Reroofing over old shingles, known as a layover or overlay, is allowed by building codes in many areas under certain conditions, but it is generally not recommended as best practice by roofing professionals and shingle manufacturers. The International Residential Code typically allows a maximum of two layers of asphalt shingles, meaning you can install new shingles over one existing layer if the existing roof is relatively flat without severe curling or buckling, the deck is in good condition without rot, and the structure can support the additional weight of about 350 to 450 pounds per square. The main advantage is lower upfront cost, saving on tear-off labor and disposal. However, disadvantages are significant. A second layer adds weight that stresses the structure, traps more heat which shortens new shingle life by 20 to 30 percent, prevents deck inspection for hidden damage, often voids or reduces manufacturer warranties, and results in a less attractive wavy appearance because new shingles cannot lay flat over the uneven old layer, with reduced wind resistance. If the existing roof already has two layers, has extensive damage, or the deck is rotted, overlay is prohibited. For long-term value, full warranty coverage, and ability to install new underlayment and properly inspect the deck, a complete tear-off to the deck is the recommended method, even though it costs more initially.
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