UK Airbrick Sizes & Ventilation Rates

The standard airbrick is 215 x 65 mm, which fits in place of a single brick. For higher ventilation rates, 215 x 140 mm (two-course) airbricks are available. The choice depends on the required ventilation area, and the free area varies enormously by type, so always take it from the manufacturer datasheet: a clay square-hole 215 x 65 mm airbrick gives about 1,290 mm² (Forterra List No. 350; Hepworth quote 1,300 mm²); a plastic louvred airbrick of the same face size gives about 6,450 to 7,750 mm² (Manthorpe G930 6,450 mm², Rytons Multifix 7,750 mm²); self-closing flood-defence airbricks give around 1,700 to 1,800 mm² nominal (Floodgate SMART Airbrick); and a telescopic underfloor vent gives 6,500 mm² with its airbrick fitted (Timloc 1201).

For suspended timber ground floors, Approved Document C requires ventilation openings equivalent to at least 1,500 mm² per metre run of external wall, or 500 mm² per m² of floor area, whichever gives the greater opening area. Worked example: a deep mid-terrace with a 5 m frontage and 10 m depth, giving 10 m of external wall (front plus rear) and a 50 m² floor needs the greater of 15,000 mm² and 25,000 mm², so 25,000 mm². With clay airbricks (about 1,290 mm² each) that is 20 airbricks; with high-flow plastic or telescopic vents (about 6,450 to 6,500 mm²) the free-area maths alone gives 4, but NHBC Standards clause 5.2.10 also caps spacing at 2 m centres within 450 mm of wall ends, which needs (10 ÷ 2) + 1 = 6 vents, so 6 is the final count at 1.67 m centres. Position airbricks on opposite walls for cross-ventilation.

Yes. In cavity walls, you must use a telescopic duct or sleeve airbrick that bridges the full cavity width so that air can pass from outside through the cavity to the sub-floor void. Simply placing an airbrick in the outer leaf alone does not ventilate the sub-floor — the duct must connect through to the inner leaf. Common duct lengths are 225 mm (for 50 mm cavity) up to 450 mm+ (for wider insulated cavities).

Airbricks should be positioned at least 75 mm above finished external ground level to prevent water ingress and blockage by soil or debris. In practice, many are set at DPC level or one course above. They must be kept clear of obstructions — paths, patios, planting beds and extension walls are common causes of blocked sub-floor ventilation.

For a semi-detached house on a 7 m x 7 m footprint (front, rear and one side wall are external; the other side is the shared party wall and does not count), that is 21 m of external wall and 49 m² of floor area. The wall-length rule gives 21 × 1,500 = 31,500 mm²; the floor-area rule gives 49 × 500 = 24,500 mm²; the wall-length rule governs since it is greater. With 6,500 mm² telescopic vents the free-area count is 31,500 ÷ 6,500 = 4.85, rounded up to 5, but the NHBC spacing rule needs (21 ÷ 2) + 1 = 11.5, rounded up to 12 vents to keep centres at or under 2 m, so 12 governs, giving 1.75 m centres. With 1,290 mm² clay airbricks the free-area count is 31,500 ÷ 1,290 = 24.4, rounded up to 25, which is higher than the 12-vent spacing minimum, so 25 clay airbricks are needed. Run this for your own wall length and floor area with the Sub-Floor Ventilation Calculator, or see How Many Air Bricks Do I Need? for mid-terrace and detached worked examples.

Air bricks and other sub-floor ventilators must be spaced at no more than 2 m centres and within 450 mm of the end of any wall, per NHBC Standards clause 5.2.10. As a rule of thumb, for an external wall of length L metres the minimum vent count to meet this spacing is (L ÷ 2) + 1, rounded up. This spacing rule often sets the final vent count where high free-area plastic or telescopic vents are used, because the free-area total alone (1,500 mm² per metre run of wall or 500 mm² per m² of floor area, whichever is greater) would otherwise allow fewer, more widely spaced openings. Always check both rules and use whichever needs more vents. The Sub-Floor Ventilation Calculator applies both checks automatically for your own wall length and floor area.

Approved Document C, paragraph 4.14, sets the technical solution for a suspended timber ground floor: a ventilated air space of at least 75 mm from the ground covering to the underside of any wall-plates and at least 150 mm to the underside of the suspended floor, with ventilation openings of not less than 1,500 mm² per metre run of external wall or 500 mm² per m² of floor area, whichever is greater, on at least two opposing external walls. BS 5250:2021 (Annex F) gives the same free-area recommendation. NHBC Standards clause 5.2.10 adds the practical layout rule: ventilators no more than 2 m centres and within 450 mm of the end of any wall. Any pipe carrying ventilating air must be at least 100 mm diameter, and grilles must keep vermin out without unduly resisting airflow. See How Many Air Bricks Do I Need? for how these rules combine into a vent count.