Roof Sheathing Bowing

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

Roof sheathing bowing.

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. If the sheathing is properly gapped then the gaps can close causing the shingles spanning the gap to buckle. A broken sagging rafter will also cause a dip in the roof s surface. They also help your roof to hold up asphalt shingles.

Buckling can occur if the wrong type of nails were used to connect the sheathing to the rafters. The most common cause appears to be movement in the roof sheathing with changes in moisture levels. 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.