Under the covering of every pitched roof is a frame of timber: rafters running up the slope, a ridge board or beam along the top, purlins carrying the rafters at mid-height, wall plates bedded on the tops of the walls and ceiling joists tying the lower ends together. In an older house that frame may be a century or more old, built by carpenters working from experience rather than calculation, and it has carried every storm, every leak and every change of covering since.
Most of the time the frame is fine. When it is not, the signs show first in the roofline, then in cracks inside, and the cause is rarely the tiles. This guide covers the faults roofers and surveyors meet most often, which of them you can see from the street, and what a sound repair involves. It deals with all of them together. The single symptom of a dipping ridge has its own answer, why is my roof ridge sagging. The wider service is on our pitched roofing page.
How the timber frame of a pitched roof works
A pitched roof is a structure that has to resist two kinds of force. Weight acts downwards through the tiles, battens, rafters and snow or workmen. The slope converts part of that weight into an outward push at the foot of each rafter, which is why the frame includes something to hold the feet together.
The parts, briefly
- Rafters run from the wall plate at the eaves up to the ridge.
- Purlins are horizontal timbers that carry the rafters part-way up their length, held up by struts or by cross walls.
- Collars tie opposite rafters together higher up.
- Ceiling joists, or separate ties, stop the walls being pushed outwards.
- Wall plates are the bedded timbers that share the roof's weight along the wall head.
Remove or weaken any of these and the load finds another route, commonly into a timber, or a stretch of wall, that was never sized for it. Whether the frame was cut by a carpenter on site or built from factory trusses makes a difference to how it fails, and our answer on cut roofs and trussed roofs explains that distinction.
Roof spread: when the walls are pushed outward
Roof spread happens when the rafters push the tops of the walls apart instead of being held in. It develops slowly, over years, and can be hard to spot from ground level.
What causes it
The usual cause is a missing or failed tie at the foot of the rafters. A ceiling joist that has been cut for a loft hatch or a pipe run, a joint that has pulled apart, or a tie removed during a conversion all let the rafter feet slide outward. On a roof without proper triangulation, the weight of heavy coverings accelerates it.
Signs
The tops of the walls lean outwards, sometimes by a visible amount. The eaves sag or the wall plate slips. Cracks appear at the top of the wall inside, and the ridge may drop. Gable walls can show a crack where the roof pushes against the apex.
Assessment and repair
We measure from the loft, with a straight edge and a string line, and photograph what we find. A structural engineer's report may follow, because the repair affects the walls. Typical repairs include fixing new ties or steel straps across the rafter feet, installing a collar or a purlin brace, and in some cases re-bedding the wall plate. Where the walls themselves have moved, the work can extend to wall restraint, which is outside a roofer's scope and is handled with an engineer.
Sagging rafters, purlins and ridge lines
A sag is a curve in a timber that should be straight. It shows from outside as a dip in the slope between the ridge and the eaves, or as a wave in the line of the ridge itself.
Causes
Rafters sag when they are too small for the span, when they have been notched or cut, or when the purlin that should carry them has failed. Purlins sag because they are undersized for the load, because their support has been removed or because they have rotted at their ends where they sit in the wall. A heavy covering that replaced a light one is another common reason; stone slates and some concrete tiles weigh more than the clay or slate that the original carpenters sized for. A ridge that dips is often the symptom of a failed purlin, not a fault in the ridge itself.
Assessment
We sight along the line of the slope from the ground and from inside the loft, and compare each rafter. A sag of a few millimetres in an old timber is common and often stable. A sag that is increasing, or one that is accompanied by cracks or by a gap opening between the rafter and the wall plate, needs further investigation. See the answer on the ridge for the full list of signs.
Repair
A sistered rafter, a new timber bolted to the side of the old one, restores strength without stripping the whole roof. A failed purlin may be replaced or supported with a new strut taken down to a load-bearing wall. Steel flitch plates are sometimes used where space is tight. These repairs usually mean lifting some of the covering to get at the timbers, and they are best done when the roof is being re-tiled anyway.
Overloading after a heavier covering
The frame of an old roof was built for the covering it had. When that covering changes, the load changes.
