Open the loft hatch on a house in Basingstoke and you are looking at one of two things. In an older home, usually pre-war, it is a cut roof: rafters, purlins and braces assembled on site by a carpenter. In a home built since the 1960s, it is more likely a trussed roof, a set of factory-made triangles lined up along the building. Both do the same job of holding up the tiles, and they work in very different ways. Knowing which you have, and what each timber does, makes it far easier to judge a quote, spot a fault and avoid disturbing something that is holding the roof together. This guide names the parts and says what goes wrong with them.
The basic idea: a frame that pushes down and out
A pitched roof is a frame of sloping timbers meeting at the ridge. The weight of the tiles, the wind and any snow all load that frame, and the load wants to do two things: push the walls outward at the eaves, and push the timbers down onto what supports them. Everything under the tiles exists to manage those two tendencies. Some timbers resist the outward push by tying the bottoms of the slopes together. Others stop long timbers from sagging by supporting them part-way along. Once you see the roof as a triangle held by its base, the names fall into place.
The main timbers and what each does
These are the parts you will hear named, in roughly the order water would meet them.
- Wall plate. A long timber bedded on top of the outer wall. It spreads the load of the roof along the masonry and gives the rafters something to sit on and be fixed to.
- Rafters. The sloping timbers that run from the wall plate up to the ridge. The battens and tiles are carried on them.
- Ridge board. A horizontal board at the top where pairs of rafters meet. It locates the rafters but, in a properly built roof, does not carry load.
- Ceiling joists. Horizontal timbers that tie the bottoms of the rafters together. They carry the ceiling and, crucially, stop the walls being pushed apart.
- Collar ties. Short horizontal timbers linking pairs of rafters higher up. They help stop the rafters spreading.
- Purlins. Horizontal timbers running along the roof mid-slope, under the rafters. They hold the rafters up so they do not sag between wall plate and ridge.
- Struts and braces. Diagonal timbers that carry the purlin's load down to an internal wall.
- Hip and valley rafters. Diagonal rafters running up the line of a hip or down the line of a valley, where two slopes meet.
The exact set varies with the house. A small roof may have rafters and ties only. A larger one adds purlins, struts and more. The point is that every timber has a job, and cutting or notching one without understanding it can bring a roof down.
Cut roofs and trussed roofs
A cut roof is built on site, rafter by rafter, from timber cut to the house. It dominates houses built before the postwar period and is still used for complex or period-style roofs. Its great virtue is open loft space: there are few diagonal webs in the way, which is why cut roofs are the easier ones to convert. The disadvantage is that it depends on the carpenter's skill, and on old houses the joints and ties may have moved or been altered over the decades. Our general guide to types of roof structures shows the main forms.
A trussed roof is made from prefabricated triangles called trusses, built in a factory from lighter timber joined by nail plates, and spaced along the building. Each truss contains rafters, a bottom chord that acts as the ceiling joist, and a web of diagonal timbers between them. The trusses work as a unit. Cut or remove one of those web members, and you have broken the triangle. Trusses are efficient and quick to put up, which is why the great majority of postwar estates use them, and their web of diagonals is why the loft is cluttered.
| Feature | Cut roof | Trussed roof |
|---|---|---|
| Built | On site by a carpenter | In a factory, craned or lifted into place |
| Typical age | Pre-war and period homes | 1960s onward |
| Loft space | Open, with purlins and few obstacles | Crossed by diagonal webs |
| Altering it | Possible with a structural design | Only with an engineer's design |
| Main weakness | Movement, decay at joints, past alterations | Damaged or removed webs, poor bracing |
Common faults in roof timbers
Most roof timbers last a very long time if they stay dry. The faults that matter are almost always about water, movement or alteration.
Rot. A long-running leak or persistent condensation lets wet rot or dry rot take hold. It is usually found at the eaves, around chimneys, under valleys and wherever a gutter has overflowed onto the wall plate. Soft, crumbly or cube-cracked timber is the sign. The affected section is cut out and replaced, usually with new timber spliced to the sound part, once the water has been stopped. Our guide to roof damp and condensation surveys covers tracing the source.
