Ventilation openings to a cold pitched or flat roof void

25 mm continuous at the eaves; 5 mm at the ridge for pitches of 15° or more; 25 mm at both sides of a cold flat roof

Cold roof construction with insulation at ceiling level

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

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Continuous free-air openings needed to ventilate a cold roof void, expressed as an equivalent continuous gap.

Basis in regulation

Part F
Approved Document F
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How to check it on site

Look into the void from the loft hatch before insulation is topped up. Check eaves trays are in place and insulation has not been pushed tight into the eaves.

Common failure

Loft insulation stuffed over the wall plate, closing the eaves gap and causing condensation on the underlay.

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.