Quality.

    Ventilation — critical dimensions

    Extract rates, purge areas, background ventilators and the void ventilation that keeps floors and roofs dry.

    Minimum

    Background ventilator equivalent area

    8000 mm² equivalent area to habitable rooms; 4000 mm² to kitchens, utility rooms and bathrooms

    Dwelling with background ventilators, including window replacement work

    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

    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

    Intermittent extract rates by room

    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

    Intermittent mechanical extract in a wet room

    Minimum

    Purge ventilation opening area

    One twentieth (1/20) of the floor area of the room for a hinged or pivot window opening 30° or more

    Habitable room ventilated by openable windows

    Minimum

    Sub-floor ventilation opening area

    1500 mm² per metre run of external wall, or 500 mm² per square metre of floor area — whichever is the greater

    Suspended ground floor with a ventilated void

    Minimum

    Ventilated batten space beneath a roof underlay

    25 mm

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

    Minimum

    Ventilated void depth under a suspended timber floor

    75 mm to the underside of the wall plate and 150 mm to the underside of the joists

    Suspended timber ground floor over a ventilated void

    Minimum

    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

    All critical dimensions · All topics

    These entries are our own editorial summaries of publicly available regulation. They are not building control approval, a warranty determination, or a substitute for the full source document.

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