Hybrid roof ventilation with air permeable coverings

25 mm

the presence and continuity of the 25mm clear air gap, the specified eaves and ridge ventilation, and the correct installation of the AVCL.

Every figure for this topic is on one page: Roof void ventilation, access and fire compartmentation

Read the full answer

Hybrid roof designs, such as room-in-roof spaces, which incorporate an air permeable outer covering and HR type underlay, require specific ventilation measures to ensure moisture control. A continuous clear airway of at least 25mm should be maintained, with corresponding eaves ventilation of 25,000mm²/m and ridge ventilation of 5,000mm²/m. Additionally, an Air and Vapour Control Layer (AVCL) is essential on the warm side of the insulation.

Basis in regulation

BS 5250
Read the source
Part C
Approved Document C
Read the source

How to check it on site

Verify the presence and continuity of the 25mm clear air gap, the specified eaves and ridge ventilation, and the correct installation of the AVCL.

Common failure

Obstruction of the ventilation pathway by insulation or sarking boards, or gaps in the AVCL leading to interstitial condensation.

Last updated 28 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.

Related dimensions at this stage

Minimum

Minimum eaves ventilation for pitched roofs

7,000 mm

For specific pitched roof constructions, a minimum continuous eaves ventilation opening of 7000 mm²/m is a common requirement

Minimum

Minimum strap dimensions for holding down

30 mm

A strap used for holding down purposes, particularly in roof structures, must have a minimum cross-section of 30mm x 2.5mm and be at least 1000mm long to effectively resist uplift forces.

Minimum

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.

Minimum

Clear air gap above insulation in a cold flat or shallow roof

50 mm

Cold flat roofs and cold roofs pitched below 15 degrees.

Minimum

Ventilated batten space beneath a roof underlay

25 mm

Roofs designed with a ventilated space beneath the underlay, including counter-battened details.

Minimum

Minimum tile or slate width at a verge

150 mm

All verges on plain tiled, interlocking tiled and slated roofs.