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I am planning to have a metal roof installed. My local supplier stated the metal may be installed over one layer of asphalt shingles. What is the disadvantage for installing over the shingles?
Many metal roofs are installed over a layer of asphalt shingles. In most areas, building codes allow for two layers of roofing. It is generally suggested to not apply standing seam over old shingles as it could force ripples (aka "oilcanning") into the standing seam panels. However, metal shingles and corrugated roof panels can be installed this way. I interpret the building code as requiring a new layer of underlayment between the two layers of roofing, and I also find that to be a good practice for many reasons.
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