What width DPC do I need? DPC sizes explained
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DPC (damp proof course) is a flexible polythene strip laid in a mortar joint to stop rising damp wicking up a wall. The width you need matches the wall thickness: 100mm for a single-skin (half-brick) wall, 150mm for a standard cavity-wall leaf, 300mm for solid or wide walls, and 450 to 600mm for wide walls and cavity trays. It comes in 30m rolls at 500 micron gauge. Match the width to the wall, keep it at least 150mm above outside ground, and use vertical DPC around openings.
What is a DPC, and what does it do?
A damp proof course is a continuous waterproof layer built into a wall to block ground moisture rising up through the brickwork. It sits in a mortar bed joint near the base of the wall, and around window and door openings, so damp cannot wick up from the ground into the walls above. Our Capital Valley DPC is a flexible polythene DPC in 30m rolls, 500 micron gauge, made from recycled polythene and made to meet British Building Regulations, with an embossed finish for a good grip in the mortar.
What width DPC do I need?
Simple rule: the DPC width matches the thickness of the wall it sits in, so it covers the full width of the masonry with a little to spare. Pick from the roll widths:
| DPC width | Wall it suits |
|---|---|
| 100mm | Single-skin (half-brick) wall, narrow blockwork |
| 150mm | A standard leaf of a cavity wall |
| 300mm | Solid one-brick walls, wide blockwork |
| 450mm | Wide walls and cavity trays spanning the cavity |
| 600mm | The widest walls and full cavity trays |
Too narrow and the DPC does not cover the full wall, leaving a damp bridge at the edge; a touch wider than the wall is fine, you trim or tuck the excess. Measure the wall and match it.
Horizontal or vertical DPC?
DPC does two jobs on a build:
- Horizontal DPC is the main run, laid in a bed joint near the base of the wall, at least 150mm above the external ground level, all the way round, to stop rising damp.
- Vertical DPC goes up the sides (reveals) of window and door openings and at cavity closers, to stop damp tracking across where the outer and inner leaves meet at an opening.
Both come off the same roll, you just cut the length you need.
What is a cavity tray, and which DPC?
A cavity tray is a DPC stepped up across the cavity, over openings, and where a roof or floor abuts a cavity wall. It catches any water in the cavity and throws it back out through weep holes instead of letting it run down onto the lintel or inside. Cavity trays span both leaves and the cavity, so they take a wider DPC, typically 450 or 600mm. Get the width right or the tray does not reach across.
What does 500 micron (gauge) mean?
The micron figure is the thickness of the polythene. 500 micron (also written 500mu, and roughly 2000 gauge) is the standard trade weight for DPC, tough enough to lay in a mortar joint and take the weight of the wall above without splitting. It is the everyday choice for domestic and general building work.
How to lay DPC
- Bed it in fresh mortar in a level bed joint, at least 150mm above finished external ground.
- Keep it continuous and lap joints at least 100mm, more where it is exposed, so damp cannot track through a gap.
- Link it to the DPM in the floor so the two damp barriers join up with no gap.
- Do not bridge it afterwards with render, mortar, a path or soil piled up the wall, a bridged DPC is a useless DPC.
Common mistakes to avoid
- DPC too narrow for the wall. It leaves a damp bridge at the uncovered edge. Match the width to the wall.
- Bridging the DPC. Render, mortar snots, a raised path or banked soil let damp jump the DPC. Keep the line clear.
- Not lapping joints. Butt joints leak. Lap at least 100mm.
- DPC too low. Keep it at least 150mm above external ground or splashing gets above it.
- Forgetting vertical DPC and trays. Openings and cavity abutments need their own DPC, not just the base run.
The short version
DPC stops rising damp, and the width matches the wall: 100mm single-skin, 150mm a cavity leaf, 300mm solid walls, 450 to 600mm wide walls and cavity trays. Lay it in a bed joint at least 150mm above ground, lap the joints, and never bridge it. Browse the full DPC range in every width, and the wider damp proofing range.
By Steve, Builders Hardware Direct. Steve runs the BHD trade counter and has spent years supplying DPC, membranes and damp-proofing to builders and bricklayers across South Wales. Not sure which width? Call the counter on 01269 267030 or order online for fast UK delivery.
