Why do my shingles look wavy?

Shingles look wavy for several reasons ranging from normal new-roof settling to underlying deck or installation problems. For new roofs, temporary waviness is common and usually resolves within 2 to 6 weeks in warm weather. Shingles are stored in bundles and can have a slight curve from packaging, and they need sun heat to soften, relax, and seal flat to the deck. If installed in cold weather, they may look wavy until spring warmth arrives. Persistent waviness after a full warm season indicates other issues. The most common cause is uneven roof decking, such as plywood sheets installed without the required 1/8 inch expansion gap that have swollen and buckled in humidity, or older plank decking with uneven boards that telegraph through shingles. Wrinkled underlayment or felt that was not installed flat will also cause a wavy appearance as wrinkles show through. Installing new shingles over an existing layer during a layover causes waviness because new shingles cannot lay flat over the uneven old layer. Overdriven or underdriven nails can create slight distortion. For older roofs, waviness that develops over time is often buckling due to moisture in the deck or attic ventilation issues. If waviness is severe, uniform, and does not improve after warm weather, have a professional inspect the deck and underlayment installation.