I recently received an email asking:
“I hear differing opinions on installing metal over old shingles. Can the metal be too much weight on a home?”
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. 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.