They also help your roof to hold up asphalt shingles.
Roof sheathing bowing.
When plywood or osb sheathing picks up moisture it expands.
A broken sagging rafter will also cause a dip in the roof s surface.
Roof sheathing helps distribute weight evenly across your 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.
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.
The most common cause appears to be movement in the roof sheathing with changes in moisture levels.
Buckling can occur if the wrong type of nails were used to connect the sheathing to the rafters.