Underfloor Ventilation Calculator
Calculate the required ventilation area, number of air bricks, and spacing for suspended timber floors — fully compliant with UK Building Regulations Approved Document C.
Calculate your underfloor ventilation requirements
Enter your floor dimensions and conditions below. We’ll calculate the minimum free ventilation area required, number of air bricks needed, and recommend a suitable layout — all to Approved Document C standards.
Floor & building details
All fields used to calculate ventilation requirements
Internal floor area of the suspended timber floor zone.
Total length of external walls the floor runs alongside. Usually 2×(length + width) minus any internal party walls.
Measured from ground level to the underside of floor joists. Minimum 150mm required by regulations.
Wetter ground conditions require additional ventilation above the minimum requirement.
Party walls shared with neighbours reduce the number of external walls available for air bricks.
Standard clay 215×65mm is most common for Victorian / Edwardian properties.
Ventilation Requirements
Based on Approved Document C, UK Building Regs
Enter your floor dimensions, site conditions, and air brick type, then click Calculate to see your ventilation requirements.
Underfloor ventilation explained
Adequate underfloor ventilation prevents moisture build-up, protects timber joists from rot, and maintains healthy indoor air quality — all governed by UK Building Regulations.
Why Ventilate?
Without adequate airflow, moisture vapour rising from the ground becomes trapped beneath a suspended floor, causing timber rot, fungal decay, and structural damage. Proper ventilation creates cross-flow to carry moisture away.
Building Regulations
Approved Document C (Site preparation and resistance to contaminants and moisture) requires a minimum free area of 1,500mm² per metre run of external wall, with cross-ventilation paths no longer than 2 metres without a break.
Air Brick Types
Standard clay air bricks (215×65mm) provide around 3,250mm² of free area. Double-course or terracotta bricks offer more free area per opening. Choose to match your existing brickwork bond and course height.
Sleeper Walls
Internal sleeper (or honeycomb) walls supporting floor joists must include openings to allow air to pass through the void. Without cross-ventilation, isolated pockets of damp air cannot escape even with external air bricks present.
Approved Document C requirements
The table below summarises the key ventilation requirements from Approved Document C for suspended timber ground floors.
| Requirement | Minimum standard | Notes |
|---|---|---|
| Free ventilation area per metre run of wall | 1,500mm² / metre | Applies to each external wall the floor abuts |
| Minimum void height (ground to joist underside) | 75mm (absolute min); 150mm preferred | 75mm with ground DPC; 150mm without |
| Maximum distance for air to travel | 2 metres without obstruction | Sleeper walls must have openings every 2m |
| Air brick position | Above external ground level DPC | Minimum 75mm above finished ground level |
| Ventilation with ground DPM installed | Minimum 500mm² / metre run | Reduced rate acceptable where DPM covers entire ground area |
| Ventilation for high moisture ground | Enhanced — designer’s specification | Consult BS 5250 and structural engineer |
Source: Approved Document C — Site Preparation and Resistance to Contaminants and Moisture (2004, amended 2013). Always verify requirements with your local building control office.
Regulation-compliant results in seconds
Our calculator applies the Approved Document C formula — 1,500mm² of free area per metre of exposed wall — and then accounts for your ground conditions, building type, and DPM status to recommend the exact number and distribution of air bricks.
We also flag any compliance issues with your void height and provide a wall layout diagram showing where to position your air bricks for optimal cross-ventilation.
- ✓Approved Document C minimum free area formula applied
- ✓Ground condition multipliers for wet / clay sites
- ✓Air brick count per wall with layout visualisation
- ✓Void height compliance check (150mm minimum)
- ✓Sleeper wall cross-ventilation openings calculated
- ✓DPM reduction factor correctly applied
Underfloor ventilation FAQs
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