Quality.

    Cavity and weather resistance — critical dimensions

    Keeping water out of the cavity and out of the inner leaf: trays, laps, weeps and the clear width the cavity has to keep.

    Minimum

    Air temperature limits for laying masonry

    Stop at 3 °C and falling; do not restart below 1 °C rising

    Masonry and render work in cold weather.

    Minimum

    Breather membrane lap on a timber frame

    100 mm

    Horizontal laps shingled downwards; vertical laps over a stud, taped where the manufacturer requires it

    Minimum

    Cavity barriers around openings in a cavity wall

    Continuous around the full perimeter of the opening

    Includes windows, doors, and any duct or vent passing through the cavity

    Minimum

    Cavity tray upstand height

    150 mm

    Cavity tray over an opening, abutment or cavity interruption

    Minimum

    Clear cavity behind the breather membrane in timber frame

    50 mm

    Clear residual width, measured from the face of the breather membrane to the back of the outer leaf

    Minimum

    Minimum tile or slate width at a verge

    150 mm

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

    exact

    Nominal mortar joint thickness

    10 mm nominal

    Brick and block masonry laid to a gauge rod.

    Minimum

    Residual clear cavity width with partial fill insulation

    50 mm

    Masonry cavity wall with partial fill insulation board

    Minimum

    Roof underlay horizontal lap

    100 mm at pitches of 15° and above; 150 mm below 15°, and at least 150 mm where the underlay is unsupported

    Pitched roof underlay beneath tiles or slates

    exact

    Thickness of Code 4 milled lead sheet

    Code 4 = 1.80 mm

    Milled lead sheet to BS EN 12588 used for flashings and weatherings.

    Minimum

    Wall tie embedment into each masonry leaf

    50 mm

    Cavity walls built with ties to a suitable length for the cavity width.

    Maximum

    Wall tie spacing at reveals and movement joints

    Additional ties at 300 mm vertical centres within 225 mm of the opening

    Within 225 mm of an unbonded vertical edge — reveals, movement joints, wall ends

    Maximum

    Wall tie spacing in a standard cavity wall

    900 mm horizontally, 450 mm vertically (2.5 ties per square metre)

    Masonry cavity wall, cavity width up to 100 mm, wall height up to 15 m

    Maximum

    Weep hole spacing above a cavity tray

    900 mm maximum centres, with at least two weep holes over every opening

    Over lintels, cavity trays and at the base of the cavity

    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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