Sagging Roof Repair

Common Symptoms

• ✓Visible dip or sag in the roofline when viewed from the street
• ✓Soft, springy feeling underfoot when walking the roof surface
• ✓Interior ceiling line bowing or cracking along load-bearing walls
• ✓Daylight visible between rafter and ridge board in the attic
• ✓Multiple rafters with visible cracks, splits, or downward deflection
• ✓Decking sheets visibly delaminated or separated at seams when viewed from the attic

What Causes This

Roof sagging in St. Louis homes has four primary causes: water-damaged decking that has delaminated and can no longer span between rafters; rafters that have cracked, split, or sagged under long-term load or point-load damage; engineered truss members that have failed, been cut during a renovation, or experienced moisture damage to the connector plates; and in older homes, ridge board deterioration that allows the ridge to drop, splaying the rafters outward. In most cases, multiple factors are present simultaneously — water damage to decking is almost always accompanied by some degree of rafter deterioration, because the same moisture source that rots the decking also affects the framing.

When to Call Immediately

A sagging roof that has been deteriorating for years can reach a point of sudden progressive failure — particularly under snow load or after a saturating rain event. If you have visible exterior sag and the attic shows multiple cracked rafters, this is an urgent structural issue, not a cosmetic one. Do not defer it indefinitely.

How Revolve Fixes It

1. 1Conduct a thorough attic inspection under all affected areas — document every rafter, truss member, and decking panel condition.
2. 2Identify the primary structural failure mode: decking delamination, rafter failure, truss failure, or ridge deterioration.
3. 3For rafter failure: sister new full-length rafters alongside damaged members using structural lumber of equivalent size and species; nail and tie per IRC/IBC requirements.
4. 4For truss failure: engage a structural engineer for a repair design; do not proceed with truss repairs without engineering documentation.
5. 5Remove and replace all delaminated or deteriorated decking in the affected zone using 7/16" or 5/8" OSB or plywood to match existing thickness.
6. 6Install new underlayment and shingles over repaired structural areas; verify roofline is restored to original geometry before covering.

How to Assess the Severity of a Sagging Roof

Not all sag is equal. A single soft spot on a 20-year-old roof with a localized area of delaminated OSB beneath it is a straightforward decking replacement — the structure is sound, only the decking panel has failed. This is a $400–$800 repair. The same visible soft spot with cracked rafters beneath it and a roofline that dips 3 inches over a 10-foot span is a structural repair requiring sistered rafters, engineering review potentially, and full decking replacement in the zone.

The attic inspection tells the full story. We look at every rafter in the affected zone, probe for soft spots in the decking from below, check the ridge board for splitting or lateral movement, and look for connector plate corrosion on any trusses. This takes 30–60 minutes to do thoroughly and it is the only basis for an accurate repair scope.

We also look for the water source. Structural sag almost always traces to a sustained moisture problem — a flashing failure, a boot that has been leaking for years, a valley that was improperly installed. Repairing the structure without also fixing the moisture source means the same damage will recur. We address both in the same scope.

Rafter Sistering — What It Is and When It Is Used

Rafter sistering is the standard structural repair for a cracked, split, or deflecting rafter. A new full-length rafter of the same dimensions is installed alongside the damaged member, tied to the ridge board at the top, nailed to the wall plate at the bottom, and fastened to the existing rafter along its full length. The new member carries the load while the damaged one is stabilized in place.

Sistering is appropriate when the existing rafter has cracked but has not fully separated, the ridge board and wall plates are sound, and the number of rafters requiring repair is limited — typically three to five in a localized zone. When the majority of rafters on a slope are compromised, or the ridge board has failed, the repair scope becomes a larger structural intervention.

Rafter sistering does not require an engineer in most residential applications where the rafter size and span are within standard IRC tables. We pull the appropriate permits and work within the code framework for the municipality. Some municipalities in St. Louis County require permits for structural roof repairs — we handle that process.

Preventing Structural Roof Deterioration

The common thread in virtually every structural roof failure we have diagnosed in 17 years of St. Louis roofing is sustained moisture intrusion that was not caught and addressed early. A small flashing leak that drips onto the same rafter for three winters can cause enough rot to compromise the structural capacity of that member. The shingles above look fine. The interior ceiling has a small stain. The attic has a brown streak on one rafter that gets worse each year.

Annual attic inspections — particularly after winters with significant ice damming or after any known leak event — are the most cost-effective way to catch structural deterioration before it becomes an expensive repair. A flashlight walk through your attic in March will tell you more about your roof's condition than any exterior inspection.

Roof ventilation plays a role too. An under-ventilated attic accumulates moisture from interior humidity — particularly in St. Louis winters with tight building envelopes — and that moisture condenses on cold roof sheathing, gradually degrading both the OSB and the rafters below it. If you are seeing widespread decking deterioration without a clear point leak source, ventilation may be the underlying cause.