Roof Sheathing Bowing

The nails slip out of the sheathing and create small visible bumps on the surface of your roof.

Roof sheathing bowing.

They also help your roof to hold up asphalt shingles. This is the most common cause of sagging sheathing and will usually be limited to the area around the leak while poor attic ventilation the second most likely reason can cause the problem over a larger swath of the roof. 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. Buckling can occur if the wrong type of nails were used to connect the sheathing to the rafters.

Since sheathing boards are affixed to your roof s trusses and joints its strongest supporting beams they help ensure that heavy loads of snow and precipitation don t cause bowing or sagging. When plywood or osb sheathing picks up moisture it expands. If the sheathing is properly gapped then the gaps can close causing the shingles spanning the gap to buckle.