Fix it
Why is my roof sagging?
A straight ridge line is structural health you can check from the street. A dip or wave in it means something under the shingles is giving — and the causes range from cosmetic to urgent.
What it usually is
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Long-term moisture softening the deck
Plywood or plank decking that has cycled wet and dry loses stiffness and dips between rafters, reading as a gentle waviness across the whole plane.
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Undersized or spaced-out rafters carrying modern loads
Older framing built for lighter roofing sags under layered re-roofs. Two or three layers of shingles roughly doubles the dead load the frame was designed for.
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A cracked or shifted rafter or truss member
One sharp dip localized over a single bay usually means one broken member — often traceable to an old leak, a snow event, or a cut someone made for an attic fan.
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Foundation or wall movement transferring upward
When the walls that carry the roof move, the ridge telegraphs it. Doors sticking on the same side of the house is the corroborating clue.
What you can check yourself
Sight along the ridge from across the street and photograph it — a repeat photo in six months tells you if it is moving. In the attic, look for cracked rafters, moisture stains on the deck, and any member someone cut. Count shingle layers at the roof edge.
When it’s a job for a pro
Any localized sharp dip, visible crack in framing, or sag that grows between photos needs a structural assessment. Sistering a cracked rafter is routine work caught early and a rebuild caught late.
Reinforcing a few rafters typically runs $500 to $2,000; deck replacement during a re-roof adds $2 to $4 per square foot. A ridge left to sag eventually prices like a new roof plus framing.
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