Cavity tray installation and weep holes

450 mm

Weep holes at not more than 450 mm centres, with at least two over every opening

Every figure for this topic is on one page: Cavity trays and DPC: where are they needed and how high?

Cavity trays are essential elements for preventing moisture penetration, particularly over openings or where the cavity is bridged. They must be installed correctly to the specified profile, with upstand ends to contain water, and kept free of obstructions. Weep holes are critical for allowing water collected by the tray to drain externally, typically spaced at maximum 450mm centres with a minimum of two per tray. Preformed trays are often used for complex geometries like bay windows, and the detail should be chosen based on the site's exposure conditions.

Basis in regulation

Part C
Approved Document C
Read the source

How to check it on site

Check correct installation and profile of cavity trays, ensure ends are turned up, the tray is clean, and weep holes are present, clear, and correctly spaced.

Common failure

Blocked or incorrectly installed cavity trays and missing or blocked weep holes, leading to internal dampness.

Last updated 2 September 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.

Related dimensions at this stage

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

Cavity wall tie vertical spacing

450 mm

vertical spacing of cavity wall ties does not exceed 450mm.

Maximum

Cavity wall tie horizontal spacing (partial-fill insulation)

600 mm

horizontal tie spacing for partial-fill insulated cavities does not exceed 600mm.

Maximum

Cavity wall tie horizontal spacing (full-fill insulation)

900 mm

For cavity walls incorporating full-fill insulation, wall ties must be placed at maximum 900mm horizontal centres

Maximum

Lowest fixing position for straps

150 mm

For any structural strap, the lowest fixing into the masonry or supporting structure must be positioned no more than 150mm from the bottom of the strap

Minimum

Chimney wall thickness and construction

100 mm

The minimum wall thickness for block, brick, or stone chimneys, excluding flue lining, is typically 100mm