What causes roof decking to warp?
Short answer. Roof decking warps mainly from moisture: humid attic air the wood absorbs, leaks, or panels installed without the small expansion gap between sheets. Long spans between rafters, missing edge clips and sagging framing make it worse, and the waves show through the shingles as a lumpy or ridged roofline.
The common causes
- Attic humidity. A poorly vented Georgia attic traps warm, wet air. Plywood and OSB absorb that moisture, swell and cup between rafters. In winter, condensation on the cold underside of the deck does the same.
- No expansion gap. APA, the engineered wood association, recommends a 1/8 inch gap at panel ends and edges. Sheets butted tight have nowhere to go when they swell, so they push up at the seams and show as straight ridges in the shingles.
- Long spans and no clips. Thinner panels over 24 inch rafter or truss spacing need edge support, usually small metal H-clips between rafters. Without them, panel edges sag.
- Leaks. Water from a failed pipe boot or flashing soaks a patch of decking, which swells, delaminates and goes soft.
- Framing problems. Sagging rafters, cracked truss members or a dipping ridge make the deck wavy even when the wood is sound.
Sorting out which one you have
Straight ridges along panel seams point to missing gaps. Dips between rafters point to thin panels or moisture. A spot that gives underfoot points to rot. From the attic, look for dark staining, rusty nail tips, frost in winter and water marks around penetrations.
The fix
Warped or rotted panels are replaced during a reroof; they cannot be flattened in place. The cause needs fixing too, which usually means improving attic ventilation. See OSB vs plywood roof decking and roof ventilation basics.