Cut roof or trussed roof: what's the difference?

A cut roof, sometimes called a traditional or purlin roof, is framed by carpenters on the building itself. Pairs of rafters, the sloping timbers, meet at a ridge board along the top and sit on a wall plate along the walls. Ceiling joists tie the feet of the rafters together so the walls are not pushed outwards. Long slopes would sag, so horizontal timbers called purlins run along the middle of each slope, held up by struts that carry the load down to internal walls.

Because each timber is cut and fitted by hand, a cut roof adapts to odd shapes, unusual spans and old walls that are never quite square. That is why it is the usual form on older houses, including many stone cottages and terraces on the hillsides around Stroud. The result in the loft is often a clear space broken by a few large timbers rather than a web of small ones.

A trussed roof is made of trusses: triangular frames built in a factory from slender timbers joined by metal plates, then craned or carried into position and set at regular spacings along the walls. Each truss carries its own share of the load straight down to the external walls. Because the triangles do the structural work, a trussed roof normally needs no internal load-bearing wall under it.

The standard truss in a house has a rafter on each side, a bottom chord that doubles as the ceiling joist, and diagonal webs forming a W or N pattern between them. This form became common on modern houses and extensions because it goes up quickly, with less skilled carpentry on site. The tiles or slates, battens and underlay are then laid over it just as they would be on a cut roof.

Look in the loft from the hatch, with a torch, and stay on the joists. Do not step between them. A trussed roof shows a regular row of frames, each with thin diagonal webs, repeated at close intervals along the whole length. A cut roof shows heavier timbers, a ridge board, purlins crossing the slopes and, often, rafters of different sizes. If you can walk between big timbers with room to stand, it is probably cut.

Some roofs mix both, especially after extensions, and old roofs may have been altered over the years. A photograph from the survey settles most doubts.

A house with a hipped end, or a roof over an L-shaped plan, often has a cut section where the hips and valleys meet, even when the main run is trussed. The rafters at those junctions are individually cut because no factory frame fits the angle. If you are not sure what you are looking at, take photographs from the hatch rather than climbing in, and send them to us. We can usually tell which kind of framing it is from a handful of clear pictures taken from different angles.

The framing decides how freely a loft can be changed. In a cut roof, purlins and struts can sometimes be reorganised by a structural engineer to make a room. A trussed roof relies on every web staying in place, so cutting one out, or adding weight to the lower chord, can upset the whole structure. Trusses are not usually a barrier to a conversion, but the work normally involves a structural engineer, and new beams are designed to carry the load before any webs are removed.

There is a smaller everyday point too. The bottom chord of a truss is also the ceiling joist, and it is sized for a ceiling and insulation, not for a loft full of stored furniture. Heavy loads on it can sag the ceiling below. In a cut roof the ceiling joists are often lighter again, so if you plan a floor in the loft, ask for the joists to be checked before boards go down. Ridge, rafter or web problems found during a roof repair are mentioned in your survey notes with photographs.

A conversion that adds a dormer (a vertical window box built out from the slope) also raises a planning question. Dormers are not permitted development in a conservation area or the Cotswolds National Landscape, so check with Stroud District Council's planning team first. Our guide to pitched roof shapes covers the outside forms, which are a separate matter from the framing hidden beneath them.

Framing also shapes how problems appear. A cut roof with an undersized purlin or a pushed-out wall plate may show a dip in the slope or a sagging ridge. A truss roof that has been overloaded or had a web removed can show uneven ridge lines or cracking at the junctions with internal walls. In both cases, battens that move with the structure open the courses, and tiles start to drop out of line. See why tiles move in our answer on slipped roof tiles and what we do when a single tile breaks in the replacement of a cracked tile.

Before any re-roof, the timbers get as much attention from us as the tiles on top of them. If the new tiles or slates would be 15% heavier or more than the old, building control approval and a structural check apply. We talk you through that and send the notification when the job needs one. The quote after the survey covers every part of the work, and what we fit is covered by a 10-year workmanship guarantee that you receive in writing. For repairs and re-roofing generally, see pitched roofing.