How Many Air Bricks Do I Need?

How We Calculated This

For a detached house measuring 8m x 6m on plan with a suspended timber ground floor, the sub-floor void must be ventilated to Approved Document C. First, work out the two quantities the rules are based on:

• External wall length (the perimeter): 2 × (8m + 6m) = 28m
• Ground-floor area over the void: 8m × 6m = 48m²

Approved Document C (paragraph 4.14(b)) requires ventilation openings of not less than either1,500mm² per metre run of external wall or500mm² per m² of floor area — whichever gives the greater opening area:

• Wall-length rule: 28 × 1,500 = 42,000mm²
• Floor-area rule: 48 × 500 = 24,000mm²
• Required free area = the greater = 42,000mm² (wall-length governs here)

Now convert that free area into a vent count, then check the spacing cap:

• Vents for free area: 42,000 ÷ 6,500 = 6.5, round up = 7 vents
• Vents for spacing: NHBC Standards cap timber-floor ventilators at 2m maximum centres, within 450mm of wall ends — one vent per 2m of wall plus one to close the run: (28 ÷ 2) + 1 = 15 vents
• Vents needed = the greater of 7 and 15 = 15 vents, giving centres of 28 ÷ 15 = 1.87m — within the 2m maximum

Note how the spacing rule, not the free-area rule, sets the final count with high-free-area telescopic vents. That’s normal: the 2m-centres cap exists to stop stagnant air pockets forming in the void, even where the free-area maths would allow fewer, wider-spaced openings.

What the Rules Say

Approved Document C (2004 edition incorporating 2010 and 2013 amendments), paragraph 4.14, sets out the technical solution for a suspended timber ground floor. As well as the free-area requirement above, it requires a ventilated air space of at least 75mm from the ground covering to the underside of any wall-plates and at least 150mm to the underside of the suspended timber floor (or insulation if provided); ventilation openings in two opposing external wallsso air has a free path between opposite sides and to all parts; any pipes carrying ventilating air to be at least 100mm diameter; and grilles that keep vermin out without unduly resisting airflow. BS 5250:2021 (Management of moisture in buildings, Annex F) makes the same recommendation: vents of not less than 1,500mm² per metre run of external wall or 500mm² per m² of floor area, whichever is greater. NHBC Standards clause 5.2.10 adds the practical layout rule used above: ventilators at not more than 2m centres and within 450mm of the end of any wall.

Air Brick Free Areas Compared

The count depends heavily on the free area of the vent you fit — always check the manufacturer datasheet:

• Clay square-hole airbrick, 215 x 65mm (e.g. Forterra List No. 350): 1,290mm² free area
• Telescopic underfloor vent (e.g. Timloc 1201): 8,000mm² free area, or 6,500mm² with its matching 1201AB airbrick fitted; suits 50–100mm cavities and adjusts over 3–5 brick courses

A single clay airbrick doesn’t even satisfy one metre of wall: 1,290 ÷ 1,500 = 0.86m of wall per airbrick. Re-running the worked example with clay airbricks alone needs 42,000 ÷ 1,290 = 32.6, rounded up to 33 airbricks— which is why telescopic vents with far larger free areas are the standard choice for new work. For the full range of standard UK airbrick sizes, types and free areas, see our reference table.

Different House Types

Only externalwalls count — party walls don’t. Using 6,500mm² telescopic vents and the same two-rule method:

• Mid-terrace (5m frontage x 8m deep): 10m external wall (front + rear), 40m² floor. Wall rule 15,000mm² vs floor rule 20,000mm² — the floor-area rule governs on deep, narrow plans. Free area needs 4 vents; spacing needs (10 ÷ 2) + 1 = 6 vents
• Semi-detached (7m x 7m): 21m external wall, 49m² floor. Wall rule 31,500mm² governs. Free area needs 5 vents; spacing needs 11 + 1 = 12 vents at 1.75m centres
• Detached (8m x 6m): 28m external wall, 48m² floor — 15 vents as calculated above

Tips for Getting It Right

Place vents on two opposing external walls as a minimum so air sweeps right across the void — on complex plans it’s good practice to vent the full perimeter. In cavity walls, fit a telescopic liner or sleeve with each airbrick so the air is carried across the cavity into the sub-floor void rather than escaping up the cavity. Keep every airbrick above finished external ground level and clear of soil, raised paving and render — blocked airbricks are a leading cause of sub-floor damp and timber decay, so never seal them up in winter. Where the floor level must sit close to the ground for level access, Approved Document C allows offset (periscope) ventilators instead. If the void is divided by sleeper or compartment walls, provide openings through them so air can reach all parts.

Airbricks and liners are counted units, so round up rather than down and allow 5–10% spare material — at least one extra vent and liner — to cover breakages; clay airbricks chip easily and telescopic sleeves can crack on site. Prices vary widely by region and specification, so treat any merchant figure as indicative and use the calculator’s optional price fields for a budget estimate.