If you have ever stood in a loft or watched a new roof going up, you will have heard words thrown around that mean very little unless you work in the trade. Rafters, purlins, ridge boards, wall plates, collars, binders. They all describe timbers doing a particular job, and once you know what each one does, the whole structure starts to make sense. This guide explains the main terms in plain English so you can follow a conversation with your carpenter and ask better questions about your own roof.

The main timbers in a traditional cut roof

A cut roof is one built on site from loose lengths of timber, rather than delivered as factory-made trusses. Most older Hampshire houses have one, and it is still the usual choice for extensions, loft conversions and anything with an awkward shape. Here are the parts you will hear named most often.

Wall plate: the horizontal timber bedded on top of the masonry. Everything above it is fixed down to this, so it needs to be dead level and properly strapped to the wall.
Rafter: the sloping timbers running from wall plate up to the ridge. They carry the tiles, battens and felt, plus wind and snow loads.
Ridge board: the timber running along the top of the roof that the rafters land against on both sides. It is not really a beam in the structural sense, more a fixing point that keeps opposing rafters aligned.
Purlin: a horizontal timber running along the roof slope partway up, supporting the rafters so they can span further without sagging.
Collar: a horizontal tie between opposing rafters, usually in the upper third of the roof, stopping the rafters from spreading apart.
Ceiling joist: the horizontal timbers at the foot of the rafters, forming the ceiling below and tying the feet of the roof together.
Binder and strut: smaller supporting timbers that stiffen joists and transfer load from purlins down to a suitable wall below.

Why rafters and purlins work together

A rafter can only span so far before it starts to deflect. Push the span too far and the roof line begins to dip, which you can often spot from the pavement on older terraces. The purlin solves that by giving the rafter an intermediate support, effectively cutting the span in half. The purlin then needs something to carry its own load, which is where struts come in, transferring the weight down onto a load-bearing wall or a substantial binder rather than into the middle of a bedroom ceiling.

Cut roof or trussed roof?

Trussed rafters arrive as pre-made triangles, craned in and fixed at regular centres. They are quick, engineered and economical on a straightforward new build. The catch is that the webs inside each truss fill the loft space, so you cannot easily use the room later without a structural redesign.

A cut roof takes longer to build and needs a carpenter who can set out and cut accurately, but it gives you a clear space between the collars and a structure that can be adapted to unusual plan shapes. On extensions that tie into an existing pitched roof, a cut roof is almost always the practical answer. We go into more detail in our guide on what a cut roof is, and if you are weighing up budget there is a companion piece on cut roof costs in Hampshire.

Terms you will hear on a loft conversion

Loft work brings its own vocabulary. Sistering means fixing a new joist alongside an old one to strengthen it. Trimming refers to framing around an opening, for a stairwell or a rooflight, using trimmers and trimming joists that are usually deeper than the standard members. A steel, or RSJ, often appears in loft conversions to carry new floor joists independently of the old ceiling joists, which were never designed to take furniture and footfall.

Hampshire roofs and their quirks

A lot of the housing stock around Hampshire is Victorian or interwar, and roof structures from those periods vary enormously. Undersized rafters, purlins bearing on nothing very convincing, and previous alterations made without much thought are all common finds. Timber of that age is often perfectly sound, but it was cut to different standards and has usually been drilled or notched by later trades running services through it.

The sensible approach is to open up, look properly, and design the new work around what is actually there rather than what the drawings assume. That is true whether you are converting a loft, adding a rear extension or simply replacing rotten rafter feet where an old gutter has been leaking for years. For repair work of that kind, our repairs and maintenance page covers what we typically deal with.

Talk it through with a carpenter

Knowing the terms means you can ask the right questions: what is carrying that purlin, how far are those rafters spanning, what happens to the load around the new opening. A good carpenter will be happy to walk you through it on site, because a client who understands the structure tends to make better decisions about the rest of the project.

PRM Carpentry builds cut roofs, loft floors and structural first fix carpentry across Hampshire. If you have a project coming up, take a look at our first fix carpentry services or get in touch for a free, no obligation quote. Every job differs, so we prefer to look at the actual roof before putting a figure to anything.

Roof Carpentry Terms Explained: Rafters, Purlins and Ridge Boards