can you please offer your opinion? I’d be happy to send you a picture of the home as well.
Thanks Carole. Sounds like a great home and a great project. There are many things to consider here including moisture control. Do you know if the existing shakes are in spaced sheathing or solid decking? That is a big question. Furthermore, is there any actual ventilation on the attic / cathedral space including intake and exhaust vents. Here is my concern — the existing shakes are likely on spaced sheathing, meaning that they “breathe” very well. Anything you do to the roof, including covering over the shakes or removing them and installing solid decking will significantly tighten the home. Unfortunately that could set you up for condensation issues that did not occur before. There are a lot of unknowns here for me … I am happy to try to help but really need more information on the current situation. My email, if that is easiest, is todd@asktoddmiller.com Thanks so much.
If a steel roof is installed over shingles with some moss I know the moss dies but will it cause any rotting of shingles or wood underneath or will it dry and be inert. Thank you for a wonderful site.
In 30+ years I have never known it to become a problem. I do recommend a layer of underlayment on top of the old shingles and beneath the metal. Unless it is specifically a non vented situation by design, then the attic needs to have proper ventilation as well. A great source of ventilation information is at http://www.airvent.com
I have a double wide mobile home. Can I put a metal roof over the old shingles or will the metal be a weight factor?
Metal roofing is much lower weight than asphalt shingles. Aluminum roofing weighs about 45 – 65 pounds per 100 square feet and steel from 80 – 120. Asphalt shingles on the other hand weigh from 275 – 425. I can’t say this for certain but chances are that your home was built to handle the weight of at least two layers of asphalt shingles. Additionally, your existing asphalt shingles have lost weight due to granules wearing off and oils evaporating.
One thing that you will need to know is the pitch of the roof on your home. Sometimes, double wides have lower pitch roofs. Most metal roofs can be installed down to 3:12 pitch but for lower pitches, you will need to choose a metal roof that is appropriate for your roof pitch.
My roof has a lot of vents on it for the roof to vent I see lots of videos showing removing the vent caps and leaving the holes and just going over the old vents doesn’t that create a problem with the roof venting and getting too hot in the inside?
What you have described should not be done. In some cases, existing vents may be replaced on the roof by a different type of vent but in that case the old vent holes should be covered / blocked. Attic vents should come through the roof surface.
Thank you in advance. Our house was built in the early 30s. It currently has one layer of shingles over top of decking that is the original pine boards. 75% of the roof is straight and relatively flat. The remainder has a dip in it probably due to that being an addition. One roofer insists that 1 x4 underlayment be attached to the roof because the old boards are dry and screws may not hold in them. He also believes that the dip can be corrected with the underlayment of wood. The second roofer believes that the underlayment is an unnecessary expense and the dip would not be noticeable anyway. I really hate to have two opposite opinions. Can you offer yours? Thank you.
There are several considerations here including how bad the dip is and what type of metal roof you plan to install. Right off hand, I do not feel the boards are essential but I agree they could remove the visibility of the dip. In most cases, dips are more apparent rather than less once the new roof is installed. Feel free if you wish to email photos to me at todd@asktoddmiller.com
Todd,
I have a question concerning installation of metal roofing.
Have metal roofing on existing singles with an underlayment garment on top of singles
The question is concerning the ventilation.
With singles – there were 3 openings which was removed. Underlayment garment installed over the openings and then metal roofing. Is this correct? What is the correct procedure?
Or should new ventilation be installed on each opening that pre-existed. But we metal roofing do not see a lot of openings on photos and other homes.
Thanks. I do not know the full details of your home’s construction but, with most homes, the following applies:
1) Metal roofs do not reduce the need for attic ventilation.
2) Proper attic ventilation requires intake and exhaust vents. Intake vents are usually in the eave soffits and exhaust vents are in the roof at or near the peak / ridge.
3) Proper ventilation helps make a home more energy efficient in the summer. It also reduces the risk of high moisture levels and possible condensation in the attic. In northern climates, it also reduces the possibility of winter ice dams on the roof.
4) Attics generally require insulation on the floor, ventilation above that, and a vapor barrier behind the ceilings. You might get by without one of those things but rarely without two of them.
I hope this helps. Please feel free to contact me anytime.
Hi Todd
I have a large one story building that is 85 years old. It is solid structurally, but needs a new roof. It is a pitched roof, with shingles. I don’t know how many layers but I’m guessing more than one. I want to go with metal roofing for this (church) roof. I’m not sure about the integrity of the roof trusses. How should I approach this project. I’m in a cold climate so it won’t be done before spring or early summer.
