They also help your roof to hold up asphalt shingles.
Roof sheathing bowing.
The most common cause appears to be movement in the roof sheathing with changes in moisture levels.
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.
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.
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.
If the sheathing is properly gapped then the gaps can close causing the shingles spanning the gap to buckle.