How many needles and props do I need?

Needles (strong-boy or timber) are positioned at approximately 900mm centres across the opening. Each needle needs two Acrow props — one each side of the wall. For a 3m opening: 4 needles × 2 props = 8 props. If there are two storeys above, you may need props at both levels. Always follow the structural engineer's temporary works design.

What is the bearing for a lintel?

The minimum bearing (end support) for a lintel is 150mm each end — so the lintel length = opening width + 300mm (150mm each side). For wider openings or heavier loads, the structural engineer may specify 200-300mm bearing. The bearing must sit on adequate masonry or a padstone to spread the load.

Do I need a padstone?

Padstones are needed when the lintel or beam load exceeds the bearing capacity of the masonry below. Concrete padstones (typically C40) spread the load over a larger area. 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.

Can I form a structural opening myself?

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.

How long do I need Acrow props for?

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 at £5-10 per prop per week.

TradeCalculator

Structural Opening Calculator — Needles, Props & Lintel

Calculate needle positions, Acrow prop count, lintel/beam sizing and making good for structural openings.

Opening Width

Opening Height

Wall Type

Storeys Above

Number of floors above opening

Prop Hire Price

Hire Duration

How We Calculate This

This calculator determines the temporary support requirements and lintel sizing for forming a structural opening in a load-bearing wall.

Temporary support

Needles = ceil(Opening Width / Spacing) + 1
Props = Needles × 2 (one each side) × Storeys
Needles at 900mm maximum centres.

Lintel

Lintel Length = Opening Width + (Bearing × 2)
Minimum 150mm bearing each end. Padstones at both bearings.

Frequently Asked Questions

Related Calculators

Concrete Volume Calculator

Calculate concrete volume in m³ for foundations, slabs and paths.

Concrete Mix Calculator

Work out cement, sand and aggregate ratios for different concrete mixes.

Block Calculator

Calculate how many concrete blocks you need per m² of wall.

Brick Calculator

Work out how many bricks you need for walls, piers and features.

Mortar Calculator

Calculate sand and cement quantities for bricklaying and blockwork mortar.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.