Beneath the shingles or tiles on your roof is a layer of thick plywood, which is known as the roof sheathing. This is the first layer of the roof, and it is the layer that provides the nailing surface as well as the main support for the rest of the roof. Sagging and low areas on the roof are often a sign that the sheathing needs to be replaced. 

1. Shingle Leaks

The most likely cause of damaged sheathing will be damage to the shingles above. Old deteriorating shingles or those that have cracked and come loose will allow water to seep beneath them. Over time, the sheathing absorbs the moisture leak and the wood begins to warp and rot. There may be water damage inside the attic, and the roof may sag in the area of the leak as the sheathing weakens.

2. Attic Condensation

Any moisture in the home will tend to be carried into the attic by the natural rising of warm air. If the attic isn't well vented, then that condensation will collect on the underside of the sheathing. Over time, the moisture absorbs into the wood. Rot and warping of the sheathing follows, which can only be repaired by replacing the damaged sheathing boards. Adding ventilation to the roof, or installing an attic fan or dehumidifier, can prevent the issue.

3. Weight Issues

Too much weight on the roof can damage sheathing. The problem occurs when the roof supports and sheathing thickness aren't rated for the amount of weight put upon them. Too much weight can be the result of using the wrong roofing material, such as installing heavy tiles on a roof made for lightweight asphalt shingles, or due to a weather event like heavier than normal snowfall. The sheathing can begin to bow or even crack, which poses a collapse risk if it isn't fixed right away.

4. Punctures

A puncture is when something punches through both the shingles and the sheathing, such as can occur when a tree branch falls during a storm. The damage tends to be severe and immediate. If the roof is in otherwise good repair, your roofer may cut out the damaged section and replace it. They will then shingle over the repaired sheathing. For older roofs or those with other damages, full replacement of the sheathing and shingles may be the best way forward.

Contact a roof repair surface if your roof has sagging areas and you need roof repairs.