Thanks,
Allen
Thanks Allen. Metal roofing’s low weight can be a benefit to aging structures. And, of course, churches often have the long rafter lengths which can heighten weight concerns. This may be something that you’d want to have a structural engineer inspect. In many cases, metal roofs can be installed over old shingles but code requirements call for no more than two layers of roofing. I am going to email you direct with a copy of my ebook on How To Buy A Metal Roof. I think it may be helpful. Additionally, these videos may be helpful: https://www.youtube.com/playlist?list=PLMw9xVe_yEaVt4yv1i4LoDTlNKNd6boRd
I have a 30×30 hip roof with at least 2 layers of shingles possibly 3. 2 of the 4 sides have have a pretty big dip in it, I live in Ontario de we get lots of snow in the winter, would you recommend stripping the shingles before I put steel on? I have had contractors say yes definitely it’s to much wait and that many layers will curate moisture and rot the steel from the bottom up and others say it’s fine.
Any dips or swales present in the roof will affect the metal roof installation and could in fact be quite visible and troublesome depending on their severity and the type of metal roof you install. Additionally, because the International Building Code does not allow more than two layers of roofing, I really caution folks on adding more layers. A home inspector, when you sell the house, could raise a fuss about it. As far as moisture and condensation … that needs to be addressed through attic ventilation. The number of layers of roofing really does not impact that.
Hello Todd. I recently bought a lake house in western MD and the shingle roof is at the end of it’s useful life. The house has a simple saltbox roof design with a cathedral ceiling (no attic space) and two sky lights. There is evidence of ice dams that may have led to some minor leaking on one side of the house. I’m thinking of removing the sky lights but not sold on that decision yet. I’m seriously thinking of a metal roof but have a few questions. 1) Should I have the old shingles removed given evidence of some possible leaking on one side of the house and possibly some damaged decking? 2) If I keep the old shingles should I insist on the roofer installing an under-layment on top of the old shingles. 3) If I decide to keep the sky lights should I buy new ones or keep the old ones? 4) If I decide to remove the old sky lights should the roofer open the cavity along the joists to ensure ventilation where the old sky lights were located?
Thanks so much. I am happy to share my thoughts.
First of all, I think it would be good if you check out this article I’ve written on ice, snow, and metal roofing: https://www.asktoddmiller.com/misc/snow-ice-dams-and-metal-roofing/
Next, I am very much in favor of looking at decking if there have been leaks or water intrusion in the past. A metal roof is a long term thing … you do not want to cover up damaged lumber.
Underlayment should be installed, in my opinion, whether or not the old shingles are removed. I interpret the international building code as requiring it and I also just think it’s a very inexpensive safeguard.
I always recommend replacing any skylights that are not functioning well or that are 10 years old or older. Most skylights do not last longer than 20 years. Installing a metal roof around skylights requires careful flashing — something you do not want to have to re-do before necessary.
If ventilation can be created or enhanced with the new roof, I am in favor of that.
I hope this helps. Please contact me anytime. I can be emailed at todd@asktoddmiller.com
If there is moss grieving on some asphalt shingles, how do you deal with that before putting a metal roof overtop
If there is moss growing on some asphalt shingles, how do you deal with that before putting a metal roof overtop
I would sweep it off as best as possible. You could apply a fungicide to kill it but most people do not. Once underlayment and new roofing is installed over it and it is isolated from sunlight and from moisture, it will die. If the moss creates a very uneven surface on the roof, that could affect the ability to install some types of metal roofs, especially standing seam, over the old shingles.
Hi: What about metal roofing over existing cedar shingles?
I expect I would need to remove any moss?
Thanks.
Doug, this gets a little complicated. I would not install a vertical seam product over cedar shingles unless you put down battens first and, realistically, to smooth it out, you may need vertical battens first and then horizontal.You will also probably want vertical battens first if the cedar shingles are on spaced sheathing instead of solid decking. If you do this, make sure that the vertical seam metal roof you are installing is approved for installations over battens, and make sure the battens are properly spaced and secured.
Now, some metal shingles can be installed over cedar shingles up to maybe 3/4 or 1″ thickness. It will impact the appearance a bit in all likelihood. If the cedar shingles are on spaced sheathing though, not all metal shingles would be approved by their manufacturers for that application. (And they may not all be approved even if the cedar is on solid decking.) There re some metal shingles that can be installed on decking (per the above) but most cannot be.
Also, if the cedar shingles are on spaced sheathing, please pay attention to your attic ventilation. Roofs on spaced sheathing naturally breathe a lot. You will be closing most of that off. So, you will likely need to increase the attic ventilation, looking for a balance of intake and exhaust.
If you leave the cedar shingles, yes, clean them off as best you can and install underlayment over them, whether or not you are putting down battens.
