A garage conversion lives or dies on the quality of its stud walls. The masonry shell was built as an unheated, unventilated store for a car, so almost every surface inside the new room has to be re-built in timber studwork – an insulated lining against the external walls, a partition separating the new room from any retained garage space, and often internal divisions for a cloakroom or utility. Get the studwork wrong and the room will feel cold, sound hollow, fail Building Control, or grow damp behind the plasterboard within a couple of winters.
This is a guide to how we build timber stud walls during garage conversions across Sussex, written from the perspective of the carpenters actually on site. It covers stud sizes, sole plate detailing, insulation, fire protection, noggins, and the Building Regulations that govern each of those decisions. For the wider picture on how the structural carpentry fits together, see our guide to timberwork in a garage conversion.
Where are timber stud walls used in a garage conversion?
Most garage conversions need timber studwork in three places: the internal face of the existing external masonry walls, any partition between the new habitable room and the remaining garage space, and internal divisions that split the new room into smaller zones. Each serves a different job, so the spec changes for each.
The external masonry walls in a typical 1970s or 1980s garage are a single skin of brickwork or blockwork with no cavity, no insulation, and no internal finish. You cannot hit a modern U-value by painting them, so we build an insulated studwork lining on the inside face. Where the conversion keeps part of the garage as storage, the new partition between the two spaces has to handle fire protection to Part B, which drives the plasterboard spec. Internal divisions (a cloakroom wall, for example) are simpler but still need noggins, proper fixings, and the right board for the room use.
What size and grade of timber should stud walls use?
The standard spec for a domestic stud wall in a garage conversion is 89x38mm CLS C16 softwood at 400mm centres, with head and sole plates in the same section. On thicker insulated linings against external walls we sometimes step up to 100x50mm or build a separate inner frame clear of the masonry. Studs should be planed, regularised, and kiln-dried to below 20% moisture content.
CLS (Canadian Lumber Standard) is the usual choice because the sections are consistent and the edges are eased, which makes plasterboard fixing cleaner. C16 is adequate for non-load-bearing partitions; anything carrying roof or floor loads (rare in a garage conversion but possible where a flat-roof garage is being re-roofed) needs C24 and a structural calculation.
The 400mm stud centre is important. Standard 12.5mm plasterboard is rated for studs at either 400 or 600mm, but at 600 centres the board noticeably deflects under a knock, and fire-rated board performance tails off. For habitable rooms we always use 400.
How is the sole plate fixed and protected from damp?
The sole plate is the timber that sits on the concrete slab and runs along the whole base of the stud wall. It has to be pressure-treated softwood bedded on a damp proof course (DPC) strip, and mechanically fixed to the slab with concrete frame fixings or SDS-driven expansion bolts at roughly 600mm centres.
This is the single most important detail in the whole wall. The original garage slab was poured without a damp proof membrane (DPM) underneath, or with a much thinner one than Building Regs now require. Moisture rises through the slab, and any untreated timber sitting directly on it will rot within a few years. Sussex conversions are particularly vulnerable because the winter ground stays wet for months.
The correct build-up, bottom to top:
- Existing concrete slab (inspect for cracks, dampness, and level)
- New DPM across the whole floor, lapped up the masonry walls by 150mm minimum
- Floor insulation (typically 75-100mm PIR between battens or under a floating chipboard deck)
- New floor finish
- DPC strip (150mm wide polythene) under the sole plate position
- Treated sole plate, fixed down
If the new floor build-up pushes the sole plate above the surrounding floor, the DPC strip can be laid directly between the sole plate and the floor deck provided the DPM below is continuous. Where the garage slab is being retained with only a thin overlay, the DPC under the sole plate is non-negotiable.
What insulation goes between the studs?
For the insulated lining against external masonry walls, the target is a U-value of 0.18 W/m²K for new elements or 0.3 W/m²K for upgraded existing walls, set by Part L of the Building Regulations. Most Sussex garage conversions achieve this with one of two build-ups:
| Build-up | Stud depth | Insulation | Additional layer |
|---|---|---|---|
| Mineral wool | 89mm studs | 90mm Rockwool Flexi between studs | 25-37.5mm insulated plasterboard on service face |
| Rigid PIR | 89mm studs | 75-80mm PIR between studs, foil-taped | 12.5mm plasterboard |
PIR is more expensive but gets you a higher U-value in a thinner wall, which matters in a garage where floor-to-ceiling height is already tight. Mineral wool is cheaper, better for acoustics, and forgives small gaps around services.
A vapour control layer (VCL) on the warm side of the insulation is mandatory. This is either a 500-gauge polythene sheet lapped and taped at joints, or the foil face of PIR boards if the tape lines are continuous. Without a VCL, warm moist air from the room passes into the insulation, hits the cold masonry, and condenses. That is how you end up with black mould appearing on skirtings two years after the job.
There should also be a ventilated air gap of at least 15mm between the back of the studwork and the existing masonry wall. This lets any moisture that does get behind the frame evaporate rather than sit trapped against the timber. For more detail on the thermal side of the job, see our guide to Part L thermal performance in timber builds.
When is fire-rated plasterboard required?
Any stud wall separating the new habitable room from remaining garage space must achieve 30 minutes fire resistance from both sides, which is what Building Regulations Approved Document B (Part B) requires for an integral garage separating wall. On a standard timber stud, that means either one layer of 15mm Fireline plasterboard each side plus skim, or two layers of 12.5mm plasterboard with staggered joints.
