Install Over Old Shingles

September 1, 2009 | Filed under: Misc, Roofing Answers, Roofing Installation

Q: We’re looking to replace our asphalt shingle roof with a 26 gauge galvalume metal roof.
In your opinion should we have the old shingles removed or just have the metal place over the shingles? Also, if leaving the shingles should we have anything between the shingles and the wood?
We live in central Texas and we want to be sure the metal doesn’t conform to the shape of the shingles. Is this a common problem? Any help or suggestions would be greatly appreciated.
Thanks for your time!

A: First of all, whether you remove the old shingles or not, I suggest a new layer of underlayment, either 30 pound felt or one of the synthetics. In your area, the synthetics are great because they act as a slip sheet and an underlayment. Slip sheets are important in warmer climates.

I am not sure if the roof you are installing is standing seam or through fastened. Most of the through fastened profiles have enough corrugation to be pretty forgiving and not have the profile of the old shingles telescope through.

With standing seam, it can be an issue and frankly just depends a lot upon how curled the old shingles are.

Ultimately I would suggest sending a picture of the existing shingles to the manufacturer of the roofing and getting their input before proceeding.

Also, keep in mind that in most areas the building code specifies no more than two layers of roofing.

The low weight of metal does encourage installation over the existing shingles though, even multiple layers.

I hope this helps.

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