Q: A reader asks if you can install metal roofing over shingles. Tearing off shingles is messy, expensive, and bad for the environment.He says, "I read an article by you on metal roofing installed on top of existing shingles.. your only concern seems to be the weight... any thought to the idea that the 2nd layer of roofing acts like a 2nd vapor barrier whereby dry rot is likely 15 years down the road with this approach?"
The reader goes on to say that one roofer recommended against installing metal roof over shingles, but that another roofer said it was plausible.
A: As it turns out, both roofers are right (Or wrong. However you prefer to look at it).Water vapor in conjunction with metal roof on existing shingles is a problem. The kind of build-up of vapor and attendant mold and rot mentioned by the reader is a very real concern.
But that doesn't mean you can't install metal over shingles. Roofers can install a vented metal roof can be installed over existing shingles which would obviate the vapor problem. Also, installing the metal roof on top of furring strips (1x3's or similar) will raise the metal and provide an air pocket between it and the shingles.