Roof Ventilation Calculator — Building Regs Compliant

This calculator determines UK roof ventilation requirements based on BS 5250:2021 (the code of practice for managing moisture in buildings), alongside Approved Document C (resistance to moisture). In England it is Part C, not Part F, that governs the roof void: Approved Document C2 accepts a roof built to BS 5250 as meeting the duty to control condensation, while Approved Document F deals with ventilating the occupied rooms below. The eaves gap depends on roof pitch, and a high-level (ridge) gap is added for steep or wide roofs.

BS 5250 requirements

• Cold pitched roof (>15°): 10mm continuous eaves gap (10,000 mm²/m)
• Cold pitched roof (≤15°): 25mm continuous eaves gap (25,000 mm²/m)
• Cold pitched roof (>35° or span >10m): add a 5mm ridge gap (5,000 mm²/m)
• Warm pitched roof: 25mm eaves + 5mm ridge, with a 50mm clear air path
• Cold flat roof: 25mm gap on two opposite sides
• Warm flat roof: no ventilation required ONLY if a vapour control layer is correctly fitted below the insulation
• Lean-to / mono-pitch: low-level eaves gap (by pitch band) + 5mm high-level gap at the abutment

Free vent area calculation

The required ventilation is set per metre run of eaves and ridge, not from the roof area. Each equivalent gap converts directly to a free area: a 10mm gap = 10,000 mm²/m, a 25mm gap = 25,000 mm²/m and a 5mm ridge gap = 5,000 mm²/m. The calculation is then:

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

Worked example: an 8m-long cold pitched roof at 30° has two eaves totalling 16m. At 10,000 mm²/m that is 160,000 mm² of free area. Divided by a 2,150 mm² soffit vent, about 75 vents are needed, roughly 5 per metre at 200mm centres, or a continuous strip certified to deliver 10,000 mm²/m.

What “free area” means

“Free area” (net free area) is the actual open area that air can pass through, always less than the vent's physical size due to mesh, grilles and louvres. BS 5250 figures are stated as free area, so you must use the certified value, not the diameter of the hole. A 70mm circular soffit vent has a ~2,150 mm² free area (e.g. Manthorpe G700 / Airflow AIR700), not the 3,848 mm² geometric area of the opening (π × 35²). Using the geometric figure under-specifies vents by roughly 45%. Always confirm the certified free area on the product datasheet, and note that a low-resistance (breathable) underlay can reduce the ventilation required under BS 5250.