Cavity drainage and DPC extension

150 mm

Cavities in external walls must extend at least 150mm below the damp-proof course (DPC) level to allow for proper drainage and collection of moisture

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Cavities in external walls must extend at least 150mm below the damp-proof course (DPC) level to allow for proper drainage and collection of moisture. The cavity must be kept clear of obstructions and equipped with weep holes or other suitable means to permit moisture to escape externally.

Basis in regulation

Part C
Approved Document C
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How to check it on site

Check that the cavity extends sufficiently below the DPC and that weep holes are correctly spaced, unobstructed, and clear of mortar.

Common failure

Blocked weep holes or an insufficient cavity depth below the DPC can lead to moisture accumulation within the cavity.

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

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