I hope this helps. Always make sure that what you’re doing is in accordance with local building codes.
i am contracting out a metal roof over a relatively new asphalt roof. we are attaching a garage to the building and since it is getting a metal roof he just wants us to go over the new roof with metal and get it done with. county building inspector says we must have the manufacturer of the metal tell us we can do so as they may not be liable if something happens. reading your thoughts on this tells me that there isn’t really a good reason why metal cannot go on top of the asphalt shingles. our one and only hardware store that is supplying the metal for us cannot get the company to write us a note saying we can put metal on top of asphalt. we now have to put up 1 x 4’s on 2 foot centers to put the metal on. to me this is a waste of time and money on everybody’s part. your thoughts?
Thank you for contacting me. I have several comments …
1) Regardless of whether you use battens or not, a layer of underlayment should be placed over the old shingles.
2) If you use battens, using cross battens (vertical followed by horizontal) will help a great deal with energy efficiency in that you could even vent the resulting chamber.
3) Regardless, even if you just use horizontal battens, it will help summer efficiency.
4) I do not know what style / profile of metal roof is being installed.I do not know if it is approved by its manufacturer for installation over battens. If I were you, I’d want that to be in writing. Do you know who the manufacturer of the panels is?
5) If you’re installing a clip-fastened standing seams, not going direct over the old shingles can help avoid ripples (oil canning) in the panels.
I hope this helps some. Feel free to respond with details as to what style of metal roof you’re buying and who the manufacturer is. That may allow me to provide additional insight.
I noticed that a layer of synthetic underlayment is mentioned many times to be installed between old shingles and the new metal roof. Why is an underlayment layer needed?
Thanks for your question. There are several reasons for this:
1) I interpret the International Building Code as requiring some sort of underlayment. Not all folks interpret it that way.
2) It does provide protection against water infiltration into the home should you have a flashing or something that occasionally bleeds some rain water.
3) It protects the back side of the metal roof panels from abrasion against the granulated shingle surface.
4) It can increase worker safety during the installation process, especially if the old shingles are cracked or falling apart.
Here’s a link to a good line of underlayments: http://www.roofaquaguard.com
Hi Todd,
What a great forum for homeowners. Its a noble deed devoting your time.
I live in S. Fla. and need a new roof. Been doing my research on underlayments. My current roof could be original from 1969. I bought house 6 years ago.
It has a white roof but I am thinking about going grey for a modern look plus the white gets so dirty.
I do have a few questions…..
1) I am interested in the Layfast TU 43, (if available here), or another similar synthetic and would like to know how it can be double layered applied? When you nail down the 1st layer do you put anything in between or do you just nail down the 2nd layer? Can the 2nd layer be hot moped?
If you do nail down the 2nd layer right on top, what benefit is that?
Wont too many nails be concerning for leaks from far more nails than traditional 30/90 hot mop?
2). My roofer, very well respected, been in business 30 years, never did synthetic before. His knowledge and experience is strictly 30/90 hot mop. For that matter, I got 8 different quotes, everybody recommended 30/90 hot mop. He is open to doing synthetic but am a little worried it may not turn out as good with lack of experience using synthetic.
After all, much like AC installation, the brand isn’t as important as who does the installation and how good the installation is.
3). What about a double layer of #40 or #43 felt, so nail 1st layer than hot mop 2nd layer? Is that a decent proposition for possibly better lasting underlayment than 30/90? Or possibly going 1st layer nailed ASTM #40/#43 then synthetic over it? Would that make any sense?
4). Since I am probably going a darker that current white concrete tile, wouldnt added heat from tile possible cause a faster degradation of the felt over the synthetic?
Do we even have 10 years++ data on synthetic to judge?
These crazy high warranties offered on Synthetic I think is a BS Marketing ploy, how the heck do they know? Its not like anybody in the company have been using them since 40s or 50s to have any real world data points or experiences. Are they nothing more than “fantasy” warranties much like the Chinese Solar panels offering lifetime warranties, then next thing you know many are out of business. So I am not buying these 50 yr warranties on Synthetic. #FakeNews
Also, The warranties are so restricting and the roofing work done by the contractor will be scrutinized severely by the manufacturer that I feel they have easy way “out” from honoring warranty. The other thing that worries me, is the many companies jumping into the synthetic underlayment business just like the Chinese solar panel example.
5). Lastly, I also have a flat roof that needs to be done, can I get a few recommendations for making this roof last as long as
possible and more importantly waterproof
I was given the following by 3 different contractors
A. GAF #80 base sheet
GAF Modified #20
1 ply mineral surfaced Cap Sheet
B. #75 Glass base sheet
1 ply of ElastoBase
1 ply of Elastoflex SAV self adhered
1 play of PolyGlass cap sheet SAP self adhered
C. #75 GAF base
#20 GAF Ruberoid Modified
1 layer GAF Mineral Cap Sheet
Thank you for any help you can provide me.
