Why is my roof sagging?
Short answer. A roof sags when the decking or framing can no longer hold its shape: water rotted or delaminated decking dips between rafters, rafters that are undersized or missing ties spread apart and let the ridge drop, trusses get cut or overloaded, or layers of old shingles add weight. Where the sag is tells you which problem you have.
Read the sag by its shape
- Dips between every rafter, like a washboard: decking that is too thin for the rafter spacing, missing edge clips, or OSB that has absorbed moisture. Usually a decking fix during a reroof.
- One soft, low area: a long term leak that rotted the decking, often near a chimney, skylight, valley or pipe boot.
- Swayback ridge, low in the middle: a sagging ridge board or beam, often in older stick framed homes. A structural issue.
- Walls bowing out at the top with a dropped ridge: rafters spreading because ceiling joists or rafter ties are missing or were cut. Structural.
- Sag near attic storage or equipment: trusses overloaded or with webs cut to make room. Trusses should never be cut without an engineer's repair design.
Other contributors
- Two or three layers of old shingles.
- Poor attic ventilation keeping decking damp.
- A tree limb strike that cracked a rafter.
What to do
- Look in the attic for water stains, cracked rafters, cut truss webs and wet insulation.
- Keep people and heavy items off the roof and out of that attic area.
- Get a roofer to assess the decking and leaks, and a structural engineer if framing is involved.
Sudden sagging after a storm or tree strike may be part of an insurance claim; slow sag from age generally is not. See OSB vs plywood roof decking and trees and your roof.