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My question had a obvious motive, a maybe a potential new marketing message,,, replacing your roof with new shingles? is GM present? We will inspect it for free. Do roofers always do a tear off when GM is present, no way, most of them don't even know what it is, most shingle salespeople don't know what is causing roof stains, most builder's just recommend a AR shingle that drives up costs, and performs poorly in most cases.

Why trap all that moisture under your new shingles? Who would advise that? Have your algae eradicated prior to your overlay.

Would you finish your basement wall with drywall where mold and moisture is present?

Give your local roof cleaner a call prior to letting a roofing company overlay your infested shingle.

I had a roofer tell me today that he overlays on GM all the time, no sun, no life, moss comes out with pressure washer and brush, he says, another roofer said that he does not recommend an overlay in general, likes to inspect entire roof, logic best option.

Mr. roof expert said GM it is cosmetic, no evidence of shingle fatigue or failure from it, and said that mold is almost never found on shingles, lastly, the inference that attic temp increases and related increase energy cost have never been proven, and was disingenuous.

I'll never ask a person that sells new roofs what time is, let alone a question about shingles.

It is never good to put a second or third layer over the existing shingles. I always explain to the homeowner that the new shingles automatically loose half of their life span when you install them on top of existing shingles.

If the existing shingles are infested with mold, algae, moss and lichen, I would advise... to clean or remove them before installing the new roof.