Having two layers of shingles on your roof, known as a layover or re-roof, is legally allowed in many jurisdictions under building codes, which typically permit a maximum of two layers of asphalt shingles, but it is generally not recommended by manufacturers and roofing professionals. The main advantage is lower upfront cost, as you save on tear-off labor and disposal fees, and it is less messy. However, disadvantages outweigh savings for most homes. Two layers add significant extra weight, about 350 to 450 pounds per square, which can stress older roof structures. It also traps more heat, which can accelerate aging of the new shingles. You cannot inspect the wood deck for rot or damage, and any existing problems remain hidden underneath. Manufacturer warranties are often reduced or voided for overlay installations, and the new shingles may not seal or lay flat properly over the uneven old layer, leading to a less attractive appearance and reduced wind resistance. Building codes prohibit overlay if the existing roof already has two layers, if the deck is rotted, or if the existing shingles are severely curled. For a long-lasting, warrantied roof that can be inspected properly, a complete tear-off to the deck with new underlayment is the best practice, even though it costs more initially.
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