What size airbrick do I need?

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: each standard 215 x 65 mm airbrick provides approximately 7,500 mm² of free area (terracotta) or up to 5,000 mm² (louvred plastic), though this varies by design.

How many airbricks do I need for sub-floor ventilation?

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 number). For a typical terrace house with 50 m² floor area, that is 25,000 mm² total — approximately 3-4 standard airbricks per external wall. Airbricks should be positioned on opposite walls for cross-ventilation.

Can I use telescopic duct airbricks in cavity walls?

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).

Do airbricks need to be above ground level?

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.

UK Airbrick Sizes & Ventilation Rates

Standard airbrick sizes, types, ventilation rates and installation requirements for UK sub-floor and room ventilation.

Standard Airbrick Sizes

Size (mm)	Brick Courses	Approx. Free Area (mm²)	Equivalent Bricks
215 x 65	1 course	5,000–7,500	Replaces 1 brick
215 x 140	2 courses	10,000–15,000	Replaces 2 stacked bricks
215 x 215	3 courses	15,000–22,500	Replaces 3 stacked bricks

Airbrick Material Types

Material	Free Area	Pros	Cons
Terracotta (clay)	Higher (square holes)	Traditional look, durable, matches brickwork	Can be bridged by mortar droppings, no pest guard
Plastic (louvred)	Lower (louvre slots)	Pest-proof mesh, weather-resistant, colours available	Lower free area, less traditional appearance
Cast iron	Medium	Heritage appearance, very durable	Expensive, may corrode without maintenance

Ventilation Rates (Approved Document C)

Application	Minimum Ventilation	Notes
Suspended timber floor	1,500 mm²/m run of wall, or 500 mm²/m² floor area	Whichever is greater, on at least two opposite sides
Suspended concrete floor (void)	Same as timber floor	If void exceeds 150 mm depth
Room ventilation (combustion air)	Varies by appliance kW rating	See Approved Document J for combustion appliances

Related Calculators

See the UK Brick Dimensions for sizing airbrick openings, or the UK Block Sizes for blockwork integration.

How We Calculate This

Frequently Asked Questions

Last updated: April 2026

All calculations are estimates. Verify with your supplier.

UK Airbrick Sizes & Ventilation Rates

Standard Airbrick Sizes

Airbrick Material Types

Ventilation Rates (Approved Document C)

Related Calculators

How We Calculate This

Frequently Asked Questions

Related Calculators

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