The short answer
A sagging roof line is usually caused by rotten or overloaded timbers, a covering heavier than the roof was built for, spreading walls or supports that have failed. Some dips are old and stable, and others are still moving. Because the cause is structural and hidden, a sag should be looked at from the loft by someone qualified, and a fast-changing one treated as urgent.
What a sag tells you
A pitched roof should show straight lines. The ridge, the long line at the top, should run level. Each slope should be flat, or at most gently curved if the building is old. Dipping along the ridge, a belly in a slope or a ripple across the courses means the framework beneath has changed shape.
The roof framework is built from rafters, the sloping timbers, and purlins, the level beams that prop them partway down the slope. Together they pass the load of the covering on to the walls. A sag appears where that chain is weakened, overloaded or pushed out of position.
It helps to separate old from new. A shallow dip of long standing, on a roof that stays dry, may be settled and stable. A dip that is deeper than it was, or that has appeared since the last winter, suggests movement, and movement needs attention.
Rot and beetle in the timbers
Timber that has been wet for long enough loses strength. Rafters, purlins and wall plates, the beams bedded along the head of the wall, can all rot at the points where they are damp, particularly at the ends where they sit in or against masonry. Years of leaking, a split lead gutter, a clogged valley or condensation in a poorly ventilated roof space can each supply the moisture.
Wood-boring beetle, such as the common furniture beetle, can also eat out the inside of old timber and leave it hollow. Both problems leave the timber carrying less, so it bends under the weight of the covering and the roof dips.
A roofer looks for darkened timber, softness when probed, powdery dust, flight holes and fungal growth. Where the timbers are affected, sound wood can often be kept and the bad sections repaired, with new timber spliced in or fixed alongside. Where the damage is deep, whole members are replaced. This is the reason we always want to see the loft, not only the outside.
A covering that is too heavy
Roofs are sized for the weight they were built to carry. When the covering is changed for something heavier, such as replacing slate with concrete tiles, the timbers carry more than they were designed for and can sag over time. Even a heavy covering on the right roof can produce a dip if the timbers are already tired.
Extra layers matter too. Some roofs have had a new covering laid over the old one, and that adds a good deal of load. The Planning Portal sets a Building Regulations trigger at a covering 15 per cent heavier than the existing one, with a structural check, which is the reason weight is not a casual decision. Concrete is the heavier choice, as we explain on the concrete roof tiles page.
If a sag appeared soon after a change of covering, weight is a likely cause, and an engineer's view is sensible before anything else is done.
Spreading walls and failed ties
A roof pushes outward as well as down. The rafters on either side lean on each other at the ridge and thrust against the tops of the walls. In a well-built roof, horizontal ties such as ceiling joists, collars or purlin supports hold that outward push.
When those ties are cut, corroded, rotted or were never adequate, the walls can spread slightly at the top. The ridge then drops, because the rafters have slid apart. Alterations are a common cause. A joist cut for a loft hatch or a stair, or a structural member removed during building work, can take away a tie that was doing important work.
The same effect can come from movement in the walls themselves, from subsidence, from poor pointing or from a chimney stack that has shifted. If the sag is accompanied by cracks in the walls or ceilings, or by doors and windows that no longer fit, tell whoever surveys it.
How serious is it?
Seriousness depends on what is happening and how fast.
- Stable and old. A long-standing, shallow dip with sound timber and no leaks is usually monitored, and the covering is looked after in the meantime.
- Growing. A sag that is clearly deeper than before needs a prompt look, because the structure is moving.
- With a leak or rot. Water in the timbers is both a cause and a consequence. Dealing with the water comes first.
- With cracks or deflection inside. Cracked plaster, gaps at ceiling lines or stuck doors suggest the wall tops are affected.
- After storm, fire or an alteration. Any of these can weaken timbers suddenly.
If the roof seems to be moving quickly, keep people out from beneath the affected area and get it looked at. Where a structural engineer is needed, we will say so, and calculations and design come from them.
What a survey checks and what it leads to
We look from three positions. From the ground, we check the lines of the ridge and slopes. From the loft, we check the rafters, purlins, wall plates and ties for rot, beetle, splits and movement, and look at the underlay for related signs. Our page on mansard roof planning is useful if your roof has the broken profile of a mansard, where the steep lower slope carries unusual loads.
We also check the felt, because a sag in the underlay can accompany a sag in the timbers, though not always. See sagging roof felt for how to tell them apart.
The outcome ranges from keeping watch, to repairing a section of timber, to reinforcing or replacing rafters, to re-roofing with a lighter covering. Each is explained using pictures of your roof, and our free survey ends with a written fixed price. Where the repair works, we repair. If building control is involved, we explain the rules that apply and manage the submission if the work is notifiable. The pitched roofing page has the wider service.
