A roof is supposed to be straight. The ridge should run level from end to end, and the roof planes should look flat from the street. When a ridge dips, a roof plane dishes inward, or the surface looks wavy, the structure underneath is telling you it is carrying more than it can support.

Sagging happens at two levels. The roof decking can sag between the rafters, or the rafters and ridge themselves can sag. The causes, and how serious each one is, are different.

Sagging Decking

Roof decking, also called sheathing, is the layer of plywood or OSB nailed across the rafters that the underlayment and shingles sit on. When decking sags, it usually shows up as dips between the rafters, giving the roof a wavy or rippled look.

Common causes include:

• Moisture and rot. Long-term leaks, condensation from poor attic ventilation, and failed flashing soften decking until it can no longer hold its shape.
• Delamination. Plywood that stays wet can separate into layers and lose its strength.
• Decking that was too thin. Panels that were undersized for the rafter spacing from the start can bow over time.
• Older materials. Some decking products used in older homes, such as fiberboard, don't hold up well over the life of a house.

Sagging decking is typically addressed by replacing the affected panels, which is covered in more detail in why damaged roof decking is best replaced.

Sagging Rafters and Ridge

When the rafters or ridge sag, the problem is structural. This usually shows up as a ridge line that dips in the middle or a roof plane that bows inward across a large area.

Missing or Failed Rafter Ties

On a traditional rafter roof, the weight of the roof pushes the rafters outward against the tops of the walls. According to InterNACHI, rafter ties prevent the weight of the roof from spreading the tops of the walls and causing the ridge to sag. In homes with flat ceilings, the ceiling joists usually serve as those ties. When ties are missing, cut during a remodel, or poorly fastened, the walls can spread and the ridge drops. An outward bow at the top of exterior walls is a sign this is happening.

Undersized or Overspanned Rafters

Rafters have to be sized for the distance they span and the load they carry. Older homes were sometimes framed with 2x4 or 2x6 rafters that are smaller than current standards call for. Rafters spaced farther apart than designed, or spanning too far without support, can bow over time. InterNACHI notes that rafters are typically installed on 24-inch centers, and wider spacing is a reason to look for signs of sagging.

Rot and Insect Damage

The same moisture that ruins decking can reach the rafters. Wood that has been wet for years, or damaged by insects, loses strength and can sag or crack.

Removed or Improper Supports

Attic remodels, HVAC installations, and DIY projects sometimes remove braces or supports without replacing them. Braces that rest on ceiling joists instead of bearing walls can also push roof load onto framing that was never designed to carry it.

Heavy Loads

A roof carries its own weight plus temporary loads like snow, ice, workers, and bundles of shingles during a re-roof. A structure that is already marginal can show sag after a heavy snow or a winter with repeated ice buildup.

Truss Roofs vs. Rafter Roofs

Most newer homes use engineered trusses rather than site-built rafters. Trusses are designed as a complete system, so they are less prone to the spreading that causes ridge sag. They can still sag if a member is cut, damaged, or overloaded. The differences between the two systems are covered in roof rafters vs. trusses.

Warning Signs

• A ridge line that dips instead of running straight
• Roof planes that look bowed, dished, or wavy from the street
• Soft or spongy spots when walking on the roof
• Daylight through the roof boards in the attic
• Water stains, dark spots, or mold on decking and rafters
• Cracked, split, or disconnected rafters in the attic
• Exterior walls bowing outward near the top
• Interior ceiling cracks or doors that start sticking on upper floors

Is a Sagging Roof Dangerous?

It depends on the cause and how far it has progressed. Mild, long-standing sag in an older home can be stable for years. Sag that is new, getting worse, or paired with cracked rafters, bowing walls, or active leaks is a structural concern, and heavy snow puts the most stress on a roof that is already weakened.

InterNACHI's guidance to inspectors is direct: when rafters are sagging, stabilization should be done by a qualified contractor. Depending on what's found, that can mean sistering new lumber alongside damaged rafters, adding or repairing rafter ties, replacing rotted decking, or bringing in a structural engineer for larger problems.

The Bottom Line

A sagging roof comes down to two things: decking that has lost strength, usually from moisture, or framing that is undersized, damaged, or missing the ties that hold it together. Decking sag is typically handled during a re-roof. Ridge and rafter sag is structural and should be evaluated before winter loads add to it. Understanding the available roof repair options starts with finding out which kind of sag the roof actually has.