What ventilation does BS 5250 require at the eaves?

For cold pitched roofs, BS 5250 requires a continuous 10mm gap at the eaves (or equivalent free area of 10,000mm² per metre run). This allows air to enter at the eaves and exit at the ridge, preventing condensation.

How many soffit vents do I need?

If using circular soffit vents (70mm diameter), you need enough to provide the equivalent free area of a 10mm continuous gap. Each 70mm vent provides about 3,850mm² of free area. For 10m of eaves, you’d need approximately 26 vents.

Do I need ridge ventilation as well as eaves?

Yes. Eaves ventilation works with ridge ventilation to create airflow through the roof space. BS 5250 requires a 5mm continuous gap (or equivalent) at the ridge for pitched roofs over 35°, or 10mm for shallower pitches.

What is an over-fascia ventilator?

An over-fascia ventilator is a continuous strip that fits between the top of the fascia board and the underside of the first tile batten. It provides the required 10mm ventilation gap and is often easier to retrofit than soffit vents.

Does a warm roof need eaves ventilation?

Warm roof designs (insulation above the rafters) generally don’t need conventional eaves-to-ridge ventilation. However, a ventilated batten space above the insulation and breathable membrane is recommended.

TradeCalculator

Eaves Ventilation Calculator — BS 5250 Compliance

Calculate eaves ventilation to BS 5250: 10mm continuous gap or equivalent, soffit vent count, over-fascia ventilators and ridge vent pairing.

Total Eaves Length

Combined eaves run (both sides)

Roof Type

Determines ventilation requirement

Vent Type

Method of providing ventilation

Price Per Vent/Metre (optional)

How We Calculate This

This calculator determines the eaves ventilation provision needed to comply with BS 5250 (Management of moisture in buildings).

The formula

Required free area = Eaves length × Continuous gap (mm)

Circular vents = Required free area ÷ Individual vent area

BS 5250 requirements

Cold pitched roof: 10mm continuous at eaves + 5mm at ridge
Lean-to / mono-pitch: 10mm at both high and low ends
Flat/warm roof: Ventilated batten counter above membrane

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.