Cavity trays and DPC: where are they needed and how high?
A cavity tray is required wherever the cavity is bridged or closed — over openings, at abutments, above lintels and at changes of level. It needs a minimum 150 mm upstand into the inner leaf, sealed stop ends at each end, and weep holes at not more than 450 mm centres with at least two over every opening. The DPC itself must sit at least 150 mm above finished ground level.
The figures on one page
| Item | Limit | Figure |
|---|---|---|
| Cavity tray upstand heightCavity tray over an opening, abutment or cavity interruption | Minimum | 150 mm |
| Cavity tray installation and weep holesWeep holes at not more than 450 mm centres, with at least two over every opening | Maximum | 450 mm |
| Damp-proof course height above finished ground levelExternal masonry wall, adjoining finished external level | Minimum | 150 mm |
| DPC and Cavity Tray LocationsDamp-proof courses (DPCs) must be installed at the base of walls, at least 150mm above the external ground level, and linked to the damp-proof membrane (DPM) in solid floors to prevent rising damp | Minimum | 150 mm |
| Cavity drainage and DPC extensionCavities in external walls must extend at least 150mm below the damp-proof course (DPC) level to allow for proper drainage and collection of moisture | Minimum | 150 mm |
| Cavity Fill Below DPCWhere partial or full fill cavity insulation is used, it should terminate at least 150mm below the lowest damp-proof course (DPC) | Minimum | 150 mm |
| Damp-proof membrane lap and sealGround-supported slab, membrane above or below the concrete | Minimum | 150 mm |
| DPC and DPM Lapping in Timber Frame | Minimum | 100 mm |
| External drainage outlet and DPC clearanceExternal rainwater downpipes must discharge at least 150mm below the damp proof course (DPC) level, typically into a gulley or soakaway, to prevent moisture bridging across the DPC into the substructure | Minimum | 150 mm |
| Garage wall weather resistance and DPC | Minimum | 150 mm |
| Landscaping Levels and DPCExternal ground levels, including landscaping finishes such as turf, must be maintained at a minimum of 150mm below the damp proof course (DPC) level | Minimum | 150 mm |
| Minimum thickness of a polythene damp-proof membranePolythene sheet used as a damp-proof membrane below or above a ground-bearing slab. | Minimum | 0.3 mm (1200 gauge) |
| Sole plate DPC height in a timber frame wallMeasured from finished external ground or paving level to the underside of the sole plate DPC | Minimum | 150 mm |
| Weep hole spacing above a cavity trayOver lintels, cavity trays and at the base of the cavity | Maximum | 900 mm maximum centres, with at least two weep holes over every opening |
What this means on site
The tray and the DPC are one continuous water path: the tray drains to the outer leaf, the weep holes let it out, and the stop ends stop it running off the end of the tray into the cavity below.
Where a roof abuts a wall, the tray steps with the roof line and each tray laps the one below by at least 100 mm. Retrofitted trays into an existing wall follow the same geometry — the common shortcut of omitting stop ends is the reason most retrofit trays leak.
What actually gets picked up
- No stop ends, so water discharges into the cavity at the ends of the tray instead of through the weep holes.
- Weep holes filled with mortar, or fewer than two over an opening.
- Ground or landscaping raised after handover so the DPC ends up less than 150 mm above finished level.
- Tray upstand cut short so it finishes below the level of the bed joint it should be built into.
Questions we get asked
- How high should a cavity tray upstand be?
- At least 150 mm, built into a bed joint of the inner leaf so water on the tray cannot rise above it.
- How far apart should weep holes be?
- Not more than 450 mm centres, with a minimum of two weep holes above every opening.
- How high must a DPC be above ground level?
- A minimum of 150 mm above finished ground level, measured after landscaping and paving are complete.
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.