Question answered
Cut roof or trussed roof: what is the difference?
The short answer
A cut roof is assembled on site from separate timbers: rafters, purlins, a ridge board and ties, each cut and fitted by hand. A trussed roof uses factory-made triangulated frames set side by side. Cut roofs are older and leave open space in the loft, with structural timbers running through it. Trussed roofs are lighter and quicker to build, but they are full of diagonals and must not be altered without an engineer.
Two houses can look alike from the street and be built very differently under the tiles. Knowing whether yours has a cut roof or trusses changes what you can safely do to it.
How a traditional cut roof goes together
Carpenters used to frame the roof where it stood, working from a pattern in their heads as much as from drawings. Pairs of rafters rise from the wall plate to the ridge, and heavier horizontal timbers called purlins support them part-way up the slope, resting on struts that pass the load down to internal walls. Collars or ties stop the rafters pushing the walls outward.
Each roof of this kind is slightly different, because the carpenter fitted every timber to its place. From the loft hatch you see large-section timbers, sometimes with pegged joints, laid out to follow the walls of the house below.
That is typical of Victorian and Edwardian terraces and many pre-war houses along the coast. The advantage is flexibility and open floor space between the purlins, but every element is doing a job, and taking one out without replacing its role causes trouble. Strutted purlins in particular should never be cut for convenience. We cover the consequence in roof spread.
What makes a trussed roof different
A truss is a ready-made triangle of slim timbers joined with metal plates, built in a factory to the span of the house. Trusses are set at regular intervals along the building, braced together at ridge and eaves, and the battens run across them. There are no purlins, because each truss carries its own load.
You can spot them from the hatch. The timbers are thin and uniform, the spacing is even, and a web of diagonals fills the space. Houses built from the later twentieth century onward often have this form, and so do many extensions.
Trussed roofs go up fast and use less timber. The price of that economy is that every member is part of a calculated frame, so cutting a diagonal to fit a water tank, a loft hatch or storage, or for a conversion, is something to take advice on first and never to improvise.
What it means for repairs and re-covering
For everyday repairs, the frame matters less than the condition of the timber. Slipped tiles, flashings and valleys are the same job on either roof. What changes is how much reserve the structure has.
When the covering itself is replaced, weight becomes the question. The weight test is 15%: Approved Document A counts anything above it as a significant change, and the Planning Portal wants approval once a covering is 15 per cent or more heavier. A structural engineer or surveyor should be consulted beforehand. Our comparison of interlocking or plain tiles shows how the choice of covering plays into that.
A cut roof that has moved over many decades needs a different eye from a truss that is simply carrying its load. Both are read in the loft at survey, before a price is given, and the notes from that visit go into the quote.
Loft use and conversion
A cut roof often leaves room for a storage floor or a conversion, although the purlins and struts then have to be dealt with by a structural engineer. A trussed roof is harder to convert, because the diagonals fill the space, and the usual route replaces the trusses with a purpose-designed frame of steel and timber.
Either way, a loft room is a Building Regulations matter, with structure, insulation, stairs and fire escape all in scope, and the covering has to stay weather-tight while the work goes on. The structural side belongs to the engineer and the building control body.
If you only want to put boxes up there, treat both types with care. The diagonals of a truss are not designed to take a person's weight, and storage loads need checking too. As for the roof outside, walking on roof tiles explains why that is never a good idea either.
How we tell which one you have
We start in the loft. Large timbers, purlins and struts point to a cut roof, and slim, repeating triangles joined with plates point to trusses. We then look for signs of movement in either: gaps at the wall plate, cracked ceilings, bowed rafters, a sagging ridge and tiles that have shifted.
Photographs from your own loft arrive with the written quote, which gives one overall price after the survey. If the survey raises a structural question, you hear it straight away, with a suggestion to bring in an engineer rather than a guess at the answer.
Roof form also bears on ventilation, tile choice and cost. For the shapes you may meet from outside, look at pitched roof types, and for everything we do on tiled and slate roofs, the pitched roofing page.
Roof frames: further questions
May I remove a timber that blocks my storage space?
Not without advice. In a cut roof, a strut or tie may be what stops the walls being pushed outward. In a truss, each member is part of a calculated frame. Ask a structural engineer first, and speak to your building control body if the change is more than minor.
Do trusses need different tiles from cut roofs?
Not in themselves. Tile choice follows pitch, exposure and weight, and the frame has to carry whichever covering you pick. What differs is the spacing of rafters or trusses, which the batten sizes follow, so the survey records it before anything is ordered.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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