Cut roof or trussed roof: what is the difference?

In a cut roof, a carpenter builds the roof frame on the house, timber by timber. Sloping rafters run from the wall at the eaves up to a ridge board along the top. Heavier horizontal timbers called purlins support the rafters part way up, and they in turn rest on struts that carry the weight down to walls inside the house. Ceiling joists tie the foot of the rafters across the house so the walls are not pushed outwards.

Every cut roof is slightly different, because it was cut to fit its own house. That has a useful side effect: the loft is often open, with room between the struts and fewer diagonal timbers in the way. Older houses, and many extensions, use this method. The weight is spread through a path of timbers and walls, so you cannot judge what holds what without looking carefully.

A trussed roof is made from prefabricated triangles that arrive on a lorry and are lifted into place. Each truss combines the rafters, the ceiling tie and a pattern of diagonal timbers, usually a W shape, that braces the whole frame. Metal plates with teeth press the joints together. The trusses stand a short distance apart across the house and are tied with battens and bracing, and the structure carries its load as a single engineered frame.

The result is quick to build and uses lighter timbers, but the loft is full of diagonal webs that cannot be moved. Each timber has a job. Cutting or notching a web, or taking out a section of the frame to make room, can weaken the whole truss. That is the most important thing to know about a trussed roof, and the reason a casual alteration here is risky. A visible dip in the ridge is a sign to take seriously, and our guide to a sagging roof covers the causes.

Put your head through the hatch with a torch, and stay on the joists. A trussed roof looks like a row of identical zigzag frames marching away from you, with W-shaped webs and small metal plates at each joint. You will have little clear floor, and moving about means stepping over timbers. A cut roof looks more varied. You see long purlins along the slopes, struts running down to internal walls, and bigger timbers with more open space around them.

Photographs help a lot. Take wide pictures down the length of the loft and close-ups of any plates or joints, and keep them with the house papers. If you are not sure, do not guess. Any structural change should begin with someone who has seen the roof, and our roof spread signs answer explains what movement in the walls or ridge can mean for either type.

A cut roof is generally the easier one to adapt to a room, because the open space is already there. It may still need new beams, and a structural engineer should design them. A trussed roof is harder, because the webs fill the space and each truss must be replaced or supported by a new structure before anything is removed. Dormers and roof windows both involve opening the slope, which means cutting rafters, so the engineer's advice comes first for either roof. Our dormer roofs guide sets out the choices.

One of the triggers the Planning Portal gives for Building Regulations approval of roof work is structural alteration, and it also recommends engineer or surveyor input where a covering's weight changes. Taking out a chimney breast always needs approval too, with a party wall notice where it sits on a shared wall. Check with your building control body before any such work begins.

For everyday repairs such as slipped tiles, ridges and flashings, both types are dealt with in much the same way. The differences appear when the work touches the structure. A roofer on a trussed roof must never notch a web or cut a chord to run a pipe or fit a vent, and loft boarding laid across the trusses should be done only as the truss designer allows. Heavy loads such as water tanks need their weight carried properly.

At survey, the framing gets as much attention as the covering, and each finding goes into the photographs. Sagging, splits or loose plates are reported plainly, and an engineer is suggested for the structure before the covering is touched. Our pitched roofing page shows the repairs and replacements we carry out, and once we have seen the roof, the figure comes to you in a written quote, one sum for the job.