The garage side is the critical face. That is where the fire is assumed to start (a car battery, a petrol can, a lithium tool battery), and the board spec has to hold the fire back long enough for anyone in the house to get out. All board joints must be taped and filled, with no unfilled screw heads, and service penetrations (sockets, switches) must be sealed with intumescent sleeves or pads.
The same 30-minute rule applies to any door leading from the habitable space into the retained garage. It must be an FD30S rated fire door with intumescent seals and a self-closing device. It is easy to spec the wall correctly and then hang a standard internal door – that alone will fail the Building Control inspection.
For the detail behind the fire regulations and how they apply to timber construction, see our guide to Part B fire safety regulations.
Why do stud walls need noggins?
Noggins are short horizontal timbers fixed between the studs. Their job is to stop the studs twisting as they dry out, to provide rigid fixing points for anything mounted on the wall, and to support plasterboard edges on taller walls. Every stud wall we build has noggins at three heights as a minimum:
- At 300mm above floor for skirting fixings and low-level sockets
- At 1200mm for mid-wall sockets, radiator brackets, and plasterboard mid-joints where sheets run horizontally
- Just below ceiling level for curtain pole fixings and any high-level services
If the wall is going to carry a kitchen unit, a wall-hung basin, a TV bracket, or radiator brackets, the noggin positions get set out to match those fixings before the board goes on. Trying to find a noggin behind plasterboard six months later with a magnet is a waste of an afternoon.
Noggins should fit snugly between the studs (cut at 346mm for 89×38 studs at 400 centres) and be fixed with at least two 100mm nails each end, skew-nailed or shot in with a first-fix nailer.
What are the most common mistakes?
Having built stud walls in garage conversions across Sussex for years, the same errors come up on projects where the work was done in a hurry or by someone outside the trade:
- Sole plate on bare concrete with no DPC. Seen it dozens of times. The wall is fine for a year or two, then the bottom 50mm of the studs turn black and the skirting starts to lift. It is a full strip-out to fix.
- Untreated sole plate. Saves a few pounds on materials, costs thousands when the wall fails.
- No vapour control layer. Leads to hidden condensation in the insulation and mould appearing on the warm side of the wall two winters later.
- Stud walls built hard against damp masonry. Without the 15mm ventilated air gap, any moisture in the existing brickwork transfers straight into the timber frame.
- Standard plasterboard on the garage separation wall. Fails Building Control immediately. The inspector will check the board markings before sign-off.
- 600mm stud centres in a habitable room. The walls feel flexy when you lean on them, and fire-rated board performance drops below the 30-minute requirement.
- Noggins missed at first fix. Usually discovered when the kitchen fitter tries to hang a wall unit and there is nothing solid behind the board.
- Missing FD30S door to retained garage. The wall spec is right but the door downgrades the whole separation.
Most of these come from treating a garage conversion like a domestic partition job. It is not. The existing shell is uninsulated, uncovered by DPM, and historically fire-separated from the house by a whole set of walls that are now being changed. The studwork has to do the work those walls used to do.
Why it matters
A stud wall is hidden the moment the plasterboard goes on, and nobody will ever see what went into it. That is why the temptation to cut corners is so strong, and it is also why getting it right matters. The sole plate DPC, the VCL, the fire-rated board, the noggins in the right places – every one of those details is something you do once and forget about for the next 30 years.
If you are planning a garage conversion in Sussex and want a clear picture of what the carpentry involves, get in touch. We will walk you through the spec, explain which parts of the Building Regulations apply, and tell you honestly where the budget needs to go.
Frequently Asked Questions
Do I need building regs for a stud wall in a garage conversion?
Yes. Any conversion of a garage into habitable space is a change of use under the Building Regulations and the whole project requires a Building Control application. That covers the stud walls, insulation, ventilation, heating, floor build-up, and fire separation. Permitted Development rules may allow the work to go ahead without planning permission, but Building Regs approval is always required.
How thick is a stud wall in a garage conversion?
A typical insulated lining against an external masonry wall finishes around 115-130mm thick including plasterboard and skim, based on 89mm studs plus a 15mm air gap and 12.5mm board. A fire-separating partition to a retained garage is around 125mm thick with 15mm Fireline each side. An internal non-load-bearing partition using 63x38mm studs comes in at around 90mm finished.
Can I build a stud wall directly on a garage concrete floor?
Not without a new damp proof membrane under the floor build-up and a DPC strip under the sole plate. Garage slabs are rarely built with adequate DPM, so moisture rises through the concrete into any timber in contact with it. The correct sequence is: inspect the slab, lay a new DPM lapped up the walls, build the floor, then fix the sole plate on a DPC strip with a pressure-treated timber.
What U-value does a garage conversion wall need?
Part L of the Building Regulations targets a U-value of 0.18 W/m²K for new walls and 0.3 W/m²K for upgraded existing walls in a garage conversion. In practice, an insulated studwork lining with 90mm of mineral wool or 75-80mm of PIR between 89mm studs, plus a service cavity or insulated plasterboard, will achieve the target. Your Building Control officer or SAP assessor will confirm the exact figure for your project.
Do stud walls in a garage conversion need to be fire-rated?
Only the walls separating habitable space from retained garage space need fire-rated plasterboard, and those must achieve 30 minutes fire resistance under Part B. That means 15mm Fireline board plus skim on each face, or two layers of 12.5mm board with staggered joints. Internal partitions between two habitable rooms use standard 12.5mm plasterboard. A fully converted garage with no retained storage space removes the fire-separation requirement for that wall, but the wall between the new room and the existing house may still need to meet Part B depending on the layout.
Woodies of Sussex – Carpentry and construction based in Partridge Green, serving Horsham, Mid Sussex, and across the county. Contact us to discuss your garage conversion.
