It would be best to tear off all the old roofing. That would give you the chance to inspect the roof decking and repair it as needed. Also the new tar paper laid down before the new shingles would give you your best bet against future leaks.
Dan's right, it is best to do a "tear-off" but sometimes , either money is short, or lots of nice landscaping or awnings and other protrusions don't allow for it. If the shingles are worn but not curling up and there are no leaks seen in the attic or ceilings, I'll do a roof-over and use new felt paper and 2" roofing nails. Valleys can be re-flashed and chimneys can have the aprons re worked with aluminum coil stock to truly seal the deal for another 15-20 years...save your dough because
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codes prevent a 3 layer roof-over. Hope this helps, best to you..JL
You will need to verify that the existing roof decking and shingles were nailed according to current codes. If not most municipalities will require you to tear off existing roofing and bring it up to code. This can be offset by doing a wind mitigations inspection on the newly nailed off decking & roofing, thereby saving you thousands on your homeowner's insurance. Check with a qualified local roofer and your buidling department. Our MR Roofing division can help you as well as provide
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you with the wind mitigations forms. Check with your insurance company to see what discount they offer.
Only consider a roofover if it is a budget issue. Otherwise, recommend taking it to the sheathing. Would you put on a clean pair of underwear without taking the old off and showering first? I rest my case...:) Tim
It would be best to tear off all the old roofing. That would give you the chance to inspect the roof decking and repair it as needed. Also the new tar paper laid down before the new shingles would give you your best bet against future leaks.