Thank you for your kind words. Unfortunately, you’re delving into an area of roofing where my expertise is limited / non-existent. My experience has been with “steep” slope metal roofing, not with “low” slope built-up roofing. That said, I will say this … “synthetic underlayment” generally refers to polymer products — no asphaltic layer at all. The synthetics do have long warranties but, yes, that’s because, in a proper installation, they are completely shielded from all elements of weather.
I am sorry … I really can’t weigh in on your three options. Many low slope roofs are having good success today with single ply products such as PVC and TPO membranes. These products generally have a life of 15 – 20 years in a harsh environment like yours.
I am sorry I can’t be of more help. I would suggest contacting some of the manufacturers of the products you’re considering, in order to gain their expertise.
Thanks Todd for the reply.
Maybe this will be closer to your expertise, while not metal, still some Self Adhesive’s I’m looking to possible use that seem to work well under metal as well.
1. In order for me to get credit/reduction off my $600/mo Hurricane Insurance, the insurance companies require 1 of 2 options:
A. Self Adhesive to Decking
B. If no SA to decking, then at least 6 inch wide strips of SA across all Plywood seams
2. Miami Dade Code, requires at least #30 nailed to Decking, followed by approved cap sheet like #90 hot mop, or a Self-Adhesive
Yes, there is a conflict between what the insurance companies want vs. what the code is??
My roofer has never laid down SA before in 30 years, but has done 30/90 in his sleep. He open to doing SA.
I’ve decided on SA after #43 ASTM basesheet. I’m a little concerned in his lack of experience with laying down SA.
The other option was 2 ply of #43, with SA over the 2ply in areas like Valleys, eaves, drip edge, flashings, etc..
I have done research, narrowed it down to the following:
-Atlas Weathermaster TU Ultra SE, 50 yr warr
– Tarco PS200HT, 30 yr warr
– CertainTeed Winterguard HT, 50 yrs warr
-Polystick TU Plus, 20 yrs warr
Have you had any experience with these 4 ? Any thoughts?
I know many people recommend Grace I&W, which I can still keep as option, but its a lot more than these products above and probably they all very good.
Any other recommendations?
Thanks Todd
Thanks. I will share a few thoughts.
If you’re in Dade County, then you must do what Dade County says or you will never get the job permitted and approved. The insurance company should not argue with doing what Dade County says.
I am unaware of any self-adhering product warranted for any application unless it is being adhered directly to decking. Personally, I think the products can work well when applied over other products but their manufacturers, to the best of my knowledge, all require installation direct to a clean roof deck.
I can’t really speak specifically to the four products you mentioned but, frankly, the main marker of the quality of a self adhering product is its thickness. All products should state the thickness in their specs. The thicker the product, the more expensive it is, and the higher quality it will be.
Look also at the top surface of the product and make sure that it is compatible with whatever might be installed on top of it. For example, I am not a fan of granulated top surface products beneath metal nor beneath single ply membranes.
I hope this helps some.
Good stuff Todd. That all makes sense, and it is baffling how Miami Dade Code and insurance companies require different techniques. I’ve been told, Miami Dade Code requires a #30 is due to the SA applications, that is, they believe down the road it would be difficult to do repairs without ripping up the decking. With the #30 you can simply uplift that 1st application then repair/replace much easier.
As far as the products, that makes sense the thicker/heavier the better products, but just maybe that’s the opposite??
So another words, maybe the thicker and the heavier has the cheaper products??
Let me explain, as I reviewed the specs of all the different SA’s, the “Gold Standard” is Grace Ice & Water
Their data sheet shows :
Thick: 40 mils
weight: 55lbs
top: polyethylen film
Now the Grace Ultra which is 2x thicker than Grace I & W is only 30 mill thick
Here are the rest:
Certain Teed Winter Guard HT (High Temp)
Thick 45 mills
Lbs: 54 (F
(Sand & Granular Top Layer both 68 lbs)
Polystick TU Plus
thick: 80 mil
70lbs
Top: polyster film
Tarco Leak Barrier PS200HT
Thick : 60 Mil
55lbs
Top : Polyster Film
Atlas Weathermaster TU Ultra SE
Thick: 60 mil
Weight: ??
Top: Polyester Film
Warranties:
Grace products= ? Couldn’t find
Atlas Weathermaster TU Ultra SE- 50 yrs
CertainTeed Winterguard= 50 yrs
Tarco LeakBarrier PS200HT= 30 yrs
Polystick TU Plus- 20 yrs
When looking at all this, the heaviest & thickest of the SA’s is Tarco and yet they have the shortest warranty. This was my thinking to state maybe its all backwards, that the thicker & heavier means the lower quality of materials used?
Any thoughts?
Thank you
Thanks. There are, I believe, two basic chemistries in terms of the “sticky stuff” in ice and watershield products. The original and most common technology is a rubberized asphaltic product. The newer kid on the block is a butyl rubber. The butyl will be more melt-resistant so it is more common in high temp products. It also suffices in thinner quantities.
