How to Set Up Formwork for a Concrete Slab

✔ Step-by-Step: How to Set Up Formwork for a Concrete Slab

1. 1

   Mark Out the Slab Perimeter

   Use pegs, string lines, and a tape measure to mark the exact footprint of your slab. If your slab needs to be square, use the 3-4-5 method or measure diagonals to confirm. The formwork will sit just outside these lines. You’re building a mould for concrete, so dimensions must be accurate. This layout will guide where your boards and stakes go, so take time to get it spot on.

2. 2

   Choose the Right Board Size and Material

   For most slabs, timber boards around 6"x1" or 8"x2" (150–200mm high) are ideal. Use treated timber if the formwork will be in place for more than a few days. Make sure boards are straight and free of warping — bowed boards create wavy edges. For deeper slabs or curved pours, ply sheeting can be used with additional bracing. The board height will set your finished slab height, so match it to the spec thickness.

3. 3

   Drive Stakes or Pins at Regular Intervals

   Place timber stakes or metal pins just outside your layout lines, spaced every 600–1000mm along the slab edge. These will support the boards during the pour. Make sure each stake goes deep enough to hold firm against the force of wet concrete — at least 300mm into the ground for soft soils, deeper if needed. For corners and joins, place extra stakes or fix through multiple boards to maintain stability.

4. 4

   Fix Boards to the Inside of the Stakes

   Position your timber form boards against the inside face of the stakes so they hold back the concrete. Fix with screws or nails — screws make it easier to adjust if needed. Use a level, string line, or laser to set the top edge of each board to your finished slab height. Pack under low spots with timber shims or wedges. Check everything lines up cleanly and holds tight — concrete is heavy and pushes hard.

5. 5

   Double-Check Levels and Slab Thickness

   Before you pour, measure the internal slab depth from the top of the board down to the base — this confirms your slab will be the correct thickness across the whole area. Slab spec says 100mm? It should be 100mm deep in every corner, not just in the middle. Use a laser level or string and tape to measure in several spots. If any areas are shallow or deep, adjust the formwork before it’s too late.

6. 6

   Add Extra Bracing If Needed

   For larger pours or deeper slabs, formwork needs bracing. Add diagonal supports from the stakes to the ground or screw timber cleats across the top edge of boards to stop them bowing. Pay close attention to corners — they’re the most likely spot to blow out during a pour. If you’re using ply or thinner boards, bracing is critical. Don’t underestimate how much concrete weighs — plan for pressure and brace accordingly.

7. 7

   Seal Gaps (Optional, But Useful)

   If you want ultra-crisp slab edges, you can run silicone, filler foam, or clay in the small gaps between boards or at corners. This stops slurry from escaping during the pour and gives you sharper corners when the form is stripped. It’s not always essential, but on exposed edges (like patios or garage bases), it adds a more professional finish. Just don’t overfill — it can leave bulges if too thick.

8. 8

   Inspect and Prep for Pour

   Walk the full perimeter. Tap the boards — they should be solid, not loose. Check for high or low edges, split boards, or wobble in the stakes. Fix anything now — once the pour starts, you’re committed. Some builders spray the inside face of the boards with form release agent or diesel so they strip off easier after curing. If so, apply now. Your slab is now boxed out, level, and ready for mesh, chairs, and concrete.