Acrow Prop Spacing Calculator — How Many Acrow Props Do I Need?

This calculator determines the number of Acrow props, their spacing, and the correct prop size based on the opening width, wall type, floor type, and number of floors being supported. It also calculates needles, strongboys, and sole plates needed.

Spacing guidelines (rule of thumb)

The figures below are a conservative trade rule of thumb, a starting point only. They are notderived from a code table. BS 5975 requires temporary works to be designed to the actual loads, so the real spacing must come from the structural engineer's temporary works design for your specific wall and floor loads.

• Load-bearing, timber floor: typically no more than 600mm centres
• Load-bearing, concrete floor: typically no more than 450mm centres
• Non-load-bearing: typically up to 900mm centres
• Multiple floors supported: reduce to 450mm or 300mm centres, and use needle beams, not Strongboys

Prop sizing and safe working load

The height ranges below are the BS 4074:1982 closed-to-extended ranges. The safe working load (SWL) of an adjustable steel prop is governed by how far it is extended, not by its size number, and it falls sharply as the prop is extended. The SWLs quoted are from CIRIA Technical Note 79 (1977) for the eccentric loading case (25mm eccentricity, 1.5° out of plumb). This is the case that applies when propping masonry or timber bearers where concentric loading cannot be guaranteed.

• Size 0: 1040-1829mm
• Size 1: 1753-3124mm
• Size 2: 1981-3352mm
• Size 3: 2591-3962mm
• Size 4: 3200-4877mm

SWL (eccentric, sizes 0–3): ≈ 17 kN up to 2.75 m extended, then ≈ 15 kN at 3.0 m, 13 kN at 3.25 m, 11 kN at 3.5 m and 10 kN at 3.75 m. Size 4: ≈ 17 kN at 3.25 m falling to ≈ 7 kN at 4.75 m. Where concentric loading can be guaranteed (e.g. fork heads / proper formwork) the SWL is higher. Always read the value from the prop maker's load table at the actual extension.

Important notes

This calculator provides general guidance only. Temporary works design must be carried out or verified by a competent person (structural engineer or temporary works coordinator). This must be done in accordance with BS 5975-1:2024 (management procedures for the control of temporary works) and BS 5975-2:2024 (falsework: design and implementation). Every structural alteration is unique and must be assessed individually.

Standards

Acrow prop use should comply with BS 5975-1:2024 and BS 5975-2:2024 (Temporary works) and BS EN 1065 (Adjustable telescopic steel props; props are also marked to the older BS 4074:1982). The structural alteration itself requires Building Regulations approval and a structural engineer's design to comply with Approved Document A (Structure).