Again, I can’t stress enough … we really are outside my area of expertise in the finer details of these products. I really encourage you to contact the manufacturers.
If the chemistry is the same, though, thicker products will be better.
I would not put a great deal of stock in warranties, frankly. If the roof is performing correctly, then any product should do just fine. Some companies, though, do make a bigger “marketing game” out of warranties than others. If you’re really going to look at warranties, you need to look at exclusions, limitations, pro-rations, transferability, etc. Again, though, in terms of roofing underlayment, once the roof system is completed, assuming it’s not a tile roof, most of these products will perform similarly.
** NOTE: Meant Grace Ultra is 2x more expensive than Grace I & W Shield, not thicker.
When you install Tin over pre-existing singles will the Sun’s temperature on the metal rot the old shingles?
Thanks so much. Not that it really impacts your question but most metal roofs today are steel or aluminum. My experience has been over 35+ years and tens of thousands of installations of metal roofs over old shingles. I always suggest a layer of underlayment between the old shingles and the metal roof. I have yet to go back on a job where anything had happened to make me say that I wish the old shingles had been removed. Once the old shingles are protected from rain and sun, they stay pretty stable. I am sure that they deteriorate to some degree but you’re not depending upon them for weather tightness, so there is no real harm. Here is a link to a short video on this subject: https://www.youtube.com/watch?v=bfD-0xvdJGc
First off I applaud this fellow roofer coming out with straight answers for consumers to make better decisions that if they’d never visited this helpful blog. I had been roofing for around 40 yrs in several states including California, Oregon, Nevada, Washington and Idaho. In California in my early years metal roofs were just about nowhere to be found in residential areas. Existing roofs consisted of semi-flat rock roofs that automatically required a tear off and old Composition Roofing that only required a tear off if the combination of existing comp roofs exceeded engineering weight specs or were too heavily damaged having been left well beyond their warranties or expected life spans.
The company I worked for was family owned and we did everything from custom cedar shake (fire retardant dipped later on) , composition, slate, tile, metal, flat roof commercial jobs consisting of torch downs, etc. One of our mottos when going up against a Big Commercial Roofing Operations on bidding was, “We Bid them to Get Them”. One such time that we would push for leaving a existing comp shingle roof on was when a lot more greedy company would be pushing the customer to do a full tear off down to the wood prior to applying a new metal roof on a house.
First on my list to tout to prospective customers was if their existing roof had no serious problems….. then this would be the best choice and would save them money on top of that. I mean if an existing Comp Roof is crumbling from out living it’s expected life span….. then you definitely don’t want to trap that debris, dirt and material on your roof. Secondly is that old roofing material will end up filling a land fill at your expense. Which is a good reason to leave it in place on a reroofing job alone. But….. there are a number of other very good reasons besides cost to remove it to consider. Like this roofer says….. removing that old roof takes away R value and what you’d been hearing in the house as muffled sound of rain….. suddenly could end up being louder not just because of the Metal Roof, but because that old comp roofing material acts as somewhat of a sound insulator too.
If tearing off to the wood, there will be added costs for your metal re-roofing job to consider that wouldn’t need to necessarily pay out for unless it was a new roof instead of a reroof. But for my entire career I nearly always recommended leaving the old comp shingle roof in place if there were no signs of problems or leaks. It was the best way to save customers money over other roofers that were more interested in getting some cheap labor Mexicans or students to tear an existing roof off. It also took money out of my pockets….. since we did all our own tear offs too. And lastly Murphy’s Law seems to always visit you on a roofing with an unexpected rainstorm once you have the roof torn off. Then you are scrambling to get it covered to protect against damage and that’s one stress both the homeowner and the roofers can do without!!! ….I have no idea how many roofs I’ve gone up on in the middle of a storm to cover it before ending up with damage to fix! 😀
Great to hear from you. Sounds like you did the right thing by your customers. We need more folks like you in this industry — those who realize what they are being entrusted to whenever they accept a roof contract … and they are determined to do things the right way.Thank you for the example you set and lived.
Do i need to create an air gap between the asphalt shingles and new steel roofing. Or can i go directly over it???
Thanks so much. Here’s a link to an article I have written on this subject.
https://www.asktoddmiller.com/articles/metal-roof-battens/
I have a steel-tile roof that was installed over an existing wood shake roof, 26 years ago, and I’ve regretted it from the beginning, and I’ve had numerous problems that will probably require me to replace the roof now – 24 years before its expected lifetime. I’ll list some issues with the concept, in priority order:
1. I’ve had countless issues with rats getting in the attic, in spite of all attempts to close of an access points… See #2.
