Metal roof installed over fiberglass shingles

I inspected my roof and noticed the metal roof had been installed directly on top of fiberglass shingles. Is this acceptable? Someone had told me this is not a good method because of expansion and contraction the granules can wear on the underside of the metal roof, reducing the life of the material. This makes sense, but is it this accurate?

Thanks in advance,
Lonnie

In general, it can be done. An underlayment is necessary and you will need to confirm with the metal roof panel manufacturer to ensure this method complies with their recommended installation requirements to qualify for warranty and to ensure longevity of their roofing components. I, however, don’t recommend it. In my opinion it’s best to always strip the roof of all coverings, examine the deck, and go from there.

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Like Simon said, generally, it can be done:

Good article. How would you comment on this on your report? As a non-issue, merely pointing out as an observation, i.e. “metal roof has been installed over one layer of fiberglass shingles “?

What is typically used under metal roofing on top of shingles is what they call a slip-sheet.
Basically, all it is, is a rosin paper. That is what roofers use in the State here. Helps protect the bottom side of the metal from abrasion that could cause rust later on.
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There are other underlayments that will do the same job.

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If you don’t want to take our word for it check with your local building department. In some areas building officials only allow a re-roof over one layer of shingles. Other areas will allow you to re-roof over two layers. In most cases it’s not even an issue if you are only going over one layer of shingles.

I would follow the bolded text above and check with the building department. While you are at it, check for permits.

The big issue here is that you are required to renail the roof deck if the previous nailing pattern does not conform to the existing code. That is probably going to be somewhat difficult with a layer of shingles in the way…

And one other issue to be aware of is whether new flashing was installed with the metal roof. If not, you will have water getting onto the shingle roof and you won’t know what that condition is…

It’s usually a cheap way out of doing the job right, shortens life of the new material and many companies don’t want to warranty, but yes it can and is done. I would not recommend it.