GN 4.3 Temporary propping
What this page covers: when a deck span needs props before concreting, who designs them, and when they can come out.Refs: SCI P300 · TR75 · BS EN 1991-1-6
The short version
Most decked floors are designed unpropped; that is the point of composite construction. Props enter the picture on long spans, at large openings, and on cantilevers over 450 mm. If your layout drawing shows a chain-dotted zone marked "TP", that bay needs props before anyone pours.
The rules that matter
- Props carry the wet-concrete dead load plus construction loads, checked at ULS. They are temporary works, designed by the temporary works engineer, not by RMD.
- Bearers sit continuous across the bay on timbers at maximum 1.0 m centres, positioned, levelled and braced before the deck is laid. The design assumes zero deflection at the propped line.
- Props stay in until the slab reaches 75% of its design strength. Striking early is the single most common propping failure we see quoted in disputes.
- Propping and safety nets fight for the same space below the deck. Resolve the fall-arrest method for propped bays at planning stage, not on the morning of the pour.
Where RMD fits
Our layout drawings flag every propped zone and we'll give you the prop line loads from the deck design on request. The temporary works design itself stays with your engineer.
Need this answered for a live job? Send us the drawing and a decking engineer replies within one working day.
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