2. The wood shake roof was 26 years old and apparently had some termite and rotting issues. Aside from continuing deterioration, rodents found weaknesses in the wood shake roof and made countless holes/entrances in it. Being under the steel tile roof makes these next to impossible to find and repair. One pest control person watched a rat escape through such a hole. He went on the roof, pulled up some tiles, and showed me a huge hole. He fixed it, but there are likely too many other spots to reasonably repair.
3. The affected wood shake roof likely also affected the continued deterioration of the fascia and soffets.
4. There are recorded instances in which wood shake roofs caught fire underneath the steel tile roofs, and extinguishing the fire was either extremely difficult or futile.
5. It just does not look good to have the roof raised a few extra inches, and this, I think, is what makes #1 such a problem.
Aside from the issues with the installation on top of the wood shake roof, I likely would not go with a steel-tile roof again in the future – even if installed cleanly on a new platform rather than an old wood roof. The main issue is that no matter how much instruction I’ve given workers on how to walk on the roof, it is counter-intuitive, and they inevitably end up bending the tiles by walking on them incorrectly.
Thanks so much for your post and your comments. I am sorry to hear of your experiences. You have dealt with roof problems for a very long period.
It’s interesting that my metal roof on my home is nearly as old as yours (23 years) and was installed over a layer of asphalt shingles. We have not had any issues or concerns with it.
It sounds like you have a batten-mounted steel roof, perhaps stone-coated, over wood shakes. There is no doubt that such installations require some extra care to make sure that all is sealed up well. Unfortunately, it sounds like the installers fell short. I have seen many such installations without issues.
That said, I’d say that it is less common to do installations like yours now than it was then. Insurances companies, mortgage companies, etc. prefer to have the old shakes removed so even my suggestion today is commonly to remove them.
Additionally, the batten mount of your metal shingles does impact their walkability.
As far as fire, I have heard a couple of times over the years of what you describe. Steel roofs though do offer great protection from exterior fires. If a structure has a high risk of an interior fire for some reason, I will often suggest aluminum rather than steel for the roof as aluminum has a lower melting point. It still provides protection from external burning brands but it will burn (melt, really) through from the bottom side in the case of a structural fire.
Again, thank you for your comments. I am sorry to hear your story and I will take it and learn from it and allow it to influence my future recommendations to other property owners. I wish you and I would have met 26 years ago before this roof was installed. Hopefully, I would have provided other advice but, then again, I only had about 9 years of industry experience at the time. We are all always learning.
An invaluable thread indeed!
Ref: NO.16/17 underlayment reason/requirement
in this case, layers: existing shingle, underlayment, battens, metal
1. Surely, code is a point of contention, since it aims to make sure functional gain is the result.
But the prevailing thinking is the functionality of a particular approach.
The primary function of a roof is to keep what’s under it as dry as possible, if not 100% dry.
That includes stopping water and managing condensation.
If the installation is accurate, rarely would a storm make its way passed the metal skin, though that can and does happen.
Even if your installation IS top-notch(huhhh Mr. Roofer, it better be!), functionally the underlayment is still needed to catch condensation.
All surfaces like glass, metal, even porous ones are subject to condensation under temperature changes.
– morning dew on grass and plants is a good clue.
Even with the best ventilation, as for instance when installed on vertical battens/furring
(longitudinal to the seams on a gable/hip roof to guarantee unimpaired air flow to the ridge cap, see NO. 2 below),
condensation will build under the metal skin. drip drip drip…
…and why cutting corners by not installing a proper underlayment is not wise.
2. …Possibly introducing a new issue here…
– Standing Seam with concealed brackets to allow expansion in both(all) directions.
– battens have 2 major reasons: unimpaired air flow to ridge, an separating the metal expansion from the under-laying support
– battens installed longitudinally! Thus for instance, on a 16″ panel-width, a 1×1 in. batten is installed 8″ center to create walking support at the mid point of the metal… is this a problem? (oil-canning, other?)
– battens are counter-sunk pre-drilled on 24″ per the size of the (screw)fastener – if (screw)fastener fails a solid grab, hole is injected with tar gun… and near new location elected.
t.y.
Thank you. I appreciate your keen observations. I am unaware of any metal roofing manufacturer who has tested nor would support installation of their vertical seam panels over battens that are vertically oriented on the roof. However, in order to achieve continuous airflow beneath the roof system, one option is to install vertical battens and then horizontal battens followed by the roof panels.
Hi, Todd. We just got a quote today from a nice gentleman for a metal roof. I told him that we have a few sheets of the plywood under our existing asphalt shingles that are bowed in pretty bad. I asked him if he would like to look in the attic and he declined. He did walk on just about every inch of the roof and he did notice where the bowed spots were. He stated something along the line of the spots indeed feeling bowed but not feeling soft (which I asume he means they are not rotted since they are not “soft”). He said the sheeting would not need replaced and that using 1×4 battens over the existing roof would be fine. Should I take his word for it or should I insist that he address and fix the bowed plywood? I obviously don’t want to spend more money than I have to but I want things done right so I don’t ha e issues moving forward. Thank you for any insight you might have!
