Hybrid roof ventilation and AVCL

25 mm

Hybrid roof constructions require a minimum 25mm clear airflow path below the underlay to ensure effective ventilation

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

Hybrid roof constructions require a minimum 25mm clear airflow path below the underlay to ensure effective ventilation. When a continuous air and vapour control layer (AVCL) is not practical to install, increased high-level ventilation of 5,000mm²/m is necessary to compensate for the reduced vapour resistance.

Basis in regulation

BS 5250
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How to check it on site

Ensure a consistent 25mm clear air gap is maintained below the underlay and confirm the presence and integrity of the AVCL or compensatory increased ventilation.

Common failure

The 25mm air gap is often obstructed by insulation or battens, and AVCLs can be poorly taped or damaged, compromising moisture control.

Last updated 23 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.