Why Your Roofline Is Sagging
A roof should read as a straight line against the sky. A dip, wave or ripple means something underneath has moved, and the cause determines whether this is a weekend repair or a structural job.
The four usual causes
- Rotted sheathing between rafters, which dips under its own weight
- A failed or cut rafter, sometimes cut years ago for a vent or a skylight
- Damaged truss members, especially where plates have pulled apart
- Too much weight — layers of roofing stacked up over decades
What it is not
An old house settling is not the same as a sagging roof. Settlement shows up in walls and floors over decades; a roofline that has changed noticeably in a few years is a framing problem and should be looked at before winter.
Common questions
Is a sagging roof dangerous?
It depends on the cause and how far it has gone. A shallow dip from soft sheathing is not an emergency; a sag from a broken rafter or a failed truss plate is load-bearing and should be inspected promptly.
Can a sagging roof be fixed without replacing the whole roof?
Often yes. If the covering still has life in it, the framing can be repaired and the affected area re-covered. If the roof is also near end of life, doing both at once saves one mobilisation.