Damp Proof Course Calculator
Calculate Damp Proof Course (DPC) requirements including material quantities, overlap lengths, and a basic height check against common IRC guidance.
What is a Damp Proof Course?
A Damp Proof Course (DPC) is a horizontal barrier in a wall designed to prevent moisture rising through the structure by capillary action (rising damp). DPCs are typically installed at least 150mm above ground level.
Building Regulation Requirements
- Minimum Height: DPC must be at least 150mm above finished ground level (IRC R406.1 (6 in above grade))
- Lap Length: Joints in DPC should overlap by at least 150mm
- Continuity: DPC must extend the full width of the wall, projecting slightly beyond the face
- Cavity Trays: Required above openings and at abutments to direct water out of the cavity
Common DPC Materials
- Polyethylene: Flexible, durable, most common for standard walls
- Bituminous Felt: Traditional material, good flexibility
- Lead: Historic buildings, very durable but expensive
- Copper: Premium option, extremely long-lasting
- Engineered Brick: Dense engineering bricks as a DPC
Standard DPC Widths
| Wall Type | Recommended DPC Width |
|---|---|
| 100mm Block | 112mm |
| 100mm Brick | 115mm |
| 215mm Brick (double) | 230mm |
| 300mm Cavity Wall | 337mm |
| Cavity Tray | 450mm |
Installation Tips
- Lay DPC on fresh mortar bed, minimum 5mm thickness
- Ensure DPC is level and free from punctures
- Maintain minimum 150mm overlap at joints
- Do not bridge DPC with continuous mortar droppings in cavity
- Ensure vertical DPC is used in cavity where external ground level is higher
NAIBC / IRC codes
References & Sources
Standards Cited
- IRC R406 — International Residential Code Section R406: Foundation Waterproofing and Dampproofing
Defines moisture barrier requirements for below-grade walls including dampproofing and waterproofing methods - IRC R404.1.6 — International Residential Code Section R404.1.6: Foundation Drainage
Requires foundation drainage and moisture protection for habitable below-grade spaces
Formulas Used
- Moisture barrier width (single skin):
width = wall thickness + 20 mm
Source: IRC R406 — Extra overhang each side to ensure full coverage - Moisture barrier width (cavity wall):
width = (wall thickness x 2) + 25 mm
Source: IRC R406 — Allows for cavity overlap in double-skin construction - Number of laps:
laps = ceil(wall length / effective roll length) - 1
Source: Standard construction practice — Effective roll length = roll length minus lap overlap. Minimum 6" (150 mm) lap at all joints - Mortar bed volume:
V = wall length x barrier width x bed depth (5 mm)
Source: Standard construction practice — Barrier is bedded in mortar; 5 mm bed joint depth is standard
Key Assumptions
- NA terminology: "moisture barrier" or "dampproofing" rather than DPC; applied to below-grade foundation walls — IRC R406
- Dampproofing per IRC R406.1 required for below-grade portions of foundation walls enclosing habitable spaces — IRC R406.1
- Waterproofing per IRC R406.2 required where hydrostatic pressure exists — IRC R406.2