Why does my loft need ventilation?

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.

What ventilation does Building Regulations require?

For pitched roofs over 15 degrees with cross-ventilation (eaves + ridge): 10mm continuous gap at the eaves on both sides, plus a 5mm continuous gap at the ridge. For eaves-only ventilation (no ridge vent): 25mm continuous gap at the eaves on both sides. These are minimum requirements from BS 5250 and Approved Document C. The equivalent free area can be provided by discrete vents instead of continuous strips.

What are the alternatives to continuous eaves ventilation?

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.

Can I use tile vents instead of a ridge vent?

Yes. Tile vents installed near the top of the roof slope (within the top third) are an accepted alternative to ridge ventilation. Each tile vent typically provides around 10,000 mm² of free area. The number required depends on the roof length and the total ventilation area needed. They should be evenly spaced along both sides of the roof.

Does loft boarding affect ventilation?

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.

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

Calculate eaves and ridge ventilation areas for your loft per BS 5250 and Approved Document C. Provides soffit vent and tile vent quantities as alternatives to continuous strips.

Roof Length

Length of roof along the ridge/eaves (plan dimension)

Roof Width

Width of roof eave to eave (plan dimension)

Roof Pitch

Angle of roof slope (must be >15° for these rules)

Ventilation Type

Cross-ventilation (eaves+ridge) is recommended

Price Per Vent (optional)

Enter price per vent for a cost estimate

How We Calculate This

This calculator determines the required ventilation areas based on BS 5250 and Approved Document C requirements for cold pitched roof spaces. It then converts these areas into practical vent quantities.

Requirements (pitched roofs >15°)

Eaves + Ridge ventilation: 10mm continuous gap at eaves (10,000 mm²/m per side) + 5mm at ridge (5,000 mm²/m)
Eaves only: 25mm continuous gap at eaves (25,000 mm²/m per side)

The formula

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

Ridge vent area = 5mm × ridge length (m) × 1000 (mm/m)

Vent sizing

Typical rectangular soffit vent: ~10,000 mm² free area (250 × 70mm)
Typical tile vent: ~10,000 mm² free area
Circular soffit vent (70mm): ~3,848 mm² free area

The number of discrete vents is calculated by dividing the total required ventilation area by the free area per vent, rounded up.

Standards referenced

BS 5250:2021 (Management of moisture in buildings — Code of practice), Approved Document C (Site preparation and resistance to contaminants and moisture).

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

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

How We Calculate This

Requirements (pitched roofs >15°)

The formula

Vent sizing

Standards referenced

Frequently Asked Questions

Related Calculators

Loft Boarding Calculator

Loft Ladder Sizing Calculator

Loft Conversion Head Height Calculator

Loft Load Capacity Calculator

Dormer Sizing Calculator