In short
A timber frame extension wall is a sandwich. From the inside it runs plasterboard, vapour control layer, insulated 140 mm studs, sheathing board, breather membrane, a ventilated cavity and the cladding. Overall thickness is 250 to 350 mm depending on the outer finish.
On this page
You do not need to be able to draw these details. It helps to recognise them, because every one is something a building inspector will look for and something a rushed site can get wrong.
The wall, layer by layer
Working from the room outwards:
- Plasterboard, 12.5 mm or 15 mm. It is the finish and also the fire protection for the frame.
- Service void (optional). 25 mm battens that make a gap for cables and pipes.
- Vapour control layer. A sealed membrane, or foil-backed insulation with taped joints, that stops indoor moisture getting into the wall.
- Studs with insulation between. Usually 38 x 140 mm softwood at 600 mm centres. Mineral wool or rigid board fills the gaps.
- Sheathing. 9 mm OSB or similar, nailed to the outside of the studs. This is what stops the wall racking sideways.
- Breather membrane. Lets vapour out, keeps rain off the sheathing.
- Ventilated cavity. 50 mm behind brick, or 25 mm or more formed by battens behind boards and render.
- Cladding. Brick, render on a carrier board, or timber.
The order matters. The inside is tight to vapour and the outside is open to it, which gives stray moisture a way out.
How thick is the wall?
| Outer finish | Typical overall thickness |
|---|---|
| Timber boards on battens | 250 to 265 mm |
| Render on carrier board | 255 to 280 mm |
| Brick slips on carrier board | 265 to 285 mm |
| Full brick skin with 50 mm cavity | 350 to 365 mm |
These assume 140 mm studs with a 50 mm insulated lining inside. Thicker studs or more lining add to the figure. A masonry cavity wall to the same standard is about 330 to 350 mm.
The numbers it has to hit
In England, Approved Document L sets the most heat each new element of an extension may lose.
| Element | Maximum U-value, W/m²K |
|---|---|
| Walls | 0.18 |
| Floor | 0.18 |
| Roof | 0.15 |
| Windows and rooflights | 1.4 |
Insulation between 140 mm studs alone gives roughly 0.25 to 0.30, because every stud is a path for heat. A continuous layer across the inside face, 40 to 60 mm of rigid board, brings the wall to 0.18 and covers those paths. Wales and Scotland publish their own tables with figures at or slightly below these.
Sole plate and base
The bottom rail of each panel sits on a preservative-treated sole plate, which sits on a damp-proof course, which sits on a brick or block kerb. The plate is fixed down with stainless straps or anchors.
Finished ground outside should be 150 mm or more below the plate. The breather membrane laps down over the damp-proof course so water running down the sheathing is thrown clear. Our foundations post covers what is beneath.
Openings
Each window or door opening has a timber lintel over it, doubled studs (cripple studs) under each end, and a sill rail. Wide openings get a laminated timber or steel beam on posts.
Around the opening, the cavity is closed with a cavity barrier and the membrane is dressed into the reveal. A cavity tray over the head sends any water in the cavity back outside. Leaks at windows are nearly always a missing tray or a membrane cut short.
Cavity barriers
The cavity behind the cladding is a hidden chimney if fire gets into it. Barriers of mineral wool or an intumescent strip close it off around every opening, at the eaves, at corners in some designs, and at each floor level in a two-storey frame.
They cost little. They are easy to leave out and impossible to see afterwards, so they are one of the inspections worth attending yourself.
If a wall is within 1 m of the boundary it also has to resist fire from outside. That usually means a non-combustible outer face there, and no timber boarding.
Where new meets old
Timber and masonry move in different ways, so the joint has to give a little.
The end stud of the new wall is screwed to the house through a strip of damp-proof material. Outside, a vertical movement joint with a compressible filler and sealant separates new cladding from old brick. Where the new roof meets the house wall, a lead or preformed flashing is cut into a mortar joint, with a cavity tray built into the old wall above it if that wall has a cavity.
Skipping the tray is the classic extension defect. Rain soaks through the old outer leaf, runs down inside the cavity and appears on the new ceiling.
The roof
A flat roof is built as a warm roof: joists, an 18 mm deck, a vapour control layer, 120 to 150 mm of rigid insulation, then a single-ply, rubber or liquid membrane. It should fall at least 1 in 80 to an outlet or gutter. Nothing in the joist zone needs ventilating because the structure is on the warm side.
A pitched roof uses trussed rafters or cut timber, with insulation at ceiling level or between and under the rafters for a vaulted ceiling. The frame maker supplies and designs either type.
The floor
Usually concrete, exactly as in a masonry extension: slab or beam-and-block, insulation, screed. A suspended timber floor on joists is possible and needs a ventilated void of at least 150 mm beneath.
What to ask for
Ask the frame maker for a section drawing through the wall at the base, at a window and at the eaves. Those three show nearly everything on this page. If you would like a firm to draw and price a timber frame extension for your house, the form below is the place to start.