What Roof Ventilation Do I Need?
Quick Answer
Cold roof: 10mm continuous at eaves + 5mm at ridge
Based on: BS 5250:2021 and Approved Document C. Warm roof with vapour barrier: no ventilation required.
Cold Roof Ventilation
A cold roof has insulation at ceiling/joist level, with the loft space above remaining unheated. It requires cross-ventilation:
- Low-level (eaves): Continuous 10mm gap or equivalent area — 10,000mm² per metre run on both sides
- High-level (ridge): Continuous 5mm gap or equivalent area — 5,000mm² per metre run
Warm Roof Ventilation
A warm roof has insulation at rafter level (e.g., loft conversion). Ventilation depends on the underlay type:
- Breathable membrane (HR underlay): No ventilation gap required above insulation
- Non-breathable underlay (e.g., bituminous felt): 50mm ventilation gap above insulation, from eaves to ridge
- Vapour control layer (VCL): Must be fitted on the warm side of the insulation
Flat Roof Ventilation
- Cold deck flat roof: 25mm clear ventilation gap above insulation with openings at two opposite sides
- Warm deck flat roof: No ventilation required (insulation above the deck)
Ventilation Products
- Soffit vents: Circular (70mm) or continuous strip type
- Tile vents: 1 per 2–3m² of roof area as a rule of thumb
- Ridge vents: Dry ridge systems with integral ventilation
- Over-fascia vents: Retro-fit continuous strip ventilation
Last updated: April 2026. Ventilation requirements are set out in BS 5250:2021 and referenced in Building Regulations. Always consult a roofing specialist for complex roof designs.