Formwork Timber: When You Need It, How to Set It Up, and When to Skip It

Your builder says he needs 30 metres of shuttering timber for the slab. You have no idea what shuttering is, whether 30 metres sounds right, or how much it should cost. You sign off on it. On pour day, the concrete bursts through a corner where the boards weren't braced properly. Wet concrete flows across the sub-base, sets in the wrong shape, and the groundworker spends the next morning chipping it out with a breaker. Half a cubic metre wasted, plus a day of labour, and it was entirely preventable.

Formwork (the trade calls it "shuttering") is the temporary frame that holds wet concrete in shape until it sets. For domestic extensions, it's usually sawn timber boards held in place by wooden stakes hammered into the ground. Simple in principle. But the first question most homeowners should be asking isn't "what timber do I need?" It's "do I need formwork at all?"

Do you actually need formwork?

Most domestic extension foundations in the UK are trench-fill. The builder digs trenches, building control inspects them, and ready-mix concrete fills the trench directly, right up to within 150mm of ground level. The trench walls are the formwork. No timber needed.

You need formwork timber when:

• The slab extends above trench level. The ground-floor slab sits on top of the foundation walls. Its edges, where the concrete meets open air rather than earth, need boards to contain the pour.
• Strip foundations in firm ground. Strip foundations use a shallow concrete layer (150-225mm deep) at the bottom of a wider trench. The concrete is narrower than the trench, so boards define the edges.
• Stepped foundations on sloping sites. LABC confirms that formwork is required for the vertical risers between steps. These are the hardest sections to get right.
• Pad foundations. Isolated pads under steel columns need formwork to hold the concrete to the engineer's specified dimensions.

If your builder is pouring trench-fill in clay and the only formwork is around the slab edges above ground, you're looking at 20-25 linear metres for a typical 4m x 6m extension. Not 30 metres of boards buried in trenches.

Types and sizes

UK builders' merchants stock sawn treated timber in the sizes that matter for domestic formwork. The board width determines the slab depth it can form.

The 150 x 25mm board is the default for most extension slab edges. It's cheap, light enough to handle solo, and available in lengths from 1.8m to 4.8m at any builders' merchant. Buy treated (tanalised) timber. Untreated boards swell when they absorb moisture from the concrete, making them harder to remove.

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OSB vs sawn boards

The self-build community overwhelmingly prefers 18mm OSB or WBP plywood for formwork. Five of eight forum threads reviewed recommended sheet material over traditional boards. The logic is straightforward: a standard 2440 x 1220mm sheet costs £25-45 and gives you eight strips at 150mm wide (each 2.4m long), working out cheaper per metre than buying individual sawn boards for large jobs.

But for a typical extension slab edge, sawn boards are easier. They're the right depth already, they're rigid along their length, and you can adjust individual boards up or down on their stakes to get a level top edge. OSB is better when you need to shutter a large area (a full slab pour with no trench walls) or form complex shapes.

How to set it up

This is where most DIY formwork fails. The boards themselves are the easy part. Getting them level, straight, and strong enough to resist the pressure of wet concrete is the skill.

Mark out the perimeter

Set up a string line at the exact finished slab level using a laser level or water level. The top edge of your formwork boards defines the top of the concrete pour. If the boards aren't level, your slab won't be level. Use the 3-4-5 method to check corners are square: measure 3m along one board, 4m along the adjacent board, and the diagonal between those points should be exactly 5m. If it's not, adjust until it is.

Stake it

Drive 50 x 50mm treated timber stakes into the ground at 600mm to 1000mm centres along the outside of the boards. Each stake should go at least 300mm into the ground, with a third of its length above ground supporting the board. Cut a point on the bottom of each stake with a handsaw before driving it. In soft ground, you'll need deeper stakes or a sole plate (a horizontal board under the stake to spread the load).

Nail, don't screw

Nail the boards to the stakes from the outside. This sounds counterintuitive if you've ever been told screws are stronger, but there's a practical reason: when you strip the formwork after the pour, you need to pull the boards away from the concrete. Nails pull out cleanly. Screws grip and tear the board, sometimes pulling chunks of fresh concrete with them.

