Q & A - Raising the Roof - Plywood Buckling

Why are my shingles bulging/buckling?

Ashley Asked, "My shingled roof is bulging/buckling. It’s not all over the roof, just in one spot, affecting about 10 shingles or so in a predominantly vertical direction. The shingles are flat to the surface, just bumped out. I looked in the attic, and it appears the plywood underneath the shingles is bowed out slightly. I don’t see any indication of water damage from underneath. The house is 20 years old – not sure of the roof’s age, but assume it hasn’t been replaced – architectural style shingles."

"The house was painted within the last year, including the soffit vent areas, and we had insulation blown in our attic within the past few years, too. Could the recent paint and insulation have prevented ventilation and caused this? It seems a relatively short time to cause so much damage, but paint does appear to be clogging the vents directly beneath the bulge. If ventilation is the problem and it’s corrected, will the bulge continue to worsen anyway?"

Ashley, I think you answered your own question. Looking at the photo, it looks to me, like the plywood did not get nailed to the rafter here. Any moisture in the attic will expand the wood and do exactly what you see. If the blown insulation covered or blocked any of the soffit vents, your ventilation system is not working properly. This can cause condensation to form from heat rising and meeting cold roof sheathing. Droplets of moisture form and swell or expand the sheathing and if not nailed properly bulge and buckle the roof.

Tom Engle of Engle Insulation Inc. in Roseville California writes, “At the soffit vent areas we use bat insulation or baffles so as not to cover the intake soffit vents.”

The damp trapped air swelled the plywood and being not nailed in this four-foot area, it moved upwards. The roof can be fixed fairly easily but the source of the moisture needs to be discovered.

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