Building codes in most jurisdictions allow a maximum of two layers of asphalt shingles on a roof, meaning you can reshingled once over an existing layer, but after that a complete tear-off is required. The International Residential Code states that you may install new shingles over existing shingles only if the existing roof has only one layer, the roof deck is in good condition without rot, the existing shingles are relatively flat and not severely curled or buckled, and the structure can support the additional weight. Adding a second layer adds about 350 to 450 pounds per square, or 100 square feet, so two layers can add several thousand pounds to the roof structure. While one overlay is permitted, it is generally not recommended by shingle manufacturers and roofing professionals. A second layer traps more heat, shortens the life of new shingles, hides deck damage, prevents proper inspection, often voids or reduces warranty coverage, and results in a less attractive, less wind-resistant installation because new shingles cannot lay perfectly flat. Many homeowners associations and insurance companies also prohibit overlays. For these reasons, even though code allows two layers total, the best practice for longevity, warranty protection, and proper deck inspection is to perform a full tear-off down to the deck each time reroofing is needed.
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