Why Is My Roof Sagging?

What sagging looks like

From the street, look along the ridge and the eaves. A straight roof line that has developed a dip in the middle, a bow in a slope, or a ridge that is no longer level is a sign of sag. Tiles may look rippled, and gaps may open between the courses. From the loft, you might see rafters that bow, purlins that deflect, joints that have opened, or cracks and timbers that have pulled apart.

Inside, look for new cracks above doors and windows, doors that stick, and gaps between the ceiling and the walls. These point to movement in the structure beneath the roof as well.

Rot, damp and insects

Timber that has been wet for long enough loses strength. A slow leak from a failed flashing, a blocked valley or a slipped tile can soak a rafter, a wall plate or a purlin end for years. The wood softens and can no longer take its load, and the roof sags above it. Woodworm and other wood-boring insects thin the timber from inside, and they often go unnoticed until the damage is advanced.

The wall plate is a typical casualty. It sits on top of the wall, and a leaking gutter can wet it. Look for a dark, crumbling plate and for rafter feet that have sunk. Our page on roof timbers explained names each part, so you can describe what you see.

Too much weight

A roof designed for a light covering may sag if a heavy one is laid on it. Moving from slate to concrete tile, or adding solar panels without checking, increases the load. Under the Building Regulations, swapping to a covering at least 15 per cent above the weight of the old one means telling building control, and strengthening of the frame can follow. Our answer on how heavy roof tiles are gives typical weights.

Over time, timbers also creep under a constant load. A roof that has carried tiles for a century may show a gentle dip along each rafter without any rot, which is simple deflection.

Alterations that weakened the roof

Some roofs sag because someone has cut into them. A loft conversion may have removed a collar or a purlin support, a chimney breast may have been taken out without support for the rafters above, or a trussed roof may have had a truss cut to make room for a staircase. Trusses are engineered as a whole, and cutting one member can overload others. If your roof sagged after building work, say so at the start.

Spreading walls and foundations

If the walls are pushed outwards by the roof, the rafters lose their bearing and the roof drops. This happens when the ceiling joists that tie the walls together are missing or have failed. Movement in the foundations or the walls themselves can also tilt the roof out of line. In these cases the sag may be a symptom of a wider structural problem, and a structural engineer should look.

Old roofs and the question of whether it has stopped

Many old roofs have some sag that has been there for generations. A dip that is smooth, even and unchanged in photographs over several years is often a sign the structure settled long ago and found its balance. A sag that is new, or getting worse, is a different matter. The way to tell is to measure and record: pin a string line along the ridge, or take photographs from the same spot every season, and note what you see.

Cracks in plaster or brickwork can be monitored with a simple tell-tale across them. If the gap grows, the structure is moving. If it stays put, the movement may be historic. Share that record with the roofer or engineer, because it helps them judge the risk.

What to do next

Do not climb on the roof or the loft joists. Take photographs from the ground and from the loft hatch, and note when you first saw the dip. A roofer can survey the covering, battens and timbers, and tell you whether the sag is local and repairable, or needs an engineer. A repair could mean replacing a rotten section, adding support or strengthening the structure, and it is usually done with the covering lifted. Read our related answer on sagging roof felt if only the underlay is drooping. Our wider work is on the pitched roofing page.