Going Over Old Roofs (Roofing Weight)
December 15, 2009 | Filed under: Benefits of Metal Roofing, Misc, Roofing Answers, Roofing Installation
Q: “I hear differing opinions on installing metal over old shingles. Can the metal be too much roofing weight on a home?”
A: Most metal roofs weigh from 40 pounds per square (aluminum) up to maybe 145 (for stone coated steel). A square is 100 square feet. Asphalt shingles on the other hand weigh from 275 – 450 pounds. When you get into products like slate and tile, weights can be pushing 2000 pounds per square!
These numbers show just how miniscule the weight of metal is.
Building codes allow two layers of standard shingles – that would be up to 900 pounds per square of roofing weight. Additionally, most homes are constructed to carry at least 3000 pounds of snowload for that area.
As asphalt (or fiberglass) shingles age, they lose weight. This is from granules that wear off and oils that evaporate. I have never seen any studies on this but it would not surprise me if an aged asphalt shingle has lost 30% of its original weight.
For that reason, in many cases, putting a metal roof over an aged shingle roof may still be less total weight than when the asphalt shingle was originally installed!
Now, while building codes say no more than two layers of roofing shingles, I have had many instances over the years when building inspectors have allowed metal as a third layer, because of the weight issues.
Learn more about the weight of roofing shingles.
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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can i have a metal roof installed on top of my slate roof?
Bruce,
That is a good question. First of all I would never suggest doing anything without the approval of your metal roofing manufacturer.
That said, as a manufacturer of metal roofing, I have been involved with a few successful installations over slate. What I suggest is using a self drilling self tapping stainless steel screw for fastening. If test fasteners show that your slate is such that these fasteners can be used without cracking the slate badly then this sort of installation has potential. If not, then tear off and if necessary re-deck will be required.
One advantage of removing the slate:
You can often resell it for pretty good prices which helps pay for the cost of tearing it off.