The short answer
A cut roof, also called a traditional or purlin roof, is framed on site from individual rafters, purlins and struts, which leaves the loft fairly open. A trussed roof is assembled from factory-made triangular frames that carry the whole load through thin, closely braced timbers. Trusses are quicker to build but far less forgiving when you want a loft room or a dormer.
How a cut roof is put together
A cut roof is made by carpenters, one timber at a time. Rafters, the sloping boards that carry the covering, run from the ridge board at the top down onto a timber wall plate, a long beam bedded on the inner leaf of each outer wall. On a hipped roof, sloping hip rafters and shorter jack rafters turn the corners. Ceiling joists tie the feet of the rafters together so the walls are not pushed outwards. Because rafters of ordinary size cannot span a whole house unaided, heavier horizontal timbers called purlins sit partway up each slope, and struts carry their weight down onto an internal wall.
The result is a roof where the load travels down into specific walls, and the loft space between them is relatively clear. Every cut roof is a little different, because the carpenter adapted it to the house. That individuality is a strength when you want to alter the roof, and a worry when you want to know exactly how it behaves, since the original drawings are rarely to hand.
How a trussed roof is put together
A trussed roof is made from trussed rafters: triangular frames built in a factory from slim timber, joined with steel plates, and lifted into place on site, side by side. Diagonal webs inside each frame, often in a W pattern, share the load so that no single timber has to be large. Nothing in the frame is spare. Each member is sized for its job, and the whole frame works as one unit.
This is efficient, and it is why trussed roofs are quick to put up and use less timber. The price is a loft filled with a lattice of diagonals that cannot be moved. Cut or remove a single web and the load has to go somewhere else. That is the point where an ordinary builder's idea, such as taking out a few braces to make storage room, can damage the roof. Never alter a truss without an engineer's design.
Which houses have which
Broadly, age tells you. Victorian and Edwardian terraces and villas, and many houses from the years between the wars, were built with cut roofs, since carpenters framed them by hand. Trussed rafters became the usual choice for speculative housing after the war and are standard on the later estates, so a house on a later twentieth-century estate, or anything newer, is very likely to have them. A few mixed cases exist, with a cut roof over the original house and a trussed roof over a later extension.
You can check from your own loft with a torch and a careful look from the hatch. A forest of thin, evenly spaced timbers in W shapes points to trusses. Thick purlins, a ridge board and struts landing on walls point to a cut roof. If you cannot tell, a picture taken from the hatch is enough for us to give an opinion. Please stay on the joists or boarded areas and never step between them.
Loft boarding is a related trap. In a trussed roof the lower timbers of each frame are sized to carry the ceiling and a layer of insulation, not a stack of stored furniture. Light boarding for boxes is usual, but heavy loads, a water tank moved to a new spot or a new staircase landing are worth discussing with an engineer before anything is placed.
What it means for a loft conversion or dormer
This is where the difference bites. A cut roof can often be converted by adding steel or timber beams that take over the work of the purlins, which opens the space for a room. A trussed roof has no spare timber, so a conversion usually means replacing the trusses with a new frame of rafters and steel beams, designed by a structural engineer. It can be done, and it costs more and takes longer.
Either way, structural alteration brings the work into Building Regulations, which means an application to your building control body. A dormer adds planning questions on top, and our page on dormers and permitted development explains where the limits sit. Our guide to roof tile types helps with the covering choice, since a heavier covering changes the load on either structure.
What it means for repairs and re-covering
For everyday repairs the two types behave much alike. A tile is replaced the same way on either, and a ridge or valley is repaired the same way. Differences show up when timber is involved. On a cut roof, a rotten rafter foot or wall plate can often be repaired in place by fixing a new timber alongside, called sistering. On trusses, damaged members need advice from the truss designer or an engineer, because every piece carries part of the load.
Re-covering is the other place to pay attention. A trussed roof is designed with a particular covering weight in mind, so swapping light tiles for heavy ones needs a check rather than a guess. The same goes for pitch. Low-pitched trussed roofs call for particular tiles, which our page on low-pitch tiles goes into.
How we look at your roof
The loft is where we begin, because the timbers show what is sensible to propose. The survey brings pictures of the framing in your house, the price arrives in writing, and any work we do is backed by a 10-year workmanship guarantee that you keep on paper. Where the job alters the structure, we explain what the Building Regulations notification is for and deal with it alongside you.
Thinking about a conversion? WhatsApp a photo from the hatch to 01793 626096 and we will tell you which kind of roof it looks like. The rest of the roof-structure family lives in the pitched roofing hub.