What Causes a Sagging Roof Line?

Inadequate Fastening

• Ceiling joists are not always a single timber; they may be composed of two timbers. When properly fastened, the combined ceiling timbers hold the roof easily. However, inadequately fastened members eventually pull apart from one another and cause the roof to sag. The separation of the timbers pushes against the outside wall, causing the wall to tilt outward, which in turn shifts the rafter, pulling it away from the roof's ridge. Telltale signs of this sagging problem include cracks along the ceiling near the area where the joist timbers have come unfastened.

Structural Rotting

• Rot caused by woodworm, termites or water leaks can lead to a sagging roof. In this scenario, the structural integrity of the wood is compromised and causes the roof line to dip or sag. Rotting and insect infestations can be remedied with the right professional treatment. The sagging roof can be repaired by replacing the damaged or failed timbers.

Too Much Weight

• Excess weight applied directly to the roof is another cause of a sagging roof line. Replacing an old roof with a heavier roofing product is one way extra weight is added to a roof. For example, replacing an asphalt roof shingle with a clay roof tile can add weight to the roof. Over time, the weight of the clay tile can cause the roof to sag if the original roof structure was not meant to handle the extra weight. When planning a new roof installation, strengthening the roof timbers may be necessary to prevent a roof from sagging.

Poor Alignment

• Structural framing mistakes create poorly built structures that cause roofs to sag. Unsupported central walls that have no ridge beam -- the central peak of the roof -- should be built with support that starts at the foundation. These unsupported walls are carrying the load the ridge beam would carry and without the proper support the roof will sag. Plumbers that have made breaches in load-bearing joists by cutting into, in half or removing larger portions of the joists also can cause a sagging roof line. In many cases, framing mistakes can be shored up and repaired by replacing or reinforcing damaged or compromised elements.