Is it ok to install a 30yr architectural shingle over an existing 20yr fiberglass shingle on a 6/12 roof. the roof that is on there is in ok shape and the only reason the customer wants to change the roof is because a tree feel on part of the roof and thought might as well replace the whole thing. just wondering if anybody has done it in the past or do you HAVE TO tare off? didnt think it should be done but on the bundle of shingles it has instructions for re-roofing. thank you.