How do I build formwork for a concrete pour?
Setting boards to the finished level, staking and bracing so they cannot move, proving the box is square, and the compaction step that decides whether the edges come out solid or honeycombed.
- Difficulty
- intermediate
- Time
- 1 hr 30 min
- Read
- 5 min
- Safety
- caution
Short answer
Set straight boards on edge with their top edges at the finished concrete level, because that top edge is what you screed off. Stake outside the boards at close centres and brace back to the ground. Check the box by measuring both diagonals — equal diagonals is what proves it is square. Oil the inside faces so the boards release.
Do this first
Formwork gets built as an afterthought and it decides almost everything about how the finished slab looks. The top edge of the board becomes the finished level of the concrete, so a board with a dip in it produces a slab with a dip in it. And the moment the concrete goes in, the whole thing is under sustained outward pressure that will find any joint that is not properly held. The drawing shows the staking and bracing arrangement and the order the pour is filled, compacted and screeded.
Safety
What you'll need
- Straight timber boards, deep enough for the full slab thickness
- Timber stakes or pegs, and a hammer
- Screws or nails, driven from outside
- A long spirit level and a tape
- Mould oil, old engine oil or a release agent for the inside faces
- A straight timber for screeding, longer than the form is wide
Step by step
- Set out the finished level first.Establish the level you want the top of the concrete to be, allowing for a fall across an external slab so water runs off. Everything else is set to this. Mark it on a datum peg you will not move.
- Use boards that are actually straight.Sight along each board before you use it. A bow of 10mm in a three-metre board is a 10mm bow in the edge of your slab, and it is far more visible than you would expect.
- Set the boards with their top edge at the finished level.The top edge is the screeding rail. Check along the whole length with a level, not just at the ends, and pack under low spots rather than hoping the concrete will find the level itself.
- Stake outside the boards, close enough.Stakes go on the outside so they can be pulled without disturbing green concrete. Space them close enough that the board cannot bow between them — for a domestic slab, every half metre or so, and closer at corners and joins.
- Fix through the board from the outside.Screw or nail through the stake into the board from outside, so every fixing can be withdrawn without levering against the concrete. Never fix from inside the form.
- Brace back to the ground.An angled brace from the top of each stake to a peg further out. Wet concrete pushes sideways continuously for hours, and a straight stake alone will lean.
- Join boards with a cleat, not a butt.Where two boards meet in a run, put a stake at the joint and a short cleat across the outside face. An unsupported butt joint opens under load and the concrete finds it.
- Prove the box is square by the diagonals.Measure corner to corner both ways. Equal diagonals means square. A rectangle can have four exactly correct sides and still be a parallelogram, and there is no way to find that out from the sides alone.
- Oil the inside faces.Mould oil, a proprietary release agent or old engine oil brushed on the inside. Concrete bonds to bare timber and stripping it then breaks the arris off the slab.
- Place the concrete in layers, working back from the far corner.Do not tip the whole load in one heap and spread it. Fill in layers across the form, working back towards where the material is arriving so you are never standing in it.
- Compact each layer properly.Work a shovel, a length of timber or a bar up and down through the concrete, particularly hard against the inside of the formwork and into corners, and tap the outside of the boards. This is what drives out trapped air and works the fine material into the face.
- Screed off along the tops of the boards.Two people sawing a straight timber side to side while moving it forward along the boards, striking off the surplus ahead of it. Do it once properly rather than repeatedly, which brings water to the surface.
- Leave the formwork in place while it gains strength.The boards protect the arrises and help retain moisture. Strip when the concrete has hardened enough that the edges will not break away, working the fixings out rather than prising the boards off.
Tips
- Use boards deeper than the slab and set them so the top is at the finished level and the bottom is into the sub-base. It stops concrete escaping under the form.
- Curves are formed with thin ply or hardboard bent against closely spaced stakes, or with several saw cuts part-way through a board's back face.
- Mark the finished level on the inside face of the boards with a pencil line as well, so you can see it even when the top edge is buried in concrete.
- Have the screeding timber and floats to hand before the pour starts. There is no going back for anything once concrete is on the ground.
If it doesn't work
The finished edge is rough, stony and full of voids
Cause: Honeycombing — the concrete was not compacted against the formwork, so the mortar never worked in around the aggregate at the face — Fix: Rod and tamp hard against the boards next time. Existing honeycombing can be cut back to sound material and patched with a repair mortar and a bonding agent, but it will always show.
A board has bowed outwards and the slab edge is bellied
Cause: Stakes too far apart or no bracing — Fix: Nothing to do once it is set except cut the excess off with a grinder. Prevent it with closer stakes and angled braces.
The slab is out of square
Cause: The form was set from the sides only — Fix: Always check both diagonals before pouring. Once cast, squaring a slab means cutting it.
Concrete has bonded to the boards and the edge broke away
Cause: The inside faces were not oiled, or the boards were stripped too early — Fix: Patch the broken arris with a repair mortar and a bonding agent, and oil the boards next time.