Equivalent ventilation areas for a cold roof
10,000 mm²/m at pitched eaves; 25,000 mm²/m at low level on flat or shallow roofs; 5,000 mm²/m at the ridge
Cold roof construction with the insulation at ceiling or deck level.
Every figure for this topic is on one page: Roof void ventilation, access and fire compartmentation
Read the full answerCold roof ventilation is specified as an equivalent free area per metre run: 10,000 mm² at the eaves of a pitched roof of 15 degrees or steeper, 25,000 mm² at low level where the roof is flat or pitched below 15 degrees, and 5,000 mm² at high level where ridge ventilation is used.
Basis in regulation
BS 5250
Read the source
How to check it on site
Compare the ventilator product data sheet against the required equivalent area — a vent's physical size is not its equivalent area.
Common failure
Vents chosen on physical opening size, delivering a fraction of the equivalent area the roof needs.
Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.