Understanding timber frame construction makes the difference between a solid build and an expensive mistake.
When I first started researching how to build a garden room, I assumed timber framing was fairly straightforward. Then I opened a dozen tabs, watched a load of videos, and somehow ended up with completely conflicting advice on timber sizes, stud spacing, and construction methods.
Some people were using 3×2, others 4×2, some spacing at 600mm, some "whatever felt right". The deeper I looked, the clearer it became that a lot of advice online simply doesn't reflect how timber frame buildings are actually built in the UK.
This guide pulls everything together in one place. I'll walk you through the exact UK timber frame standards used for modern garden rooms — sizes, spacing, wall build-ups, floors, roofs, and noggins — and show how to calculate quantities properly without guesswork.
Why timber frame is ideal for garden rooms
Timber frame isn't a "DIY workaround". It's the default construction method for most UK housing and has been for decades.
For garden rooms and outbuildings, it works especially well because it mirrors standard UK house-building methods, it's very DIY-friendly (straight cuts, repetitive work), it allows excellent insulation thickness between studs, it's predictable, strong, and well understood, and materials are easy to source everywhere in the UK.
Compared to log cabins, timber frame gives you far more control over insulation, wall thickness, and internal finishes. I've covered that trade-off in more detail in the Log Cabin vs Timber Frame article, but for year-round garden offices, timber frame usually wins on performance.
Understanding CLS timber (the backbone of UK framing)
If you're building a garden room in the UK, CLS timber is what you want.
CLS stands for Canadian Lumber Standard, and despite the name, it's the standard structural timber used across the UK for stud walls, floors, and roofs.
Why CLS timber is used
CLS is regularised and planed (straight, consistent sizes), easy to work with, structurally graded, and designed to work with sheet materials and insulation.
Unlike rough sawn timber, CLS is predictable. That predictability is what makes accurate calculations possible.
Actual size vs nominal size
This catches a lot of people out.
A "47mm" CLS timber actually started life as 50mm rough sawn, then gets planed down. The finished size is 47mm thick, not 50mm.
That matters when you're calculating stud spacing, insulation fit, and overall wall thickness.
The calculator on OutbuildingPlanner uses true finished sizes, not nominal guesses.
C16 vs C24 grading
For most garden rooms:
- C16 timber is perfectly adequate
- C24 is stronger, but usually unnecessary and more expensive
Single-storey garden rooms with sensible spans don't need C24 unless you're pushing limits.
Common CLS sizes used in garden rooms
These are the sizes you'll see again and again — and the exact ones used by the calculator:
47 × 100mm CLS
Used for wall studs, sole plates, top plates, and noggins
47 × 150mm CLS
Used for floor joists and roof joists (most common size)
47 × 125mm CLS
Sometimes used for shorter floor spans
47 × 200mm CLS
Rarely needed unless spans are very large
Wall frame construction & sizing (UK standard)
A proper timber frame wall isn't just a row of vertical studs. It's a system.
Standard wall build-up
The calculator assumes this exact UK-standard configuration:
- Vertical studs: 47 × 100mm CLS
- Bottom plate (sole plate): 47 × 100mm CLS (single)
- Top plate: 47 × 100mm CLS (double top plate)
- Noggins: 47 × 100mm CLS
- Corner backing studs: 8 total for proper internal lining support
This is not overkill — it's how proper timber frame walls are built.
Why a double top plate matters
The double top plate:
- Ties all walls together
- Distributes roof loads
- Adds lateral strength
Skipping it is one of the most common DIY mistakes, and it weakens the whole structure.
Why 100mm deep walls are standard
Using 47 × 100mm studs gives you space for 75–100mm insulation, enough strength for single-storey buildings, and compatibility with UK building materials.
Once you add sheathing and cladding, total wall thickness ends up around 170mm, which is ideal for garden rooms.
Wall height and stud length (important detail)
A standard internal wall height is 2400mm.
But your studs are shorter than that, because you have a sole plate (47mm) and double top plate (94mm). Total plate thickness = 141mm.
Example:
- 2400mm wall height
- − 141mm plates
- = 2259mm stud length
The calculator handles this automatically — no mental maths required.
Stud spacing standards (400mm vs 600mm)
Stud spacing isn't arbitrary. It affects strength, rigidity, and how easy it is to line the walls.
400mm centres (recommended)
This is the calculator default — and for good reason.
- Most common in UK construction
- Stronger and stiffer walls
- Better fixing points for shelves and units
- Works perfectly with 1200mm plasterboard (3 studs per board)
- Requires 1 row of noggins
If you're unsure, choose 400mm.
600mm centres (acceptable, but compromises)
- Uses ~25% fewer studs
- Still structurally sound for single-storey buildings
- Requires 2 rows of noggins for stiffness
- Fewer fixing points internally
- Common in cheaper kit buildings
It saves timber, but makes lining and fixing more awkward.
