Causes of a Sagging Roof and Repair Options

Sagging Roof of an Abandoned House
© rusty elliott / Adobe Stock

Spotting a sagging area in your roof is always worrying. While a little sagging doesn’t necessarily mean your roof is in imminent danger of collapse, it does mean you need to act soon to save the structure. Often, a saggy roof can be repaired and reinforced. To preserve your roof, it’s worth hiring a professional to determine the exact cause of the sagging and take steps to correct it.

Water Damage to a Weak Roof Surface

Rain and snow can quickly damage a roof that isn’t optimally designed and well maintained. Shingles and other roofing material must be in good condition to allow water to run off. Metal flashing should be installed on the valleys and junctions of your roof to direct water away. Damage to any part of the roof’s surface gives water a way to get in and weaken the underlying structure.

A damaged or poorly designed gutter system can also lead to water issues. If water runoff isn’t safely directed away via the gutters, it can pool on the roof and eventually seep in.

Poor ventilation in the attic exacerbates water problems. Without enough airflow in the attic, moisture builds up on the underside of the roof, making it easier for water to permeate the roof’s surface. It also allows summer heat to build up, speeding up the deterioration of your shingles.

If water damage is causing your roof to sag, your first step will be to correct the problems that allowed the damage to occur. That might mean replacing the shingles and flashing, repairing your gutters, and improving the attic ventilation.

To correct the sagging itself, angled braces can be installed to shore up the rafters. Another approach is to give more support to the walls by installing chains attached to the wall plates and linked in the middle with a turnbuckle. Depending on the degree of sagging, your contractor might need to use jack posts to raise the roof before installing any reinforcement.

Excess Weight from Snow and Design Flaws

Every roof is designed to support a specific maximum weight and anything above this, especially for long periods, can cause the roof to sag. In cold-winter climates, snow is the most common cause of excess weight on the roof.

If you suspect snow is causing your roof to sag, for your own safety and to protect your roof, consider calling a professional to clear your roof of heavy snowfall. Incorrect snow removal can further damage your roof, making it more vulnerable to water damage and worsened sagging. A roof rake can help you deal with lighter snowfall. Keep your attic well insulated and ventilated to prevent ice dams, which can cause roof leaks.

Weight-related sagging can also be caused by design flaws that leave the roof unable to support its own weight. Common flaws include undersized rafters, insufficient sheathing or too few rafter or collar ties.

In a stick frame (non-truss) sloped roof, these issues usually cause the roof ridge to sag in the middle, giving it a swayback look, but can also cause one side of the roof to sag. Installing new rafters directly alongside the weak or damaged ones is often enough to correct this problem.

If the roof sags between the rafters or trusses, the most likely cause is sheathing that’s too thin for the area it covers. The fix here is to replace the thin sheathing with thicker, sturdier sheathing.

Multiple layers of shingles and new roofing material that’s heavier than the roof was designed for can cause sagging even in a well-constructed roof.

If your roof is sagging under the weight of too many shingles, ideally, all the shingles should be removed, and a new set installed. This gives your roofer a chance to inspect the flashing and roof underlayment for damage. If your roof started sagging after you switched to heavier roofing material, such as by replacing wood shingles with clay tiles, you’ll probably need to go back to the lighter material.

Less Common Causes of a Sagging Roof

Poor quality roofing material and faulty installation can cause a roof to slump. Low-quality materials are more prone to water and weather damage, and generally deteriorate faster.

Even the highest quality materials don’t last forever, though. On average, a roof lasts between 15 to 30 years depending on the design, the materials used, and your climate. If your roof is more than 30 years old, rebuilding it will be the most effective approach to dealing with sagging.

Problems with the roof itself aren’t the only reason a roof might sag. If your foundation has shifted or sunk, your walls and roof might have shifted along with it. In this case, you’ll probably notice tilting and cracks in your foundation. To repair major foundation damage, you’ll need to talk with a construction professional.

Once a roof has started to sag, it will only get worse and more expensive to repair. Eventually, it could cave in, causing damage to the rest your home and injury or even death to anyone in its path. To protect your home and your family, get in touch with a roofing professional who can determine the cause of the sagging, assess the damage, and recommend repair options.

Posted on Categories Roofing