First-fix carpentry is the structural timber stage of a build — the framework that goes in before any plaster, wiring or finishes. It covers the load-bearing and dividing elements hidden behind the walls: floor joists, stud partition walls, roof timbers and door linings. Once first-fix is complete and signed off, the building is "watertight and ready to close up," meaning the bones are in place for plasterboard, plastering and the later finishing work.
What first-fix carpentry actually means
The terms "first fix" and "second fix" split a build into two broad phases. First fix is everything that happens before the walls and ceilings are plastered or boarded over. Second fix is the visible, finished joinery that follows — skirting boards, architraves, hanging doors, fitting stairs and trim. The simple rule of thumb is that first-fix work disappears from view, while second-fix work is what you see and touch.
First-fix carpentry sits within a sequence shared with other trades. Carpenters often work alongside electricians and plumbers, who run cables and pipes through the same timber framework before it is closed in. That is why timing matters: holes for services are sometimes drilled through joists or studs at this stage, and getting the structure right first makes the later trades far simpler.
Because so much of this work ends up concealed, accuracy and squareness count more than appearance. A stud wall that is out of plumb (not truly vertical) or a row of joists set at uneven heights will cause problems for plasterers and floor layers down the line. The point of first fix is to leave a true, level, well-supported shell.
The structural timber that goes in first
First-fix carpentry is the structural timber stage of a build — the framework that goes in before any plaster, wiring or finishes.
Several distinct elements make up first-fix carpentry, each doing a specific structural or supporting job:
- Floor joists — the horizontal timbers that span between walls or beams to carry the floor above and the ceiling below.
- Stud partition walls — non-load-bearing or load-bearing timber-framed walls that divide internal space, ready to be boarded.
- Roof timbers — the rafters, joists and supports that form the roof structure, whether cut on site or delivered as prefabricated trusses.
- Door linings — the timber frames fixed into openings that the door itself will later hang from.
- Noggins and battens — short pieces fitted between studs and joists to stiffen the frame and give fixing points for boards and fittings.
The choice of timber is rarely arbitrary. Structural softwood used for joists and studs is usually strength-graded and stamped accordingly, so it meets a defined load capacity. Sizes and spans are set by the structural design and, where relevant, by Building Regulations rather than by preference. A reader looking at a job in progress will often see grade stamps and certification marks printed along the timber.
Door linings deserve a mention because they bridge first and second fix. The lining is fitted during first fix so the opening is correctly sized and plumb; the door, handle and architrave come later, during second fix. If the lining is set badly, the finished door will never sit or close cleanly — another reason this early work has to be precise.
Joists, studwork and roof carpentry in practice
Floor joists are spaced at set intervals, commonly around 400mm or 600mm apart, with the exact spacing and depth chosen to suit the span and the loads. Longer spans need deeper joists or intermediate support. Joists are usually levelled and held in position with timber straps, hangers built into the masonry, or by sitting on wall plates. Where pipes or cables must pass through, there are rules about where holes and notches can be cut so the timber is not weakened in the wrong place — typically away from the centre of the span and within defined zones.
Stud partition walls are built from a frame of vertical studs fixed between a horizontal head plate at the top and a sole plate at the bottom. Noggins are added between the studs to brace the wall and to provide solid backing where heavy items — radiators, wall units, sanitaryware — will later be fixed. If a partition carries load from above, it has to be designed and built to do so; a non-load-bearing partition simply divides space and supports its own boarding. Knowing which is which matters, because removing or altering a load-bearing wall later is a structural change.
Roof carpentry takes two broad forms. Traditional "cut" roofs are built piece by piece on site from rafters, ridge boards, purlins and ties, which suits irregular shapes and gives more usable loft space. Trussed roofs arrive as factory-made triangulated frames that are lifted into place and fixed together; these are quicker to erect and span wide distances without internal support, but the bracing within a truss should not be cut or altered. In both cases the roof structure is tied and braced to resist wind and spread, and to transfer loads safely down into the walls.
Across all of these elements, a few questions are worth asking of anyone carrying out the work. Is the structural timber correctly graded for the job? Are spans, spacings and openings set out to match the drawings? Have service routes been planned so holes and notches stay within safe limits? And has the work been checked for level, plumb and square before anything is boarded over? Once plasterboard goes up, putting right a hidden mistake means taking finished surfaces back down — so the value of getting first fix right lies precisely in the fact that, when it is done well, nobody ever sees it again.
In short, first-fix carpentry is the unseen framework that everything else relies on. The joists, studwork, roof timbers and door linings established at this stage define the shape, strength and layout of the building long before the first coat of plaster goes on.