Sagging Roof Deck

A sagging roof deck is any noticeable downward deflection in the roof plane. It can range from slight dips between rafters, sometimes called a wavy or "hungry horse" look, to a pronounced swayback along the ridge. Some sag is a cosmetic sign of age; other cases indicate a structural problem that needs prompt attention.

Types and causes

• Sag between framing members. Thin sheathing panels or boards spanning too far between rafters or trusses, or panels installed without the recommended edge clips or gaps, can deflect, especially when wet. Moisture from a damp attic weakens wood-based panels.
• Sagging rafters. Undersized or over-spanned rafters, notching, missing collar ties or purlin supports, and long-term creep of timber under load cause the rafter line to dip.
• Ridge sag. A ridge board or beam that is not properly supported, or rafters that push outward and spread the walls, causes the ridge to drop.
• Added weight. Multiple layers of reroofing, heavy materials such as tile or slate on framing not designed for them, rooftop equipment, or heavy snow and ice.
• Decay. Long-term leaks or condensation cause rot in sheathing, rafter ends, wall plates or the ridge.
• Truss damage from cutting or altering engineered trusses during renovations.

Inspection

From outside, sighting along the ridge and across slopes in low-angle light shows dips and waves. In the attic, the inspector looks for cracked or split rafters, sagging between supports, separation at the ridge, staining or soft wood, and walls that lean outward at the top. Probing with a screwdriver identifies rot. A level or string line measures deflection.

Remedies

Minor sheathing dips are often corrected during reroofing by replacing damaged panels and adding blocking or clips. Rafter and ridge problems may need sistering new members alongside old ones, adding purlins, struts or collar ties, or installing a supported ridge beam. Any change to structural framing should be designed or checked by a qualified person and comply with local building codes. Stopping leaks and attic moisture problems is part of every lasting repair.