A sagging roof should never be ignored, since this sort of damage may indicate an impending leak or even roof collapse. A roof can sag anywhere on its surface, with some sags being most obvious along the flat planes of the roof and others appearing as a bowed ridge line. There are several reasons a roof can sag, so it's important to determine the cause so the repair can be effective.

1. Age

Age is the most likely culprit when it comes to a sagging roof. Although roof decking, the plywood boards that make up the under-structure of your roof, can be used for several new roof installations, eventually the wood will weaken and require replacement. A roofer will need to assess the state of your decking to see if it needs to be replaced at your next shingle replacement and installation.

2. Moisture Damage

If your roof suffers damage to the shingles, then moisture can seep beneath them and affect the roof decking. Prolonged moisture exposure will cause the decking to develop moisture rot, which means the wood fibers will swell and weaken. The wet, damaged decking will then sag, both because of the rot damage and due to the weight of the absorbed moisture. Replacement of the damaged decking and shingles is necessary.

3. Design Issues

If your newer roof is sagging, the cause may be a design issue. If the roof supports are spaced too far apart, the decking may sag under its own weight between the supports. Sometimes, there are sufficient supports but overly thin decking was used or an insufficient amount of screws and hardware was used to secure the decking. Fortunately, these issues are usually covered under the warranty on the new roof, but you will need to schedule repairs within the warranty period.

4. Weight

Too much weight can lead to damaged and sagging decking. The weight could come from a single event, such as heavy snowfall, or could be systematic of an ongoing issue. For example, installing new shingles over the old ones can add additional weight that the trusses or decking can't support. Changing the shingle type could also cause weight issues, such as switching from asphalt to heavier tile shingles. Additional supports may need to be added.

5. Support Issues

Although trusses are designed to last for the life the home, damage can occur. Trusses can break or bow, leading to poor support and a roof sag. Sometimes the foundation of the home sinks, which puts undue stress on trusses. In other cases, hardware that holds the truss in place may fail and require replacement.

Contact a roofing contractor in your area for more assistance with a sagging roof.