How Much Loft Ventilation Do I Need?
Quick Answer
10mm continuous gap at eaves (or equivalent area)
Cross-ventilation required for cold roofs. Warm roofs (insulation at rafter level) have different requirements
Ventilation Requirements (Approved Document C / BS 5250)
Cold Roof (insulation at ceiling level)
- Pitched roof (15°+): 10mm continuous gap at eaves on two opposite sides
- Pitched roof (mono-pitch): 10mm at eaves + 5mm at ridge
- Lean-to (below 15°): 25mm gap at eaves on two opposite sides
Warm Roof (insulation at rafter level)
- With ventilation gap: 50mm clear airspace above insulation, continuous from eaves to ridge
- Without ventilation gap: Possible with breathable roofing membrane (BS 5534) and vapour control layer below
Equivalent Free Area Calculation
If using discrete vents rather than a continuous gap, calculate the equivalent free area needed:
- 10mm × roof span (in mm) = free area per metre of eaves
- For a 6m span: 10 × 6,000 = 60,000mm² = 600cm² per metre
- Typical soffit vent: 200cm² each — need 3 per metre
Ventilation Products
- Circular soffit vents (70mm): ~20cm² free area each
- Rectangular soffit vents (225×75mm): ~100cm² each
- Continuous soffit strip: 10mm+ continuous gap, best solution
- Tile vents: ~50–100cm² each, used at ridge or mid-roof
- Ridge vent: 5mm continuous gap with dry ridge system
Why Ventilation Matters
Without adequate ventilation, warm moist air from the living spaces rises into the cold loft and condenses on cold surfaces. This causes damp, mould, and timber rot. Proper cross-ventilation allows moisture-laden air to escape and be replaced by drier outside air.
Tips
Ensure insulation does not block eaves ventilation. Use proprietary eaves ventilator trays to maintain the gap. Do not block ventilation when boarding the loft for storage. If adding loft insulation, check that existing ventilation is still adequate.
Related Questions
Last updated: April 2026