Swollen or buckled roof decking is OSB or plywood sheathing that has expanded, ridged at its joints, sagged between rafters or delaminated. The common causes are panels installed tight with no expansion gap, decking that got wet before or during roofing, attic moisture from air leaks and exhaust ducts, and thin panels spanning rafters without edge support. You see it as straight ridges or waves in the shingles. Panels that only ridged at joints are often a cosmetic issue; soft, delaminated or rotted panels are replaced, ideally at the next reroof. This page is general information; your roofer and building department make the final call.
On this page: Signs · Causes · What to check · Repairs · Basis · When to call
Signs of deck trouble
- Straight horizontal or vertical ridges in the shingles, spaced at panel widths (about 4 or 8 feet).
- A wavy, “picture frame” look across a slope in low-angle light.
- Dips between rafters, sometimes called sheathing sag.
- In the attic: swollen panel edges, flaking or delaminated OSB, dark staining, rusty nail tips.
- Shingles near the ridges cracking or not lying flat.
Why roof decking swells and buckles
- No expansion gap. Panels butted tight have nowhere to grow when they take on moisture.
- Wet deck at installation. Rain or dew on the deck before shingles went on.
- Attic moisture. Warm, damp house air or exhaust ducts dumping into the attic wet the underside; see attic frost.
- Missing edge support. Thin panels spanning rafters without clips, blocking or tongue-and-groove edges can sag between supports.
- Uneven decking or framing. ARMA lists uneven decks and structural problems among the causes of distorted shingles.
- Old plank decks with wide gaps; several Front Range jurisdictions require overlay or new sheathing over gaps beyond a set limit at reroof.
Attic and ground checks
- From the ground in low morning or evening light, look across each slope for straight ridges in a grid.
- In the attic, standing on framing, check panel edges for swelling and look for flaking or soft spots.
- Look for clips or blocking between rafters at panel edges.
- Check for moisture sources: frost on nails in winter, ducts ending in the attic, insulation blocking soffit vents.
Repairs
| Repair | When it fits |
|---|---|
| Fix the attic moisture source and monitor | Ridging is mild and the wood is sound |
| Replace damaged panels during a repair, with proper gaps and edge support | Isolated soft or delaminated panels |
| Replace or overlay sheathing as part of a reroof | Widespread damage, sag or gaps beyond local limits |
| Have framing checked | Ridges follow misaligned rafters or trusses |
Fixes that fail: nailing ridged panels down through the shingles, shingling over soaked decking, and adding attic vents without stopping the moisture source.
Industry and manufacturer basis
APA’s builder tip Prevent Buckling with Proper Spacing (Form M300S, revised 2013) recommends a 1/8-inch space at panel edge and end joints, says spacer-type panel edge clips may be used for roof sheathing, and describes buckling as an aesthetic or serviceability issue rather than a structural deficiency. Owens Corning’s Duration instructions call for at least 3/8-inch plywood or 7/16-inch OSB spaced 1/8 to 1/4 inch, installed to APA and local code, and kept dry before, during and after installation. ARMA’s Plain Facts About Buckled Shingles (March 2024) ties shingle distortions that line up with deck joints to deck moisture and calls for at least 1/8-inch deck spacing and staggered joints. USDA Forest Products Laboratory guidance notes wood decay starts when moisture stays above about 30 percent.
When to call
Call when you see a grid of ridges, sag between rafters, or flaking decking in the attic. Related guides: buckled shingles, structural roof problems, nail pops and the roof problems guide.
Get the deck checked
Integrity Roofing and Painting is veteran-owned and family-run since 1990, an Owens Corning Platinum Preferred contractor with 650+ Google reviews. We photograph the attic side of the deck, check for moisture sources and tell you what needs fixing now and what can wait for the next reroof. Colorado Springs area: (719) 488-2800 or a free inspection. Denver metro: (303) 209-0152 or a free Denver inspection. Austin: (512) 610-5897 or a free Austin inspection. Same-business-day reply.
Sources: APA, Prevent Buckling with Proper Spacing (Form M300S); Owens Corning Duration installation instructions (roof deck); ARMA, Plain Facts About Buckled Shingles (March 2024); USDA Forest Products Laboratory, Home Wreckers in Search of Moisture.
