Roof Ventilation Requirements UK: BS 5250 and Building Regs
Quick Answer
Cold roof: 10mm eaves gap over 15° pitch, 25mm at 15° or under; ridge vent only needed if pitch exceeds 35° or span exceeds 10m
Warm pitched roof: 25mm eaves + 5mm ridge with a 50mm clear air path. Flat warm deck: none needed with a correctly fitted vapour control layer. Source: BS 5250:2021 and Approved Document C2.
Roof Ventilation Building Regulations (Approved Document C)
In England, roof ventilation is governed by Approved Document C (resistance to moisture). Approved Document C2 accepts a roof built to BS 5250:2021 as meeting the requirement to control condensation. Approved Document F covers ventilating the occupied rooms below: it is Part C, not Part F, that sets the roof void requirement.
Cold Roof Ventilation
A cold roof has insulation at ceiling/joist level, with the loft space above remaining unheated. BS 5250:2021 sets the low-level (eaves) gap by roof pitch, cross-ventilated on two opposite sides:
- Pitch over 15°: continuous 10mm gap or equivalent area (10,000mm² per metre run)
- Pitch 15° or under: continuous 25mm gap or equivalent area (25,000mm² per metre run)
- High-level (ridge): an additional continuous 5mm gap (5,000mm² per metre run), only required where the pitch exceeds 35° or the span exceeds 10m; most cold pitched roofs need eaves ventilation only
Warm Roof Ventilation
A warm roof has insulation at rafter level (e.g., loft conversion). Where a ventilated warm pitched roof is used, BS 5250:2021 sets a 25mm continuous eaves gap (25,000mm² per metre run) plus a 5mm continuous ridge gap (5,000mm² per metre run), with a 50mm clear air path above the insulation from eaves to ridge. A vapour-permeable (breathable) roofing underlay can reduce or remove the need for this air path; a low-resistance (non-breathable) underlay such as bituminous felt needs the full 50mm gap. A vapour control layer (VCL) must be fitted on the warm side of the insulation in either case.
Flat Roof Ventilation
- Cold deck flat roof: 25mm clear ventilation gap above insulation on two opposite sides (25,000mm² per metre run), plus a 50mm clear air path
- Warm deck flat roof: no ventilation required, but only where a correctly specified vapour control layer is fitted below the insulation
How Many Soffit Vents Do I Need?
Vent count depends on the certified free area of each vent, not the diameter of the hole. A standard 70mm circular soffit vent provides about 2,150mm² of free area (e.g. Manthorpe G700, Airflow AIR700), not the roughly 3,848mm² geometric area of the hole itself: using the geometric figure under-specifies vents by roughly 45%. A cold pitched roof over 15° needs 10,000mm² per metre of eaves, so that works out at about 5 vents per metre, typically fitted at 200mm centres. A continuous soffit strip certified to the required mm²/m is often simpler than fitting many circular vents. Use the eaves ventilation calculator for an exact count on your roof.
Ventilation Products
- Soffit vents: circular (70mm, ~2,150mm² free area) 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: July 2026. Figures cross-checked against BS 5250:2021 and Approved Document C, and against the free area and pitch-band figures used in our Roof Ventilation Calculator. Always consult a roofing specialist for complex roof designs.