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I want to reroof my garage, but I think my old shingles are asbestos shingles. I don’t want to remove them. The decking underneath is in fine shape. So my question is (because the shingles have a flat, even surface), can I put osb on top of these shingles with longer screws to reach the decking underneath, then tar paper and shingles? Honestly, I don’t care about codes, but I do care about a sage opinion.

If not, why not?

(I can work out the moldings and drip lines just fine if I do it over the shingles)

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  • There may be an issue with the overall weight. You'll be adding a couple of lbs per square foot of permanent weight. Jul 16, 2021 at 1:57
  • Wouldn’t that be ok because my roof 2x4s are 16” apart? I don’t know, but it seems like it would be ok to me. Good point on weight. I’m not sure. Jul 16, 2021 at 1:59
  • There is no single answer for roofs. As I understand it, the calculation includes the weight of the roof itself (which you are increasing), the weight of snow (calculation dependent on expected maximum snowfall in a given area - Arizona and Vermont have different numbers), use of the roof (for flat roofs), and possibly other issues due to hurricane or earthquake zones. Jul 16, 2021 at 2:06
  • Thanks! Glad to know more parameters to look into Jul 16, 2021 at 2:52
  • Are these tile type shingles? With the Sold shingles like that as long as you don’t break them there is not a problem. In any case I would say “are you nuts”? Wanting to put a sheeting product over the shingles sounds like it would not be allowed by building codes. I would have them tested in any case so you know for sure if it’s asbestos. But it’s not hard to remove the tiles. If you live in a state that allows disposal it is a bit more work than a asphalt roof but a better option than trying to sheet over them in my opinion.
    – Ed Beal
    Jul 16, 2021 at 3:23

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Where in the world are you? If United States..

Available information is rather contradictory. Some resources suggest that asbestos may have been used in shingles as recently as the mid- to late-1980's, for example Burns Environmental Services writes "asbestos roofing was so prevalent between 1920 and 1986 ..." On the other hand, ShingleRecycling.org writes "asbestos was rarely used in the shingles themselves."

Based on The Spruce writing "in 1989, asbestos became illegal when The Environmental Protection Agency (EPA) issued an Asbestos Ban and Phase-Out Rule" one would think that shingles manufactured after that date surely must be asbestos-free.. however, The Mesothelioma Center's advocacy site Asbestos.com warns that asbestos is still legal for use in roofing.

It seems the cost of testing may be quite minor, less than the cost of two sheets of 7/16" OSB in today's crazy lumber market. Asbestos123 says US$50-80 while Asbestos Siding claims as little as US$30.

Even if the shingles do contain asbestos I'd expect it to be non-friable and therefore minimally hazardous to remove or dispose. Testing would confirm.

You probably could do fine putting new decking over the existing roof. Crazier things surely have been done. But it seems like a lot of trouble and expense to avoid the unknown. Besides, even if testing does confirm presence of asbestos, you're not required to remove the stuff. You could still choose to shingle over it.

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