The regulatory line
On the Planning Portal, a new covering that adds 15 per cent or more to the roof's weight triggers Building Regulations approval. Approved Document A (2004 edition, amended 2010 and 2013) describes a significant change in roof loading as an increase of more than 15 per cent, and adds that strengthening follows if the structure cannot take the extra load, citing overstressed members or deflection that leads to ponding. The two sources differ slightly at the threshold, so check with your building control body.
Where it goes wrong
The obvious case is a slate roof replaced with thick concrete interlocking tiles. The less obvious ones are a roof where a second layer of tiles has been laid over the first, and a roof where heavy ridge and hip bedding has been added over the years. Our answer on concrete tiles on an old roof looks at this from the tile side. Cotswold stone slate is the heaviest covering of all: the district council's technical guidance gives about 1,000 kg for every 10 m².
What we check
Before pricing any change of covering we climb into the loft with a torch and a tape, and note rafter size, spacing, span, existing deflection, condition of the purlins and wall plates. If the margins look thin, we recommend a structural check before ordering tiles.
Rot and moisture damage in roof timbers
Timber rots when it stays damp. In a roof, the source is nearly always water: a leak that has run unnoticed for years, or condensation from warm air in the house meeting cold timber.
Where it starts
The first places to rot are the ends of timbers that sit in or against masonry, such as wall plates, rafter feet and purlin ends; the area around a chimney stack where flashings fail; valley boards under a failed lining; and the lower ends of battens where the eaves is poorly ventilated. Our guide to why tiled roofs leak covers the entry points.
Condensation and ventilation
A cold roof, one with insulation at ceiling level, needs air movement through the roof void so moisture can leave. BS 5250 sets out ventilation to manage moisture; Marley's reading is that non-breathable underlay calls for a 25 mm continuous gap at the eaves when the pitch is 15 degrees or less, and 10 mm once it is steeper. A blocked eaves, covered over by loft insulation, defeats this. Our guide to roof underlay and breathable membranes covers how the membrane affects it.
Assessment and repair
We probe suspect timber with a bradawl: sound wood resists, rotted wood crumbles. The repair is cutting out the decayed section and splicing in new treated timber, or replacing the member, and, as importantly, dealing with the moisture source. Replacing wood without curing the leak only starts the clock again.
Insect damage: woodworm and other wood-boring beetles
Wood-boring beetles lay eggs in timber, and the larvae tunnel through it for years before emerging through small round holes. In older roofs the damage is often historic and inactive. The question for a survey is whether it is still going on and how much of the section is left.
Signs
Small round exit holes in the surface of the timber, fine powder called frass below them, and sections that sound hollow when tapped. Fresh, pale frass suggests recent activity; dark, compacted dust suggests the infestation is old. Damp timber is more vulnerable than dry, so insects and rot often appear together.
Assessment
We look at the extent: whether holes are confined to the surface or run deep, and whether the timber is still sound at its core. Where a structural member is in doubt we recommend a specialist timber survey. We do not claim a treatment will cure everything, and we do not recommend one where there is no activity.
Repair
Where damage is surface-only, treatment of the timber may be all that is needed. Where it has weakened a rafter or purlin, the member is strengthened or replaced. If the roof is being re-covered, this is the moment to do both. Treating timbers before the underlay and tiles go back is far simpler than doing it later.
Old exit holes alone do not condemn a timber. What matters is how much sound wood remains and whether the damp that invited the insects has been dealt with.
How a structural problem is found, priced and put right
You cannot judge a roof frame from the pavement. A proper assessment happens in the loft and on the roof.
The survey
Photographs of your own roof and timbers are taken, and we measure the sections, look for movement and moisture, and note what the covering is. MyBuilder's inspection guide gives a standard roof survey at £150 to £250, and £150 to £500 across the survey types it lists. If the findings are serious, we advise bringing in a structural engineer.
The quote
Structural repair is hard to price before the covering is lifted, because the full extent of damage appears only when the tiles come off. Our written quote separates the covering work, which is priced, from timber repairs, which are listed as provisional with the likely scope described. Nothing is added without your agreement.
Regulations and consent
Structural alterations bring in Building Regulations approval, as the Planning Portal makes clear, and we explain the process and look after the notification if the work is notifiable. If the house is listed, repairs to the frame need listed building consent from the council; the protection extends to the entire building. Like-for-like repair is the approach: old oak stays oak where it can.
Work carries a 10-year workmanship guarantee in writing, the site is left clear and waste is taken away. Send photographs of what you can see, with the address, and a survey can be booked in.