High-level and ceiling air permeability for ventilation
5,000 mm
High-level ventilation, typically 5000mm²/m, is necessary for pitched roofs where the pitch exceeds 35°
Every figure for this topic is on one page: Roof void ventilation, access and fire compartmentation
Read the full answerHigh-level ventilation, typically 5000mm²/m, is necessary for pitched roofs where the pitch exceeds 35°. The air permeability of the ceiling beneath a cold roof significantly influences the required ventilation, with a normal ceiling being less airtight than a well-sealed ceiling, which conforms to strict air permeability standards.
Basis in regulation
BS 5250
Read the sourceBS 9250
Read the source
How to check it on site
Check for high-level ventilation where applicable and confirm the ceiling construction aligns with the assumed air permeability for the chosen ventilation strategy.
Common failure
Omission of high-level ventilation on steeper roofs or assuming a well-sealed ceiling without proper construction, resulting in inadequate condensation control.
Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.