What "centres" actually means
Centres are measured from the centre of one stud to the centre of the next.
So 400mm centres ≠ 400mm gap
Actual gap = 400mm − 47mm = 353mm
This matters for insulation sizing and sheathing.
Floor joist sizing & spacing
Your floor frame carries everything above it, so this isn't the place to guess.
How floor joists are arranged
Joists usually run across the shorter span, spaced at 400mm centres, supported by perimeter rim joists, and braced with noggins.
Common floor joist sizes
The calculator automatically selects the right size, but here's the logic:
47 × 150mm @ 400mm centres
Most common, spans up to ~3–4m
47 × 125mm @ 400mm centres
Suitable for smaller rooms (2–3m spans)
47 × 200mm
Only needed for very large spans
Floor noggins
Noggins stop joists twisting and stiffen the floor.
Typical rules:
- Joists up to 3m: 1 row of noggins
- Joists 3–4.2m: 2 rows of noggins
Again, the calculator handles this automatically.
Roof joist sizing (flat roof garden rooms)
Flat roofs work very similarly to floors, but with extra considerations.
Typical roof framing
47 × 150mm roof joists spaced at 400mm centres, supported by perimeter rim joists, designed to carry OSB decking, insulation, EPDM or felt, and snow load.
This size also allows space for 100mm roof insulation, which is key for year-round use.
Roof fall
Flat roofs aren't actually flat. The fall is created using firring strips or tapered insulation. The structural joists remain level.
Noggins & bracing explained (don't skip these)
Noggins are short horizontal pieces of timber fixed between studs or joists.
They prevent twisting, add rigidity, provide fixing points for shelves and cabinets, and improve overall stability.
Wall noggins
- Fitted horizontally between studs
- Usually at mid-height (around 1200mm)
- 1 row for 400mm spacing
- 2 rows for 600mm spacing
Floor and roof noggins
Same size timber as the joists, positioned based on span, and essential for long-term rigidity.
Many DIYers forget noggins entirely. That's a mistake.
Framing doors and windows properly
Openings need extra timber to carry loads around them.
Standard opening components
Each opening typically needs:
- 2 king studs (full height)
- 2 jack studs (support the lintel)
- Lintel/header above the opening
- Cripple studs where needed
Typical sizes
- Standard door: 900 × 2100mm
- Typical window: 1200 × 1200mm
The lintel size depends on span, but even small openings add more timber than people expect.
The calculator accounts for number of openings, their sizes, and extra studs and lintels required.
How much timber do you actually need?
This is where most DIY builds go wrong.
Manual calculation sounds simple until you factor in exact stud counts per wall, plate overlaps at corners, joist counts based on spacing, noggin rows, door and window framing, corner backing studs, and rim joists and blocking.
Example: 3m × 4m garden office
A typical 3m × 4m garden room with 2400mm walls might need roughly:
- 40–50 wall studs (47 × 100)
- 12–15 floor joists (47 × 150)
- 10–12 roof joists (47 × 150)
- 30–40 noggins (47 × 100)
- 12 wall plates (47 × 100)
- Plus lintels, backing studs, rim joists...
Miss one category and your totals are wrong.
Why the calculator matters
Rather than guessing, the flat roof calculator on OutbuildingPlanner.co.uk uses exact UK timber standards, calculates every stud, joist, noggin, plate, and backing stud, adjusts for spacing choice and openings, and gives you a complete, accurate breakdown in seconds.
It's the difference between hoping and knowing.
Buying timber: practical advice
Where to buy
- Builders merchants: best quality and pricing for bulk
- DIY stores: convenient for small quantities
- Timber yards: excellent quality if available locally
What to specify
Use clear descriptions:
- "47 × 100 CLS C16 treated" (sole plates)
- "47 × 100 CLS C16 kiln dried" (studs)
- "47 × 150 CLS C16 kiln dried" (joists)
Standard UK lengths
Common lengths: 2.4m, 3.0m, 3.6m, 4.8m
Longer lengths cost more to deliver. The calculator helps you plan quantities sensibly.
Treatment
- Sole plates: treated
- Everything else: kiln dried is fine once protected
Common mistakes to avoid
- Random stud spacing
- Skipping the double top plate
- Forgetting noggins
- Undersizing joists
- Ignoring openings in calculations
- Buying timber without a cutting plan
- Assuming zero waste
Timber frame rewards planning.
Final thoughts
Timber frame construction is proven, reliable, and DIY-friendly.
Using 47mm CLS timber, 400mm centres, and proper wall build-ups makes everything easier — insulation, lining, and long-term durability.
Plan properly once — and build it right the first time.