The weight of one layer of shingles could add a couple thousand pounds to your roof. From what I've read it's about 250 lbs. per 10' X 10' area. If you don't remove the first layer when replacing it you are doubling the stress. When we have a lot of snow in the winter everyone is worrying about roof collapses and damage. Do you really want to add a few more thousand pounds in roofing on top of the existing weight?
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