Short answer: for a normal slab, plan on roughly one prop per square metre — props set about 1 m to 1.2 m apart, both ways.
A 200 m² slab needs somewhere between 140 and 200 props. Here is how to get your own number.
The quick answer
Props go on a grid. Set them 1.0 m to 1.2 m apart in both directions and you get about 0.7 to 1.0 props per square metre.
To get your number, divide your slab area by the area one prop covers:
- 1.2 m grid — each prop covers 1.44 m²
- 1.0 m grid — each prop covers 1.00 m²
- 0.9 m grid — each prop covers 0.81 m²
Worked example: 200 m² slab, 150 mm thick
- Concrete weight: 24 kN/m³ × 0.15 m = 3.6 kN/m².
- Add formwork (about 0.5) and crew, barrows and equipment (about 1.5). Total ≈ 5.6 kN/m².
- On a 1.2 m grid each prop carries 1.44 m² — about 8 kN. Well within a normal prop.
- 200 m² ÷ 1.44 = 139 props. Order about 150 to cover edges and breakages.
- Go tighter to a 1.0 m grid and you need 200 props.
| Slab thickness | Typical grid | Props per 100 m² |
|---|---|---|
| 100–150 mm | 1.2 m × 1.2 m | about 70 |
| 150–200 mm | 1.0 m × 1.0 m | about 100 |
| 250 mm or more | 0.9 m × 0.9 m | about 125 |
What changes the number
- Thicker slab — more concrete overhead, so a tighter grid.
- Longer spans — less support underneath, so a tighter grid.
- Taller props — a prop carries less the further you wind it out.
- Soft ground — props need sole boards or they sink under load.
- Heavy work on the deck — stacked block and material all add up.
A taller prop is a weaker prop
A steel prop is a thin column, and thin columns buckle. The further it is extended, the less it can safely carry — and the drop is big, not small.
So a high ceiling needs either heavier props or more of them. Check the load table for the props you actually have, at the height you will actually use. Bent, dented or badly rusted props have lost strength you cannot see.
Do not forget these
An order with only props in it will stop work on pour morning.
- Bearers and joists — props carry these, not the slab directly.
- Sole boards — for soft ground or fresh backfill.
- Bracing — tall props need it to stay steady.
- Back-propping — the floor below often is not at full strength yet.
- Spares — for edges, openings and damage.
Common questions
- How far apart should steel props be?
- About 1.0 m to 1.2 m apart in both directions for a normal slab, which is roughly one prop per square metre. Tighter for thick slabs, long spans or props near full height. Your engineer sets the final spacing.
- How many props do I need for a 200 square metre slab?
- About 139 props on a 1.2 m grid, so order around 150 with spares. On a tighter 1.0 m grid you need about 200. Confirm the grid before you order.
- Does ceiling height change how many props I need?
- Yes. A prop carries less load the further it is extended, so a high floor-to-floor needs heavier props or a tighter grid. Read the load table at the height you will be working at.
- When can the props come out?
- Only when the concrete is strong enough to carry itself and whatever is on it. That is your engineer’s call, not a scheduling one, and back-propping is often needed afterwards.