Structural Opening Calculator — Needles, Props & Lintel

Needles (Strongboy or timber) are positioned at 900mm maximum centres across the opening. Each needle needs two Acrow props — one each side of the wall. This calculator uses needles = ceil(width ÷ spacing) + 1, so a 3m opening at 900mm centres = ceil(3 ÷ 0.9) + 1 = 5 needles × 2 props = 10 props. If there are two storeys above, you may need props at both levels. Always follow the structural engineer's temporary works design.

Under Approved Document A §2C24, the minimum bearing (end support) is 100mm each end for a clear span of 1200mm or less, and 150mm each end for spans over 1200mm — so a wider lintel length = opening width + 300mm (150mm each side). For heavier loads the structural engineer may specify 200–300mm bearing. The bearing must sit on adequate masonry or a padstone to spread the load. See our Lintel Sizing Calculator for detailed lintel selection.

Padstones are needed when the lintel or beam load exceeds the bearing capacity of the masonry below. Standard precast concrete padstones are C40 / 40 N/mm² (builders-merchant stock 215×215mm and 440×215mm) and spread the load over a larger area. This calculator assumes one padstone per bearing end (two for a single lintel or beam); multi-span, multi-storey or built-up beam arrangements may need more. They are almost always needed for steel beams and are recommended for any lintel over 2m span. Size and specification must come from the structural engineer. Use our Padstone Sizing Calculator for sizing guidance.

Forming a structural opening in a load-bearing wall requires Building Regulations approval and structural calculations by a chartered structural engineer. The temporary works (propping) must be designed before any masonry is removed. An incorrectly supported wall can collapse catastrophically. This is not a DIY job — use a qualified builder with structural engineering support.

Acrow props must remain in place until the lintel or beam is fully installed and the making good is complete and cured. For concrete lintels, allow at least 14 days. For steel beams, props can usually be removed once the beam is in position and the padstones are bearing. Typical hire period is 2-4 weeks; UK suppliers charge roughly £4-£5 per prop per week (+VAT), often with a 2-week minimum hire.