How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manufactured and modular homes
What causes shingles to buckle along a line on the roof?
Thursday, June 14, 2018
Asphalt shingles sometimes buckle slightly, and in an irregular pattern, due to moisture in a the materials of a new roofing system. The wetness gets absorbed by the felt underlayment, causing the felt to wrinkle. This then telegraphs to the shingles above. But the moisture evaporates as the materials reach equilibrium with the the surrounding air, and the shingles settle back down flat.
However, a more pronounced buckling in a line running up the roof, like in the photo above, is caused by buckling of the edges of the plywood or OSB roof sheathing, due to water intrusion or extremely high humidity in the attic. An 1/8” gap is specified by manufacturers between sheets of plywood that are nailed side-by-side over a shared top chord of a roof truss or a rafter. If this requirement is ignored and the sheathing is butted tight, the wood has nowhere to go when the moisture makes it swell, except to curl up along the shared seam.
A common cause of high humidity in an attic is lack of ventilation, which can be caused adding insulation that covers the soffit vents or lack of soffit vents and/or ridge vents altogether. A clothes dryer that terminates into the attic, or has vent through the roof that has become disconnected in the attic, is another source of water vapor and heat. And, of course, a roof leak can spread water under felts that sometimes migrates along the roof sheathing.
Eliminating the moisture cures most buckling problems. It may be necessary to wait for the materials to stabilize before determining whether any repairs or replacement is needed.
To learn about two other visible roof problems and their cause, see our blog posts What causes roof shingles to curl up at corners?
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Field Guide for Home Inspectors, a quick reference for finding the age of 154 brands of HVAC systems, water heaters, and electrical panels, plus 210 code standards for site-built and manufactured homes, and the life expectancy rating of 195 home components. Available at amazon.com for $19.95.
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To learn more about roofs and attics, see these other blog posts:
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