What is a DPC and why do I need one?

A Damp Proof Course (DPC) is a waterproof barrier built into walls to prevent moisture rising from the ground into the masonry (rising damp). UK Building Regulations (Approved Document C) require a DPC at least 150mm above external ground level. It's typically a strip of polyethylene or pitch polymer laid in the mortar bed.

How wide should my DPC be?

For a standard single-leaf wall (102.5mm brick), use 112.5mm DPC. For cavity walls, you need DPC in both leaves plus a cavity tray. For wider walls or stepped situations, 225mm or 300mm DPC may be needed. The DPC should be at least as wide as the wall or leaf it's protecting.

How much DPC overlap do I need at joints?

DPC joints should overlap by at least 150mm (6 inches). This is critical — any gap in the DPC will allow moisture through. At corners, the DPC should be folded and continued around the corner, not cut and butted. Where DPC meets a DPM (damp proof membrane), ensure they are lapped and sealed.

What height should DPC be above ground level?

Building Regulations require the DPC to be at least 150mm above external finished ground level. This is to prevent rain splash-back from bridging the DPC. If ground levels are raised after construction (e.g., by new paving), this can bridge the DPC and cause damp problems.

When do I need a cavity tray DPC?

Cavity tray DPC is required above all openings (windows, doors) in cavity walls, at abutments where a lower roof meets a wall, at changes in cavity width, and wherever moisture could track across the cavity. The tray collects moisture and directs it to weep holes in the outer leaf.

TradeCalcs

DPC Calculator — Damp Proof Course Lengths & Rolls

Calculate damp proof course quantities for walls, cavity trays and stepped DPC. UK standard roll sizes.

Wall Length

Length of wall requiring DPC

Number of Walls

DPC Type

Application

Cavity Trays Needed

Above windows, doors and abutments

Include wastage allowance

Adds 10% for cutting and fitting

Wastage Percentage

10% is standard for DPC work

Price Per Roll (optional)

How We Calculate This

This calculator works out the total length of DPC needed for your project, including allowances for overlaps at joints (150mm per join) and wastage, then converts this to standard roll quantities.

The formula

Total DPC = (Wall length + Join overlaps) × Wastage factor
Rolls needed = Total DPC length ÷ Roll length
Cavity tray = (Opening width + 300mm) per opening

Standard UK DPC sizes

112.5mm wide: Standard single leaf — 30m rolls
225mm wide: Wide walls / double leaf — 30m rolls
300mm wide: Stepped DPC — 30m rolls
450mm wide: Below ground / wide walls — 30m rolls
Pitch polymer: 20m rolls (higher specification)
Lead DPC: 3m strips (premium, cavity trays)

Always overlap DPC joints by 150mm minimum. At corners, fold the DPC — never cut and butt. DPC must be bedded on a full mortar bed and pointed flush with the wall face.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.