That said, many self-builders use screws during setup so they can adjust board heights, then pull the screws and replace with nails before the pour. "Screws are your friend so you adjust levels as you go" is common advice on the forums. Just switch to nails for the final fix.

Brace the corners

Corners are where blowouts happen. Concrete exerts hydrostatic pressure (the pressure of a liquid against the walls containing it) evenly in all directions. At a corner, two boards meet at right angles and neither one braces the other. Without diagonal bracing, the concrete pushes the corner open.

Fix a diagonal timber brace from each corner stake to a short peg driven into the ground 300-400mm out. Two braces per corner, one on each side. This turns the corner from a hinge into a rigid triangle.

Seal the gaps

Gaps between boards, or between boards and the ground, leak concrete. Wet concrete is heavy (2,400 kg per cubic metre) and surprisingly fluid at S3 or S4 consistence. Even a 5mm gap at the bottom of a board will weep liquid cement for the duration of the pour, leaving a stained mess on the outside and potentially a void on the inside.

Pack gaps with stiff clay, expanding foam, or even duct tape on the outside face. Run your hand along the bottom edge of every board before the pour. If you can feel daylight, seal it.

Apply release agent

Release agent (mould oil) creates a thin barrier between the timber and the concrete, preventing the cement paste from bonding to the wood grain. Without it, stripping the formwork tears the concrete surface, leaving a rough, pitted finish. For foundation edges that will be buried, this doesn't matter much. For exposed slab edges, it matters a lot.

Apply release agent by brush or spray at least one hour before the pour. A 5-litre container costs £10-15 and covers a domestic extension job comfortably. A 20-litre bulk container runs about £45. Don't use old engine oil or diesel as a release agent. Both work, but they stain the concrete permanently and create an environmental contamination risk in the soil.

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When to remove it

The Concrete Society says vertical formwork can be struck when the concrete reaches 5 N/mm2 (enough strength to resist the surface tearing when the board pulls away). In controlled conditions at 7-16C, that's technically 9-12 hours. In practice, on a UK building site with variable temperatures, leave formwork in place for a minimum of 24 hours. 48 hours is better and standard practice for domestic pours.

After striking, the Concrete Society recommends covering all exposed concrete surfaces with polythene or wet hessian for 5 days to prevent moisture loss. This matters because concrete cures by chemical reaction with water, not by drying. Losing moisture too quickly weakens the surface layer.

What if the boards won't come out?

Leave them. This comes up repeatedly on BuildHub and DIYnot, and the consensus is clear: if formwork boards are buried at the edges of a slab or trench and won't pull free without damaging the concrete, leave them in place. They'll rot over time but cause no structural harm and no ground settlement. "Leaving the ply in won't do any harm if you can't pull it out" is the community position, and it's correct. Timber buried in contact with concrete and soil decomposes gradually without affecting the foundation.

How much do you need

For a rectangular extension slab, measure the perimeter. A 4m x 6m extension has a perimeter of 20m. Add 10% for wastage and cutting, so 22m. If there are internal walls with separate slab pours, add their lengths too.

Boards come in standard lengths: 1.8m, 2.4m, 3.0m, 3.6m, and 4.8m. Buy the longest lengths practical for your site (fewer joints means fewer potential leak points and less bracing). For 22m of 150 x 25mm formwork, you need either eight 3m boards or five 4.8m boards with offcuts.

Stakes: one every 600-1000mm, plus extras at corners and board joints. For 22m of perimeter at 800mm centres, that's 28 stakes, plus 8 for corners (two per corner) and a few spares. Budget 40 stakes. Cut them from 50 x 50mm treated timber, 450-600mm long each.