My suggestion would be to have someone take a look in the attic to ensure that no bad decking can be seen. What he is suggesting would attach the battens to the rafters so they should be solid. The question as to whether the battens are necessary is a bit of a question. Make sure the metal panels he installs are approved for installation over battens.
Very helpful info on this topic. I have a few questions, thanks
Okay, we are making our final decision on doing the roof on the house and shop. The metal roofs we are looking at are about 25K including colors and “double bubble” insulation on purlins. The prices do not include removal of the single layer asphalt shingles, but replacement and dry-in of sheets that are damaged. We have had 5-6 estimators who are saying that there is some “spongy” plywood that would need to be replaced.
2 of the estimators have given us a price for removal of the old roof between 5K and 8.5K.
So my questions are:
1. if you don’t remove all of the old shingles, how do you know if you if they got all of the damaged plywood.
2. when they put the “rhino synthetic underlayment” over the old shingles what happens to any moisture that might be in the plywood?
The asphalt shingle roofs are about 18.5K to about 21K.
We are probably going to be putting the house on the market between 8 and 10 years so a shingle roof would be about halfway thru it’s expected life.
Last question, (for now) is there really a significant difference in heat absorption between asphalt roofing and metal roofing with insulation in between?
It sounds to me like there is enough evidence of damaged decking that all of the shingles should be torn off, no matter what type of roofing is being installed. This will also good inspection of the roof deck. Once the roof is covered, any moisture in the decking will evaporate / dry up. That happens all the time. Yes, there will be a heat gain difference. I am not sure where you’re located but you probably will have a savings of 15-20% during the cooling season,
We have a triple wide prefabed home. When the installers joined the pieces together they took the plastic sheeting from the open sides and bunched it up and placed it over the joints pryer to putting on the shingles. I wasnt here due to work and only found it out later and was told it was ok and it would settle down after a while! Well 20 years later and you can still see those bumps. there not real hugh but still noticable. Question? can i space 1×3 on both sides of these bumps and continue on spacing boards on from there on down?
I assume your plan is to install a metal roof over the 1 x 3’s? As long as the bumps do not exceed the 1 x 3’s, you should be fine. Make sure that the metal roof you’re installing is approved for installation over battens rather than solid decking.
my house has wood shingles with asphalt on top of that and I am needing a new roof because mine is 15 years old. can I lay over both existing roofs or do they need to come off?
My opinion is they both should come off. From a technical standpoint, it might be possible to go over them but I do not recommend it. Most building codes do not allow more than 2 layers of roofing. Even if you live in an area where that code is not enforced, going over two layers still concerns me. I’d be concerned it could be a problem down the road for you as a red flag thrown up by a home inspector or mortgage company. That would like arise as an issue should you sell the home.
Hi Todd,
I am currently in the planning process of replacing the roof on my Wisconsin home. The home is a ranch style home with a 4/12 pitch roof. I replaced the shingles on it about 20 years ago. At that time I peeled off all layers of shingles down to the decking and replaced it with a layer of felt and then asphalt shingles over that. I would really like to put a steel roof on the home using classic rib steel roof panels. The current shingle roof is still in pretty good shape, no curled or missing shingles and the 3/4″ plywood decking is still in very good shape. Should I put down another layer of felt before putting on the steel? Also, there are 2 roof vents currently on the roof that I would like to remove and replace with vented ridge cap when I install the steel. There are also vented soffit panels under the eaves. And my last question is in regard to the type of screws to use for fastening down the steel. I want to make sure the rubber washers on the screws are going to hold up to deterioration from the weather. Any comments or suggestions on this would be greatly appreciated!
In your case (I say that because not all folks reading these posts may have the same exact situation so I do not want them to jump to wrong conclusions), you should be okay leaving the old shingles. A layer of underlayment should go over those old shingles. Most folks have started using a premium grade synthetic underlayment beneath metal roofs, such as RoofAquaGuard UDLX. You will want to fill in the old vent holes with lumber and also underlayment or something on the surface to bring things up to the thickness of the rest of the roof. As far as screws, I am much less an expert on those but I think that generally you get what you pay for and you want a cap head on the screw that covers the washer well. Getting the screws driven in completely perpendicular to the roof is important. Here is a brand of screw that I have heard good things about: https://wtfasteners.com Please contact me anytime.
Hello, and thanks for the useful information. I’m in upstate NY.
