Roof Ventilation Calculator — Building Regs Compliant

BS 5250:2021 (and Approved Document C) set the requirements. A cold pitched roof above 15° needs a continuous 10mm eaves gap (10,000 mm² per metre run); at 15° or below it needs a 25mm gap (25,000 mm²/m). Warm pitched roofs need a 25mm eaves gap plus a 5mm ridge gap. A 5mm high-level (ridge) gap is also required on cold pitched roofs where the pitch exceeds 35° or the span exceeds 10m. These prevent condensation in the roof space, which can cause timber rot. Use our Eaves Ventilation Calculator for detailed eaves vent quantities and our Roof Insulation Calculator for insulation depth.

For a cold pitched roof up to 35° pitch and up to 10m span, cross-ventilation through eaves vents alone is sufficient: no ridge ventilation is needed. BS 5250 requires an additional 5mm continuous high-level (ridge) gap when the pitch exceeds 35° OR the span exceeds 10m, and for all lean-to / mono-pitch roofs. Warm pitched roofs (insulation at rafter level) always require both low-level (eaves) and high-level (ridge) ventilation to drive airflow through the batten cavity.

This depends on the certified FREE area of each vent, not the diameter of the hole. A standard 70mm circular soffit vent (e.g. Manthorpe G700, Airflow AIR700) provides about 2,150 mm² of free area. A cold pitched roof above 15° needs 10,000 mm² per metre of eaves, so you need about 5 vents per metre fitted at 200mm centres. (Beware the π×35² ≈ 3,848 mm² figure: that is the gross hole area, not the free area, and using it under-specifies vents by roughly 45%.) Continuous soffit strip is often simpler than fitting many circular vents.

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.

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.

In England roof ventilation is governed by Approved Document C (resistance to moisture), which accepts a roof built to BS 5250:2021 as meeting requirement C2. For a cold pitched roof with a high-resistance (non-breathable) underlay, the requirement is a continuous low-level (eaves) ventilation gap equivalent to 10mm for pitches over 15° and 25mm for pitches of 15° or less, running the full length of both eaves. A continuous 5mm high-level (ridge) gap is added where the pitch exceeds 35°, the span exceeds 10m, or the roof is a lean-to / mono-pitch. Warm pitched roofs need 25mm at the eaves plus 5mm at the ridge with a 50mm clear air path above the insulation. Note that Approved Document F (Part F) covers ventilation of the occupied rooms, not the roof void: roof-space ventilation falls under Part C.

Free vent area is worked out per metre run of eaves (and ridge where required), not from the roof area. Multiply the required gap by 1,000 to get mm² per metre: a 10mm continuous eaves gap = 10,000 mm²/m, a 25mm gap = 25,000 mm²/m, and a 5mm ridge gap = 5,000 mm²/m (BS 5250:2021). Multiply that by the total eaves or ridge length to get the total free area required, then divide by the certified free area of each vent to get the number of vents. For example, an 8m-long, 30° cold roof has 16m of eaves needing 10,000 mm²/m = 160,000 mm² total. This calculator does the per-metre conversion and the vent count for you.

Free area (also called net free area) is the actual open area air can pass through, after deducting the mesh, louvres, baffles and grilles inside a vent — it is always smaller than the geometric size of the hole. BS 5250 ventilation figures are stated as free area, so you must use the manufacturer's certified free area, not the diameter of the opening. A 70mm circular soffit vent has a geometric hole area of about 3,848 mm² (π × 35²) but a certified free area of only around 2,150 mm² (e.g. Manthorpe G700, Airflow AIR700) — using the geometric figure under-specifies vents by roughly 45%. Always check the product datasheet for the certified mm² free area.

The older convention of providing eaves ventilation equal to 1/300 of the ceiling area (for pitches over 15°) is the area-ratio equivalent of the continuous-gap figures, and BS 5250:2021 expresses the requirement primarily as a continuous equivalent gap (10mm or 25mm at the eaves, 5mm at the ridge) rather than a fixed ratio. The continuous-gap method is simpler and directly drives vent quantity, so this calculator uses it. Whichever route you take, the duty to control condensation comes from Approved Document C2, and BS 5250 is the cited means of compliance — confirm the chosen strategy with your building control officer, especially when using a low-resistance (breathable) underlay, which can reduce the ventilation needed.