New roof shingles looking wavy is a common concern after installation, and it can be due to normal temporary settling or to installation or deck issues. Temporary waviness is normal and occurs because shingles are stored in bundles and may have a slight curve from packaging, and the deck may have minor irregularities. In warm weather, this waviness typically flattens out within 2 to 6 weeks as sun heats the shingles, softens the asphalt, and allows the sealant strip to bond and shingles to conform to the deck. If your roof was installed in cold weather below 50 degrees, shingles may look wavy longer until spring warmth arrives. However, persistent waviness after a full warm season indicates other causes. The most common cause is uneven roof decking, such as plywood sheets not spaced with a 1/8 inch gap that have expanded and buckled, or old plank decking with uneven boards. Wrinkled underlayment or felt installed with wrinkles will telegraph through new shingles and cause a wavy appearance. Installing new shingles over an existing layer during a layover will also look wavy because they cannot lay flat over the old uneven surface. Overdriven or underdriven nails can also cause slight waviness. If waviness is severe, uniform, and does not improve after 2 months of warm weather, have the contractor inspect the deck and underlayment installation.
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