Minimum
England
Clear air gap above insulation in a cold flat or shallow roof
50 mm
Cold flat roofs and cold roofs pitched below 15 degrees.
Keep a clear, continuous air space of at least 50 mm between the top of the insulation and the underside of the deck in a cold flat or shallow-pitched roof, so ventilation air can actually move across the void.
Basis in regulation
BS 5250
Read the source
How to check it on site
Measure the gap at mid-span where the insulation is most likely to have bowed upward, not just at the edges.
Common failure
Insulation cut oversize and springing up against the deck, closing the ventilation path halfway along the roof.
Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.