How Much DPC Do I Need?

How We Calculated This

For a 4m × 3m extension (single-skin outer leaf):

• Perimeter: (4 + 3) × 2 = 14m
• Add 10% for overlaps: 14 × 1.1 = 15.4m
• One 30m roll is sufficient for a single DPC course

Standard DPC Widths

• 100mm: Half-brick walls (102.5mm)
• 112mm: Single-leaf with mortar overhang
• 225mm: Full-brick (one-brick) walls
• 300mm: Cavity walls (both leaves in one strip)
• 450mm or 600mm: Wide cavity walls or step/tray DPC

Where DPC Is Required

• Base of walls: Minimum 150mm above finished ground level (Building Regs Part C)
• Below window sills: Cavity tray DPC to redirect moisture
• Above lintels: Cavity tray to prevent water tracking inward
• Where floors meet walls: To link with the floor DPM
• Parapet walls: At the base and capping

DPC Materials

• Polyethylene (polythene): Most common, BS 6515 compliant, cheap and effective
• Pitch polymer: Self-adhesive, used for cavity trays and complex details
• Lead-cored: For exposed locations and under cills

Installation Tips

Bed the DPC on a full mortar bed. Overlap joints by at least 150mm. Do not puncture or tear the DPC. Ensure it projects slightly beyond the face of the wall on both sides. At corners, fold rather than cut and overlap.