Ventilated batten space beneath a roof underlay

25 mm

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

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

Where the design relies on a ventilated batten space, keep a clear gap of at least 25 mm beneath the underlay so air can pass through the space above the insulation.

Basis in regulation

BS 5250
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How to check it on site

Check counter-battens are continuous and that the underlay drape between rafters has not closed the gap.

Common failure

Underlay pulled drum-tight or drooping onto the insulation, leaving no measurable gap to ventilate.

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

Minimum tile or slate width at a verge

150 mm

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

Minimum

Holding-down strap fixing and dimensions

30 mm

Roof holding-down straps should be 30mm x 2.5mm in cross-section and at least 1 metre long