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Eaves Ventilation Calculator — BS 5250 Compliance
Calculate eaves ventilation to BS 5250: continuous gap (10mm or 25mm by pitch), soffit vent count, over-fascia ventilators and 5mm high-level (ridge) ventilation.
Combined eaves run (both sides for duo-pitch; the single low eave for a lean-to)
Cold roof: ≤15° needs a 25mm eaves gap, >15° needs 10mm; >35° also needs a 5mm ridge gap
Eaves-to-eaves span. A span over 10m also triggers 5mm high-level ventilation
Standard ridge vent roll/unit length
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How We Calculate This
This calculator determines the eaves ventilation provision needed to comply with BS 5250 (Management of moisture in buildings).
The formula
Eaves free area (mm²) = Eaves length (m) × Free area per metre
where a continuous slot of N mm provides N × 1,000 mm² per metre (a 10mm slot = 10,000 mm²/m).
Circular vents = Eaves free area ÷ Certified free area per vent
BS 5250:2021 requirements
- Cold roof eaves, pitch over 15°: 10mm continuous (10,000 mm²/m)
- Cold roof eaves, pitch 15° or less: 25mm continuous (25,000 mm²/m)
- Warm roof eaves: 25mm continuous above the insulation
- High-level (ridge): 5mm continuous (5,000 mm²/m), fixed regardless of pitch, required where pitch over 35°, span over 10m, or the roof is mono-pitch/lean-to (and above warm-roof insulation)
- Lean-to / mono-pitch: eaves gap by pitch (above) at the low edge + 5mm at the high edge
Frequently Asked Questions
For cold pitched roofs over 15° pitch, BS 5250:2021 requires a continuous 10mm gap at the eaves (10,000mm² per metre run). For low-pitch roofs (10–15°), the requirement increases to a continuous 25mm gap (25,000mm² per metre). Warm roofs typically need 25mm continuous eaves gap above the insulation. This allows air to enter at the eaves and exit at the ridge, preventing condensation.
If using circular soffit vents (70mm diameter), you need enough to provide the equivalent free area of a 10mm continuous gap. Manufacturer-certified free area for a 70mm vent is typically ~2,000mm² (the mesh and grille reduce it from the 3,848mm² geometric area). For 10m of eaves at 10mm gap (100,000mm²), you need about 50 vents.
Not always. BS 5250:2021 only requires high-level (ridge) ventilation — a continuous 5mm gap, equivalent to 5,000mm² per metre, regardless of pitch — where the roof pitch exceeds 35°, the span exceeds 10m, or the roof is mono-pitch/lean-to. A standard duo-pitch cold roof at 35° or less and a span of 10m or less is ventilated by the eaves alone. Warm roofs also need a 5mm ventilated gap above the insulation. The high-level gap is fixed at 5mm — it does not increase at shallower pitches. Use our Roof Ventilation Calculator for a full ventilation strategy including ridge and tile vents.
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 a continuous eaves ventilation gap (commonly rated at 10,000mm² per metre, equivalent to a 10mm slot, with low-pitch versions rated higher) and is often easier to retrofit than soffit vents. Check the ventilator's certified free area against the figure your pitch requires.
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 — equivalent to a 5mm continuous slot at high level — is recommended by BS 5250.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.