Insect attack. Woodworm in softwood shows as small round holes and fine dust. Light attack in old timbers is often historic and inactive, and a surveyor can tell the difference. Active infestation may need treatment.
Sagging and spreading. If the ridge dips or the slopes bow, something is overloaded or has moved. Undersized rafters, missing purlin supports, a heavy covering added later or removed ties can all cause it. Our answer on why a roof sags sets out the causes and which are urgent.
Alterations. Over the years, owners and trades cut timbers to run pipes, fit tanks, install loft hatches or create storage. A notch in the wrong place in a rafter or truss chord weakens it. Removing a diagonal web to make headroom can overload the neighbouring members.
When a structural engineer is needed
Roofers deal with coverings, battens, ridges, valleys and minor timber repair. When the job touches the structure itself, an engineer's design is the safe route. That means cutting, removing or relocating trusses or purlins, building a loft conversion, forming a large opening in the roof, or changing the roof's shape. Structural alterations are controlled by the Building Regulations, and a building control body, either the council's team or a registered building control approver, will want to see the design and inspect the work.
The same logic applies if you plan to change the covering. Swap in something much heavier, 15% or more above the old weight, and the change goes to building control while the timbers are checked or strengthened. Our page on heavier roof coverings and structural checks explains what that involves.
Reading the timbers in a Basingstoke loft
The housing stock here tells you what to expect before you open the hatch. Villa and terrace roofs in the older parts of town and the village centres are mostly cut roofs, with purlins and often a timber that has been repaired or propped over the years. The family homes of the postwar expansion, the large estates built as Basingstoke grew, are mostly trussed, with thin softwood members joined by metal plates and spaced at regular centres. Many of those estates also had a loft conversion done by a later owner, and that is where we meet the worst alterations: a truss web cut for headroom, a new floor laid straight on ceiling joists that were never meant to carry it, or a dormer cut into a roof that was never designed for one.
None of that is a reason to panic. It is a reason to look carefully, and to ask for the building control sign-off on any conversion when you buy. Our guide to roof checks when buying in Basingstoke lists what to ask for.
Timber and the covering: how they interact
Roof timbers and tiles depend on each other. The rafters and battens carry the tiles, and the weight of the tiles keeps the roof pressed down against the wind. If the battens are weak, the load path is broken at the top layer, as our explainer on roof battens shows. If the valleys leak, the valley rafter underneath is the first to rot, and the guide to roof valleys deals with that end.
Timber also moves with moisture and temperature. Over a damp winter, rafters swell. Over a hot summer, they dry and shrink. A roof that was built tight and sound will take this in its stride. A roof with loose joints or poor fixings can creak, shift and open gaps that let wind and water in. Ventilation is part of keeping timber at a stable moisture level, which is why a sealed, unventilated roof space is a risk.
What to check yourself
Simple checks from the loft hatch, with a torch and without leaning on anything, tell you a good deal.
- Look along the ridge line from one end. Is it straight, or does it dip?
- Look at the timbers near the eaves and chimney for dark staining, soft patches or fungal growth.
- Check for daylight where there should be none, which suggests slipped or missing tiles.
- Notice any timber that has been cut, notched or removed, and whether it looks recent.
- Look for sawdust-like frass under timbers, which suggests active insects.
Do not walk on ceiling joists, which are not floor joists, and do not stand on a ceiling between them. If anything looks wrong, photograph it and ask for a survey.
How we approach roof timbers on a survey
We look at the structure whenever we are in the loft, because the covering and the frame fail together. We photograph the timber as it is on your roof, show you the sound parts and the suspect ones, and explain what we recommend. Many timber faults are local and mended with a splice; the report says so. If we see signs of movement or alteration that need an engineer, we say that too and do not guess. Prices come as a fixed written quote after the survey, and our ten-year guarantee on workmanship is in writing. Full details of what we offer are on the pitched roofing in Basingstoke page.
