A sagging roof indicates a major structural problem, which could be dangerous if not addressed promptly. Sags can occur along the roof ridgeline, at the eaves, or across the expanse of the main roof plane.
1. Decking Damage
The most common cause of roof sag is damage to the plywood decking. Age can weaken decking boards, but moisture is the real problem. The plywood can absorb moisture if there is a leak. This weakens the wood fibers and causes them to warp. The result is a weakened and warped plywood sheet that then sags between the roof trusses to which it is attached. Damaged decking must be replaced to prevent a collapse.
2. Weight Overload
Sometimes the weight on the roof surpasses what the materials were designed to hold. Weight overload can occur during heavy snow accumulation, when the old shingles are left in place during a new roof install, or as a result of placing items like solar panels on the roof. The decking nor the trusses are designed to hold the extra weight, so they simply begin to sag under the load.
3. Truss Failure
Your roof is supported by a network of trusses, which are made of metal or wood. Metal trusses tend to last a long time with no chances for sagging, but wood trusses can become damaged and fail. If a truss cracks or begins to rot, the lack of support can lead to a sag along the ridge or gable ends of a roof. Sometimes a badly shifting house will also cause a truss to come out of square, which leads to a roof sag.
4. Vent Stack Damage
There are multiple vents that penetrate your roof, some from the plumbing, others from the attic, or from the bathrooms. A sag around a vent stack means that there is a leak in the roof between the vent and the shingles. Usually, this space is sealed with a rubber boot or weatherstripping, but if they fail, water will seep in. The roof around the stack weakens from water damage, causing the roof to sag around the vent.
5. Weak Materials
A roof that is sagging soon after installation can indicate an issue with the materials used. The most common problem is that new decking was made from overly thin plywood, so it can't support the weight of the shingles over the expanses between trusses.
Contact a roofing contractor if you notice a bow in the roof.