Props carry the full weight of wet concrete, steel, formwork and workers until the slab can hold itself. Get the spacing wrong and the slab can sag or, in the worst case, collapse. Here's what decides it.
What the props are really carrying
A prop doesn't just hold up concrete. The load includes:
- Wet concrete, the biggest part, which grows with slab thickness
- Reinforcement steel inside the slab
- Formwork: plates, runners and spans
- Construction load: workers, vibrators, buckets and any concrete heaped during placing
A thicker slab means more load per square metre, so props must come closer together.
Four things that decide spacing
- Slab thickness. The single biggest factor.
- Runner or span strength. Runners and acro spans must span between props without bending too much. Stronger runners allow wider spacing.
- Prop capacity at working height. A prop carries less near full extension. Pick a size whose working height sits mid-range.
- Ground or floor below. Props on soft soil need sole plates or boards so they don't sink.
Typical ranges
For ordinary residential slabs, props are often placed around 0.9–1.2 m apart in both directions, with closer spacing under beams and thicker sections. Treat this as a starting point for planning, not a design. Your engineer should confirm spacing for your slab, especially for slabs thicker than usual, long spans or heavy beams.
Common mistakes to avoid
- Extending props to their limit instead of using a taller size
- Leaving out cross-bracing on tall props, so they buckle sideways
- Props standing on loose soil or broken bricks
- Bent or damaged props put back into use
- Removing props early to return them and save rent
- Heaping concrete in one spot during placing
Plan it, then count it
Once spacing is fixed, counting props is simple: see the method in how many shuttering plates do I need, or use the slab shuttering calculator.
This guide is general information. Formwork and prop design for your slab must be checked by a qualified structural engineer.