What causes a sagging roof?
By Best Roofing Answers · Published May 2026 · Updated July 2026
Roof sag is almost always structural — the four common causes are chronic leak rot in the decking or rafters, undersized rafters relative to snow or dead load, failed truss webs or connectors, and removal of a load-bearing wall below; any visible sag warrants a licensed structural engineer's evaluation.
Diagnose before you patch
Per IRC Chapter 8 load tables, roof framing is sized for the local snow load and the weight of the roof covering itself. A sag that appeared gradually often points to slow rot from a chronic leak; a sag that appeared after a specific event (heavy snow, hail) points to overload of framing.
Cosmetic patching over a structural sag is a code violation in most jurisdictions and can void your homeowners insurance in a later collapse claim. Order the engineer's report first; then repair to the report.
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