Total material cost for a typical extension

For a 4m x 6m extension slab edge (22m of formwork):

• 22m of 150 x 25mm sawn treated boards: £53-79 at £2.40 – £3.60 per metre
• 40 stakes from 50 x 50mm timber: £15-25 (cut from 3m lengths)
• 2kg of 75mm round wire nails: £5-8
• 5L release agent: £10-15
• Total: roughly £85-130 for all the formwork materials

That's less than a single cubic metre of concrete. Formwork is one of the cheapest elements of a foundation pour, which makes cutting corners on it especially pointless. The £20 you save by skipping corner braces can cost you £200 in wasted concrete and labour.

Cost and where to buy

Forward Builders Supplies, Jewson, and Harlow Bros all stock sawn treated carcassing in these sizes. Prices vary by region and by whether you're buying tanalised (pressure-treated) or plain sawn. Tanalised costs slightly more but is worth it: the treatment prevents the timber swelling during the pour and makes boards easier to strip afterwards.

For larger jobs, builders' merchants offer better rates on bulk timber orders. For a single extension slab, the total timber cost is modest. That's a small order by merchant standards. Collect in person if you can, as delivery charges on small timber orders often approach the cost of the timber itself.

Hire vs buy: steel road forms

Steel road forms are channel-section steel formwork in 3m lengths, available in 100mm, 150mm, and 200mm depths. They're rigid, perfectly straight, and reusable indefinitely. For a one-day slab pour they're an alternative to timber.

Weekly hire for steel forms typically costs more than buying the timber equivalent outright, and you don't keep them at the end. Steel forms make sense for larger or more complex jobs where absolute straightness matters, or if you're doing multiple pours over several weeks. For a single domestic slab edge, timber is cheaper and simpler.

Alternatives

OSB or plywood sheets. Cut 18mm OSB or WBP plywood into strips at the depth you need. Cheaper per metre than sawn boards for large areas. Less rigid along their length, so needs closer stake spacing (500-600mm). Good for forming entire slab bases where you need shuttering on all sides.

K-Form plastic permanent formwork. BBA-certified recycled uPVC formwork that stays in place permanently (no stripping). Pre-drilled dowel holes at 300mm centres. Installs up to four times faster than timber. Overkill for a simple slab edge, but worth considering if your groundworker is pricing their time into the formwork setup.

Steel road forms (hire). As described above. Heavy and difficult to adjust for fine level changes, but perfectly straight and incredibly strong. No blowout risk.

Where you'll need this

• Foundations and Footings - containing the slab edge concrete pour and any strip foundation sections above trench level

Formwork timber appears during the groundwork phase of any extension, renovation, or new-build project where a concrete slab or strip foundation needs edge containment. It's a one-day material: set up the morning of the pour, stripped the next day, and often never thought about again.

Common mistakes

Using formwork when you don't need it. Trench-fill foundations pour directly into the trench. If your builder is buying timber to line the inside of a trench-fill excavation, ask why. The trench walls do the job. Formwork is only needed at slab edges above ground level, for strip foundations, or for stepped sections.

Spacing stakes too far apart. 1.5m or 2m spacing looks fine until 2.4 tonnes of wet concrete per cubic metre pushes against the boards. At 2m centres, the board flexes and the concrete bulges outward, creating an uneven slab edge. At 1m centres, this doesn't happen. At 600mm centres (the professional standard for anything above 100mm deep), it definitely doesn't happen.

Skipping corner bracing. Every experienced builder and forum contributor flags this. Corners are where the pressure acts on two unsupported board ends simultaneously. Without diagonal braces, the corner opens during the pour. You cannot fix a blowout mid-pour. The concrete is already flowing.

Not checking levels before the pour. The top edge of the formwork is the finished level of your slab. If one board is 10mm high and the next is 10mm low, your slab has a 20mm step in it. Check with a long spirit level or a laser level across every board. Adjust by tapping stakes deeper or packing underneath.

Forgetting the release agent. On buried slab edges this barely matters. On any exposed concrete face, skipping mould oil means the board grain imprints permanently into the surface and chunks of timber stay bonded to the concrete when you strip. Apply it an hour before the pour and let it soak in.