My client will often ask me while on a home inspection "how many layers of shingles do they allow on a roof?". It is ultimately best to have one layer, but most building organizations have agreed than you can have up to three (like you can see in my home inspection picture from Albany/Loudonville NY recently).
There are many downsides to having multiple layers of shingles though. 1: With each layer you put on, the life span of the shingle is shortened. 2: the shingles can hold moisture in between the layers. 3: Roof damage is harder to detect for the home owner because of the multiple layers and you don't always catch the leaks in time. 3: the cost of stripping a roof with three layers can get very pricey when you have to replace it.
Ian M. Robertson,
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