DPC Calculator — Damp Proof Course Lengths & Rolls

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.

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.

BS 8215:1991 requires a minimum 100mm lap in runs, with full laps at angles and corners. 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 (and bonded with jointing tape or heat-bonded per the manufacturer).

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.

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. See our Cavity Tray Calculator for tray lengths and weep hole quantities.