Your house has two layers of shingles because a previous owner or contractor chose to perform a layover or re-roof, installing new shingles over the existing layer instead of doing a complete tear-off to the deck. This practice was common to save money, as it avoids the labor and disposal costs of removing old shingles, which can be 1,000 to 3,000 dollars depending on roof size. Building codes generally allow a maximum of two layers of asphalt shingles, so one overlay is permitted if the existing roof has only one layer, the deck is in good condition, and the existing shingles are relatively flat. Homeowners often chose this option when the old roof was not severely curled or damaged and they wanted a lower upfront cost. However, having two layers has significant drawbacks. It adds considerable extra weight, about 350 to 450 pounds per square, which can stress older roof structures. It traps more heat, accelerating aging of the new layer. You cannot inspect the deck for rot or damage underneath. Manufacturer warranties are often reduced or voided for overlay installations. The new shingles may not seal properly over the uneven old layer, leading to reduced wind resistance and a wavy appearance. When it comes time to replace again, building codes require a complete tear-off of both layers, increasing future costs. If you have two layers, monitor for signs of premature aging.
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