Window Lintel Calculator — Size, Bearing & Cavity Tray

For steel lintels the minimum end bearing is 100mm for spans up to 1m, 150mm each side for spans up to 3m, and 200mm for spans over 3m (where an engineer is normally involved). 150mm each side is the usual figure for domestic window and door openings. Always check the lintel manufacturer’s technical data — Catnic, IG, and Keystone all publish minimum bearing requirements for each lintel type and load condition. Use our Lintel Sizing Calculator for detailed sizing guidance.

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.

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 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.

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.