Your roof shouldn't sag, so any bowing along the eaves, roof ridge, or main roof surface should be an immediate cause for concern. Some problems can be solved with a few minor repairs, while others may require the full replacement of your roof.

1. Snow Load Damage

Heavy snow and ice loads are common causes of a sagging roof. In areas where snow is common, roofs are constructed to exceed the expected weight of snow that may be on the roof at any given time. An exceptionally heavy snowfall or a winter with frequent snowfalls with minimal melt between them can quickly lead to a heavy load that is beyond the roof's weight capacity. Prompt snow removal and an assessment of damage is necessary to prevent a possible roof collapse.

2. Moisture Problems

Moisture in the roof doesn't lead to sagging right away. Instead, ongoing moisture exposure eventually causes the sheathing boards and any wooden support systems beneath them to suffer from water damage and rot. The entire roof begins to sag as these wooden structural components weaken. Eventually, the roof may cave in and collapse in the affected areas. Replacing old shingles before water leaks occur and fixing small damage or punctures can prevent moisture problems from progressing this far.

3. Rotted Roof Sheathing

A roof consists of layers. The main structure is made up of trusses, rafters, and braces. Attached to this are large sheets of plywood, called the sheathing, which provide the nailing surface for the moisture barriers and shingles on top. Water that leaks through the upper layers of the roof will eventually soak into the sheathing, causing it to swell and begin to weaken. Sagging is the result, along with water leaks into your attic. If caught early, the damaged section of sheathing can be cut out and patched. Extensive damage may require replacing the entire roof, including the sheathing and the shingles.

4. Support System Failure

Some homes feature metal truss systems, while others have wood trusses. Rafters are typically made of wood, while braces can be a combination of wood and metal. Missing braces or rusty or missing bolts can lead to problems with metal supports that cause them to sag under the weight of the roof. Wooden components can suffer issues such as rot or warping that will lead to sagging, usually as a result of roof leaks and moisture exposure. Roofs sagging due to support failures require immediate assessment and repairs so that the roof doesn't collapse.

Contact a roofing contractor in your area for more information.