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

Loft ventilation prevents condensation in the cold roof space. Warm, moist air rises from the living spaces below through cracks, loft hatches, and light fittings. If this moisture cannot escape, it condenses on cold surfaces (underside of tiles, rafters) causing timber decay, mould growth, and damage to stored items. BS 5250 and Approved Document C require adequate ventilation for all cold roof spaces.

BS 5250:2021 and Approved Document C key the requirement off pitch and span, not off whether a ridge vent is fitted. For a standard cold pitched roof with a pitch of more than 15 degrees: a 10mm continuous gap at the eaves on both sides (10,000 mm² per metre per side). A high-level (ridge or tile) vent equivalent to a 5mm continuous gap (5,000 mm² per metre) is required in addition only where the pitch exceeds 35 degrees or the span exceeds 10m. For low-pitch roofs of 15 degrees or less (and warm or mono-pitch roofs) the eaves provision rises to a 25mm continuous gap and the 5mm high-level vent is always required. The equivalent free area can be provided by discrete vents instead of continuous strips. There is no compliant strategy of 25mm eaves with no high-level vent.

Instead of a continuous 10mm gap along the eaves, you can use individual soffit vents (rectangular or circular) spaced along the soffit boards, over-fascia ventilators that sit on top of the fascia board, or ventilated soffit boards with pre-punched holes. The key requirement is that the total free area of ventilation matches or exceeds the continuous gap equivalent.

Yes. Where a high-level vent is required, tile or slate vents installed near the top of the roof slope (within the top third) are an accepted alternative to a continuous ridge vent. They supply the same 5mm (5,000 mm² per metre) high-level allowance. Free area per tile vent varies by product — typically around 10,000 to 12,000 mm² — so always confirm against the manufacturer datasheet. This calculator distributes the total high-level requirement evenly across both slopes, which is a practical spacing estimate rather than a coded BS 5250 rule.

Loft boarding can obstruct ventilation if boards are pushed right to the eaves edge, blocking the airflow path from the soffit vents into the roof space. Always leave a gap of at least 25mm between the edge of the loft boards and the eaves. Some loft boarding systems include ventilation spacers for this purpose. Blocking eaves ventilation is a common cause of condensation problems after loft boarding.