What are the UK Building Regs for roof ventilation?

UK Building Regulations (Approved Document F and BS 5250) require: Cold pitched roofs need a continuous 10mm gap at eaves (equivalent to 10,000 mm² per metre run). Warm pitched roofs need a 25mm gap at eaves plus a 5mm gap at the ridge. These prevent condensation in the roof space which can cause timber rot.

Do I need ridge ventilation?

For cold pitched roofs with a pitch above 15°, cross-ventilation through soffit vents alone is usually sufficient. Warm pitched roofs (insulation at rafter level) require both low-level (eaves) and high-level (ridge) ventilation to create airflow through the batten cavity. Ridge ventilation is always recommended as best practice.

How many soffit vents do I need?

This depends on the free ventilation area of each vent. A standard 70mm circular soffit vent provides about 3,848 mm² of free area. For a cold pitched roof, you need 10,000 mm² per metre of eaves — that's roughly 3 circular vents per metre, or you can use continuous soffit strip ventilation instead.

What if I can't ventilate at the eaves?

If soffit ventilation isn't possible (e.g., no roof overhang), alternatives include over-fascia vents that sit between the fascia board and the first row of tiles, or tile vents positioned low on the roof slope. A building control officer can approve alternative ventilation strategies.

Do warm flat roofs need ventilation?

A properly designed warm flat roof should NOT need ventilation. The insulation sits above the deck with a vapour control layer underneath, so there's no cold space for condensation to form. Cold flat roofs DO need ventilation — a 25mm gap on two opposing sides.

TradeCalcs

Roof Ventilation Calculator — Building Regs Compliant

Calculate roof ventilation requirements to comply with UK Building Regulations and BS 5250. Free area and vent quantities.

Roof Type

Determines ventilation requirements

Preferred Vent Type

Roof Length

Length along the ridge/eaves

Roof Span

Total span (wall to wall)

Roof Pitch

Angle of roof slope

How We Calculate This

This calculator determines roof ventilation requirements based on UK Building Regulations (Approved Document F) and BS 5250:2021 — the code of practice for control of condensation in buildings.

Building Regs requirements

Cold pitched roof: 10mm continuous gap at eaves (10,000 mm²/m)
Warm pitched roof: 25mm gap at eaves + 5mm at ridge
Cold flat roof: 25mm gap on two opposite sides
Warm flat roof: No ventilation required (if correctly designed)

The formula

Required free area = Required gap per metre × Total eaves/ridge length
Number of vents = Required free area ÷ Free area per vent

“Free area” refers to the actual open area that air can pass through, which is always less than the vent's physical size due to mesh, grilles and louvres.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.