The short answer
Rafters are the angled timbers beneath the battens and covering, a purlin is a level beam propping them part way up the slope, and a truss is a factory-made triangular frame that does both jobs together. Trussed rafters must not be cut or altered without an engineer's design, because every timber in the frame is carrying load.
Rafters: the traditional cut roof
A rafter is a sloping timber that runs from the wall plate, the timber laid flat along the head of the wall and rises to the ridge. Battens, the timber strips under the covering, are nailed across the rafters, and tiles or slates hang from them. In a cut roof, carpenters shaped each timber on site, often a century or more ago, to suit the house.
Older Bath houses are usually framed this way. The timbers are generally larger than modern ones and the loft is open, with space to stand and walk. Because the roof was built by hand, no two are quite alike, and the covering often shows the history: a gentle dip along a slope, a ridge that is not perfectly straight, repairs where a rafter foot rotted and a new piece was spliced in.
Rafters work as a pair, pushing outwards at the base as they lean together at the top. In a cut roof that push is held by ceiling joists or collar ties, and the rafters are supported part way by purlins. Take away one of those supports without a replacement and the roof can spread or sag.
Purlins and the rest of the cut roof frame
A purlin is a horizontal timber that runs along the length of the roof, under the rafters, about midway up the slope. It shortens the span each rafter must cross, so rafters can be a smaller section than they would otherwise need. The purlin ends rest on a gable wall, or on a strut or a cross wall inside the house that carries the load down to the ground.
Other parts have names worth knowing. The ridge board sits at the top where rafters meet. Collar ties join opposing rafters near the top. Hip and valley rafters run diagonally where roof planes meet. Wall plates spread the roof load into the masonry.
When a roofer strips a slope on a cut roof, these timbers are exposed and can be checked. We look for rot where a leak has run down a valley rafter, insect attack at the ends embedded in damp masonry, and old repairs where the load path has been interrupted. Replacing a decayed rafter foot with a sound timber joined to the old is routine work. Removing a purlin or a strut is not, and needs an engineer.
Trusses: the factory-made alternative
A trussed rafter, often just called a truss, is a prefabricated triangular frame made in a factory. Rafters at the top, a ceiling tie at the bottom, and thinner internal timbers called webs, joined with metal connector plates, form a rigid unit that spans from one external wall to the other. Trusses are set at regular centres, fairly close together, and the battens run across them just as they do on rafters.
Trusses became widespread in house building because they use less timber and go up quickly. They are common in houses built from the middle of the twentieth century onwards, and in extensions and garages. Because the trusses carry the load across the full width of the house, they often need no internal supporting walls, which is also why the loft looks so cluttered: the webs run in a repeating zigzag pattern and leave little open floor.
You can usually tell the type from the loft hatch. A forest of thin timbers in triangles, each plate-joined, is a truss roof. Big beams, a visible purlin and a more open space point to a cut roof. If you are unsure, a photograph from the hatch is enough for us to tell you.
Why trussed rafters must not be cut
Every timber in a truss has been sized for the load it carries, and the frame works as a whole. The webs may look like clutter, but they hold the rafters and the ceiling tie in the shape that stops the frame from spreading. Cutting a web to make room for a hatch, a water tank or a storage platform removes part of the load path, and the stress goes somewhere it was never designed to go.
The same applies to notching a chord, drilling through the members, or altering the connector plates. A truss that has been cut can sag, push the walls outwards, or fail suddenly under snow or wind load. It may appear sound for years before the damage shows.
The safe route is always to ask a structural engineer for a design. If a loft is to be converted or a chimney removed, the engineer may specify steel beams or new supporting frames so the trusses can be modified without loss of strength. This is the main reason a trussed roof is harder and dearer to convert than a cut one, a point our page on the roof changes in a loft conversion expands on.
What this means when you have roof work done
Re-covering a roof does not usually touch the structure, but the type of frame affects how we plan it. On a truss roof, the trusses are at regular centres, so battens can be fixed across them quickly. On a cut roof, the rafters may be uneven or at irregular spacing, and battens may need packing to give a flat slope. Neither is a problem, but both show up in the quote after the survey.
Weight matters too. A new covering that adds 15 per cent or more to the weight needs Building Regulations approval and a structural check under Approved Document A. A thin truss roof designed for lightweight tiles may not suit a switch to natural stone.
Roofers also have to think about how they climb. Tiles and slates are not made to be walked on, and the way we get onto a roof protects them, as our guide on whether you can walk on roof tiles explains. If you want the timbers checked, the free survey includes a look from the loft, and the battens that sit across them are covered in our roof battens guide. For everything else on this kind of roof, start at our pitched roofing page.
