Roof void ventilation, access and fire compartmentation
A cold pitched roof is ventilated with an equivalent area of 10,000 mm²/m at the eaves and 5,000 mm²/m at the ridge, with a clear 25 mm air path maintained above the insulation. In the roof void itself, cavities must be subdivided at a maximum of 20 m and every compartment wall must be carried up and fire stopped tight to the underside of the roof covering.
The figures on one page
| Item | Limit | Figure |
|---|---|---|
| Equivalent ventilation areas for a cold roofCold roof construction with the insulation at ceiling or deck level. | Minimum | 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 |
| Minimum eaves ventilation for pitched roofsFor specific pitched roof constructions, a minimum continuous eaves ventilation opening of 7000 mm²/m is a common requirement | Minimum | 7,000 mm |
| Cavity subdivision in a roof voidMaximum distance between cavity barriers in a concealed roof space | Maximum | 20 m |
| Hybrid roof ventilation and AVCLHybrid roof constructions require a minimum 25mm clear airflow path below the underlay to ensure effective ventilation | Minimum | 25 mm |
| AVCL jointing requirementsJoints in air and vapour control membranes must be adequately lapped, typically with a minimum overlap of 100mm, to ensure continuity of the membrane | Minimum | 100 mm |
| High-level and ceiling air permeability for ventilationHigh-level ventilation, typically 5000mm²/m, is necessary for pitched roofs where the pitch exceeds 35° | exact | 5,000 mm |
| Hybrid roof ventilation with air permeable coveringsthe presence and continuity of the 25mm clear air gap, the specified eaves and ridge ventilation, and the correct installation of the AVCL. | Minimum | 25 mm |
| Intermittent extract rates by roomIntermittent mechanical extract in a wet room | Minimum | Kitchen 30 l/s next to the hob or 60 l/s elsewhere; utility room 30 l/s; bathroom 15 l/s; WC 6 l/s |
| Purge ventilation opening areaHabitable room ventilated by openable windows | Minimum | One twentieth (1/20) of the floor area of the room for a hinged or pivot window opening 30° or more |
| Sub-floor ventilation opening areaSuspended ground floor with a ventilated void | Minimum | 1500 mm² per metre run of external wall, or 500 mm² per square metre of floor area — whichever is the greater |
| Ventilated batten space beneath a roof underlayRoofs designed with a ventilated space beneath the underlay, including counter-battened details. | Minimum | 25 mm |
| Ventilation openings to a cold pitched or flat roof voidCold roof construction with insulation at ceiling level | Minimum | 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 |
What this means on site
The ventilation path is the thing that actually fails: insulation pushed into the eaves closes the gap even when the vents themselves are present, so eaves ventilators or rafter trays are needed to hold the 25 mm clear.
Roof void access hatches must be big enough to inspect the void and be insulated and draught-sealed to the same standard as the ceiling around them.
Fire compartmentation in a roof void is the highest-consequence item on this list — a party wall that stops at ceiling level lets fire and smoke run the length of a terrace.
What actually gets picked up
- Insulation blocking the eaves gap so the ventilation path is closed even though vents are fitted.
- Party or compartment wall not sealed tight to the underside of the sarking or covering.
- Loft hatch left uninsulated and unsealed, causing condensation staining around the opening.
- Ridge ventilation omitted on a roof that relies on high-level extract to work.
Questions we get asked
- How much ventilation does a cold roof need?
- An equivalent area of 10,000 mm²/m at the eaves and 5,000 mm²/m at the ridge, with 25 mm clear air space above the insulation.
- Do I need access to the roof void?
- Yes — an access hatch is needed for inspection and maintenance, insulated and draught-sealed in line with the surrounding ceiling.
- How is fire compartmentation handled in a roof void?
- Compartment and separating walls continue to the underside of the roof covering and are fire stopped, with cavities subdivided at a maximum of 20 m.
Where this is inspected
This is ProPlot's editorial summary of publicly available UK regulation and good practice. It is not building control approval or a warranty determination, and the designer's or manufacturer's tighter figure always takes precedence.