What bearing does a window lintel need?

The minimum bearing for steel lintels in the UK is 150mm each side for openings up to about 1.8m. For wider openings, 200mm or more is typically required. Always check the lintel manufacturer's technical data — Catnic, IG, and Keystone all publish minimum bearing requirements for each lintel type and load condition.

What is a cavity tray?

A cavity tray is a waterproof membrane installed above the lintel in a cavity wall. It catches any moisture that penetrates the outer leaf and directs it outwards through weep holes. Building Regulations require cavity trays above all openings in cavity walls. The tray should extend at least 150mm beyond each end of the lintel.

Do I need a DPC above a window lintel?

Yes. A damp proof course (DPC) is fitted above the lintel to prevent moisture bridging into the inner leaf. For cavity walls, the cavity tray acts as the DPC. For solid walls, a separate DPC strip is needed above the lintel. DPC should lap 150mm past each end of the lintel.

Steel or concrete lintel — which is better?

Steel lintels (Catnic, IG) are the most common choice for UK domestic construction. They are lighter, faster to install, and come in combined cavity lintel profiles. Precast concrete lintels are heavier but may be preferred for exposed blockwork or where fire resistance is needed. Timber lintels are only suitable for internal non-loadbearing walls.

How do I choose the right lintel size?

Measure the structural opening width (brick-to-brick or block-to-block), then add twice the bearing (150mm each side minimum). Choose the next standard lintel length up. For example, a 1200mm opening needs 1200 + 300 = 1500mm minimum lintel length. Check the manufacturer's load tables to ensure the lintel can carry the load above.

TradeCalculator

Window Lintel Calculator — Size, Bearing & Cavity Tray

Calculate lintel size, bearing requirements, cavity tray and DPC lengths for window and door openings in UK walls.

Opening Width

Structural opening width (brick to brick)

Wall Type

Cavity wall is standard for UK new builds

Lintel Type

Steel is most common for domestic openings

Number of Openings

Windows and doors of this size

How We Calculate This

This calculator determines the lintel size, cavity tray length and DPC requirements for window and door openings in UK walls.

The formula

Lintel length = Opening width + (Bearing × 2)

Cavity tray length = Lintel length + 300mm (150mm extension each side)

Standard lintel lengths

Steel lintels come in standard lengths: 600, 750, 900, 1050, 1200, 1350, 1500, 1650, 1800, 2100, 2400, 2700, 3000 and 3600mm. The calculator rounds up to the nearest standard size.

Bearing requirements

Up to 1800mm opening: minimum 150mm bearing
1800–2400mm opening: minimum 200mm bearing
Over 2400mm opening: minimum 250mm bearing (check engineer's spec)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.