Guidance Note GN67 Lap Vents in Pitched Roofs

It explains the causes of roof space moisture, outlines the requirements of BS 5534 and BS 5250 in relation to ventilation and underlay performance, and clarifies why lap vents do not constitute compliant roof ventilation. The guidance highlights the technical and durability risks associated with these devices and reinforces NFRC’s position that condensation issues should be addressed through recognised external ventilation methods, such as eaves, ridge, or purpose-made tile and slate ventilators, in line with British Standards. 

This guidance explains why roof space ventilation is essential in pitched roofs to prevent condensation, damp, mould, and long-term structural damage—especially in homes that have become more airtight due to insulation upgrades and draught-proofing.