Question 1

What size airbrick do I need?

Accepted Answer

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).

Question 2

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

Accepted Answer

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 mid-terrace with 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) it is 4, though NHBC Standards also cap spacing at 2 m centres within 450 mm of wall ends, which usually sets the final count. Position airbricks on opposite walls for cross-ventilation.

Question 3

Can I use telescopic duct airbricks in cavity walls?

Accepted Answer

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).

Question 4

Do airbricks need to be above ground level?

Accepted Answer

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.

Size (mm)	Brick Courses	Clay square-hole (mm²)	Plastic louvred (mm²)	Equivalent Bricks
215 x 65	1 course	1,290	6,450–7,750	Replaces 1 brick
215 x 140	2 courses	2,900	12,900–15,500 (two interlocked 9x3 units)	Replaces 2 stacked bricks
215 x 215	3 courses	4,500	19,350–23,250 (three interlocked 9x3 units)	Replaces 3 stacked bricks

Material	Free Area	Pros	Cons
Terracotta (clay)	Low: about 1,290 mm² per 9x3	Traditional look, durable, matches brickwork	Very low free area, can be bridged by mortar droppings, no pest guard
Plastic (louvred)	High: about 6,450–7,750 mm² per 9x3	High free area, pest-proof mesh, weather-resistant, colours available	Less traditional appearance
Cast iron	Varies by pattern: check the datasheet	Heritage appearance, very durable	Expensive, may corrode without maintenance

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

UK Airbrick Sizes & Ventilation Rates

Standard Airbrick Sizes

Airbrick Material Types

Ventilation Rates (Approved Document C)

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