I am considering getting standing seam installed over one layer of asphalt shingles. The shingles were installed only did years ago, but they were installed incorrectly, so the over and water barrier doesn’t go over there drop edge. Additionally the fascia was never cut down after switching to shingles from very thick concrete tiles, so there is a slight cupping up near the drip edge where water collects.
The new standing seam roof would be installed over battens, which would raise the roof to the correct height for the fascia. My concern would be if water somehow got under the metal roof, it might sit there or continue to leak under and further damage the soffits. When a standing seam roof is installed this way, does it have a drop edge that would meet the gutter, so I wouldn’t have to worry about water getting to the roof underneath?
Or should either the fascia be corrected it the original roof torn off?
Additionally I have heard about people installing insulating boards under there metal roof. Is that a good idea, or is having room to breath better?
Lastly I think my current roof breaths ok, but could definitely be better. What would be the best way to prevent ice damming with the new roof? Would adding insulation under the roofing boards between the rafters be a good alternative, preventing the hot air from getting too the roof?
Finally, I have seen you mention underlayment many times. My roofer didn’t think that was a good idea, but might be willing to if I push him. How much would a reasonable additional cost be to add an underlayment over there existing roof?
Thanks,
Evan
Thanks Evan. It is good to hear from you. Personally I would get the fascia boards corrected. I also generally am not a fan of battens. They make the roof harder to walk and more prone to uplift issues. Tell me a bit about the ventilation in your attic. Do you have severe ice dam issues? Have you sealed all possible air leaks from the home into the attic? A lot of folks would put underlayment on top of the starters if you have any drainage. Insulation boards? On steeper pitched roofs I am okay with them but not on lower pitch roofs. Underlayment is required by building code. Please contact me anytime. You can email direct to me at todd@asktoddmiller.com if that is helpful.
I’m reading your article and I wich I would have read this before, the roofing company that install the metal roof of my house in Vero Beach, they took of all the wood roof shingle and then they install the metal. They could have done it faster and cheaper without removing the wood shingles. Now I’m working for a roofing company and I hope this will not happen me again.
Thank you for the article ….
Hello Todd,
Your website has been very helpful. I’m looking to replace the roofing on a small 24′ x 38′ barn that currently has old three-tab asphalt shingles and am looking to roof over it with at a 27 gauge ribbed metal product. My question is about the existing roof which I believe to be structurally sound but is far from flat. Running strings the 17′ from roof to eave reveals up to 2 1/2″ of belly at the most pronounced locations. I might be able to jack up and reinforce a few rafters and collar ties to address some of this, paired with layered and shimmed strapping but I don’t imagine that I’m going to get close to perfectly flat without a potentially budget-breaking amount of work. Can you offer thoughts about what amount of sag or belly would be acceptable for ribbed panels without compromising their performance? I’m not deeply concerned about the aesthetics of the roof not being perfectly flat, the whole structure has out of square character, but I’d like to avoid leak issues by trying to significantly bend the roofing. My other option would be to strip and replace the asphalt but I’d prefer to go with metal. Thanks for any advice you can lend.
Jon
Thanks Jon.
I’d certainly suggest trying to even things out if you can. But, if you’re not concerned about aesthetics, the panel you’re installing is probably pretty flexible. The manufacturer may be able to shed more light on this for you.
If your roof panels can be screwed through the overlapping seam, that will help make sure the seam stay tight.
One option would be to cross batten — vertical battens and then horizontal battens followed by the roofing. This is assuming your roof panels can be installed on battens. That should smooth things out though. Of course, battens can make the roof harder to walk and less wind resistant. Cross battening would also require gutters to be raised and could become a problem with dormers, etc on the roof.
Hi Todd, I’ve read your forum from the beginning to end but haven’t found much that addresses the issues I have. I’ve been working with a roofer who thought he’d solved a series of leaks by installing a new layer of asphalt shingles on an old shed roof. This didn’t solve the problem. He has been back numerous times but can’t find the source of the leak(s). He mentioned installing steel at one point but there is a big dip up the middle of the roof and also a change in pitch partway up from the eaves. The building was built in the 1880s so age and poor craftsmanship are at fault here. The roof is approximately 30′ wide and 65′ from eave to pitch. I’m wondering if I’ll need to have the 2 layers of shingles stripped so that the dip and swale can be corrected or if battens can be installed over the rafters in a fashion so as to eliminate both defects. I’ve shimmed plywood subfloor to resolve similar problems but don’t have much experience with roofs. I’m in Ontario and am unsure of what the building code requires. Maybe I’ll have to talk to the town for guidance. I’m also not sure if I can trust this roofer to solve the problem given the number of attempts he’s already made. I’m somewhat disabled and can’t do the required work myself. Thanks so much.
Metal roofs can be installed over your existing roof without tearing off shingles. Your roofers can install a vented metal roof that eliminates all potential problem.