This is generally not recommended. It’s best to remove old roofing first to make sure there are no other dry rot or structural issues that need to be addressed. In addition to that, layering several roofing materials may lead to a ventilation problem, as well as create a very heavy roof. Metal roofing is the only lightweight material that may be installed over an existent roof, but be sure to talk to your Seipp roofing contractor first.
A roof starts with a roof deck, which is a skeleton of the entire structure made of wooden rafters. Plywood is nailed to the rafters to create a smooth and even roof surface. The roof deck is then covered with underlayment—a leak barrier to keep the moisture out. Roofing materials, such as asphalt shingles, metal, etc., are nailed on top of the underlayment. Then flashings are installed around the chimney, plumbing vents and at the edges to ensure no gaps remain.