I received the following question recently from a homeowner named Tom. He is obviously doing his homework to ensure a successful roofing project. I like that!
Todd, I saw your info on metal roofs over shingles and have a few questions. I have a double wide low pitch roof that has some saging between the rafters. I believe from snow loads. I just bought the house mainly for the 6 acres and think it is worth rehabing the house because it sits on a nice basement. I am going to tear the ceiling down to inspect all rafters and fix any issues with cripples so the roof should be prety straight before I put on the metal. I would still like to put down 2×4 perlins and shim as needed. Is there any advantage to put styrofoam between the perlins if the attic is ventilated? If I have spray foam installed in the underside of roof deck and then use styrofoam between perlins will I have moisture issues between the 2 insulation? I plan on using synthetic underlayment. Should I put it down before purlins or up and over them. Thank you for your experenced advise. Tom
Here was my response:
Thanks so much.
There is a chance that your sagging decking is the result of weakened decking due to age, moisture, roof issues, or the original quality. I suspect you will figure that out once you open things up.
With the ventilation in the attic and the spray foam on the underside of the roof deck, I do not see a lot of benefit to insulation on top of the deck. Yes, there would be additional R Value but I think the return on investment would be very long.
I think you could install metal roofing directly on top of the decking and underlayment. However, going ahead and putting down purlins, and no insulation, will create a good thermal break which could be very effective both with energy efficiency and making sure that you do not have thermal bridging that causes condensation anywhere.
Make sure that the metal roof you select is approved by its manufacturer for installation on the pitch of your roof and also that, if you go over purlins, it is approved for installation over purlins.
I hope this helps.
Feel free to contact me anytime.