Why is my roof wavy?
Short answer. A wavy roof is usually caused by something under the shingles: decking panels that swelled at their seams because they were installed without a small gap, sheathing that sags between rafters, uneven rafters or trusses, wrinkled felt underlayment, or new shingles laid over an old layer. Brand new shingles can also look wavy for a few days in cool weather until the sun relaxes and seals them.
Match the pattern to the cause
- Straight ridges in a grid, about every 4 or 8 feet: decking seams. OSB and plywood swell with humidity, and panels installed tight together push up at the joints. Panel makers call for about a 1/8 inch gap at edges and ends.
- Dips between every rafter: sheathing that is too thin for the rafter spacing, missing edge clips, or decking weakened by moisture.
- Long, gentle humps across the slope: rafters or truss chords that are crowned or uneven, or framing that has settled.
- Random lumps that mirror old shingle lines: new shingles installed over an existing layer, which telegraphs every bump below.
- Small wrinkles running sideways: felt underlayment that absorbed moisture and wrinkled before shingles went on.
- Waviness on a brand new roof in cool weather: shingles that have not yet softened and sealed. They usually flatten after a few warm, sunny days.
Does it matter?
Some waviness is cosmetic, but certain causes do affect performance. Ridged seams can lift shingle tabs and stop the sealant strip from bonding, and sagging decking can hold nails poorly. Rippled overlays trap debris and moisture.
Georgia's humidity makes it worse
Wood sheathing in humid attics with weak ventilation absorbs moisture, swells and stays that way. Improving ventilation helps prevent new ridging but will not flatten what is already there; that is fixed at the next reroof by replacing or refastening panels.
See OSB vs plywood roof decking, roofing over existing shingles and roof ventilation basics.