Loft Ventilation Calculator — How Much Ventilation Does My Loft Need?

This calculator determines the required ventilation areas based on BS 5250:2021 and Approved Document C requirements for cold pitched roof spaces. The provision is keyed to roof pitch and span — not to whether a ridge vent is fitted. It then converts these areas into practical vent quantities.

Requirements (cold pitched roofs)

• Pitch more than 15°: 10mm continuous gap at the eaves (10,000 mm²/m per side). Add a 5mm high-level vent (5,000 mm²/m) only when the pitch exceeds 35° or the span exceeds 10m.
• Pitch 15° or less (low pitch), warm or mono-pitch roofs: 25mm continuous gap at the eaves (25,000 mm²/m per side) plus a 5mm high-level vent (always required).

The formula

Eaves vent area = Required gap (mm) × eaves length (m) × 1000 (mm/m) × 2 sides

High-level vent area = 5mm × ridge length (m) × 1000 (mm/m) (only when required by pitch or span)

Vent sizing (indicative — confirm against the datasheet)

• Continuous soffit / over-fascia strip: typically quoted as ~10,000 mm² free area per metre run, matching the 10mm cold-roof requirement.
• Tile / slate vents: typically ~10,000–12,000 mm² free area each.
• Circular soffit vent (70mm): meshed free area is product-specific and much smaller than the bore — a common Klober/Manthorpe 70mm vent provides only ~2,000 mm², so several are needed per metre.

The number of discrete vents is calculated by dividing the total required ventilation area by the free area per vent, rounded up. Always read the actual free area off the chosen product’s datasheet, as it varies widely after meshing.

Standards referenced

BS 5250:2021 (Management of moisture in buildings — Code of practice), Approved Document C (Site preparation and resistance to contaminants and moisture). Pitch/span trigger per the Klober/BMI and Marley BS 5250:2021 guidance.