Sagging Roof: Causes and Who to Call
A sag means something under the shingles has moved or softened. It is checked from inside the attic first, and it is fixed before or during a reroof, never by the new roofing alone.
What you may notice
A ridge line that dips in the middle when you look at it from the street
A shallow hollow in one roof plane, often between two rafters
Wavy rows of shingles that follow the sheathing underneath
In the attic, sheathing that bows down between rafters or looks dark and damp
New cracks in upstairs ceilings or at the tops of interior walls
How soon it needs attention
A dip that has been there for years and is not changing is not an emergency, but have it looked at before the roof is replaced, because that is the time to fix it. A sag that is new, growing, or comes with cracking inside the house needs a professional promptly. Stay out from under it, and do not let anyone walk that part of the roof.
What a sagging roof means
A sagging roof is a problem with the wood, not the roofing. Shingles have no strength of their own; they lie on the sheathing, which lies on the rafters or trusses. When the roofline dips, one of those two has bent, softened or moved.
There are two broad kinds. A dip between rafters is the sheathing, and it is often tied to moisture: a slow roof leak or years of damp attic air. A dip across several rafters, or a ridge that bows, is the framing.
What gets checked
From the attic, with a light: the underside of the sheathing for staining and softness, each rafter for cracks and splits, the ridge board, and whether bracing has been removed. From outside: how many layers of roofing are on the house. Older Portland houses that were recovered instead of torn off can be carrying more weight than the frame was built for, and the ice loads of a storm like January 2024 add to it.
How it gets fixed
Soft sheathing is cut out and replaced during a tear-off, which is why a sag is one of the clearer reasons to choose a full roof replacement over another layer. Framing repairs come first and may need an engineer’s detail. Ask for the deck and framing work as its own line on the bid. Sheathing replacement is one of the items that moves a reroof up the ranges on the cost page.
What causes it
A sag is a structure or deck problem, not a shingle problem. It can be sheathing that has stayed wet and softened, rafters that are undersized or cracked, or more weight on the frame than it was built for, such as several layers of roofing.
Sheathing that has been wet for a long time from a leak or attic condensation
Sheathing too thin for the rafter spacing, which sags between supports over the years
Rafters that are undersized, cracked or notched
Missing or cut collar ties or braces after an attic remodel
The weight of more than one layer of roofing
Exterior walls spreading outward at the top, which lets the ridge drop
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Common questions
Can a sagging roof be fixed without replacing the whole roof?
Sometimes. Rafters can be reinforced from inside the attic. Soft sheathing has to be replaced from above, which means removing the roofing over it, so that repair is usually done as part of a reroof.
Is a sagging roof dangerous?
A long-standing, stable dip usually is not. A sag that appeared recently, is getting worse, or follows a heavy load of ice or snow can be. If you are not sure which you have, treat it as the second kind and get it inspected.
Will new shingles hide the sag?
No. Shingles follow the deck. A new roof over a dipped deck shows the same dip, and it still has the problem that caused it.
Do I need a structural engineer or a roofer?
Start with a roofer for sheathing that has gone soft. If rafters are cracked, the ridge has dropped or walls are moving, a roofer should tell you to bring in an engineer, and structural repairs are the kind of work that building permits are for.
Request a Roofing Quote
Tell us about your roof and we pass your request to a local, CCB-licensed roofing contractor who can quote the work.