Damp-Proofing Requirements for New and Existing Buildings

What This Requirement Covers

Approved Document C2 requires that the floors, walls, and roof of a building adequately resist the passage of moisture to the interior. Dampness in buildings causes mould growth, timber decay, damage to finishes, and health problems for occupants. Effective damp-proofing is achieved through a combination of damp-proof courses (DPCs), damp-proof membranes (DPMs), and appropriate construction detailing.

Key Requirements

Damp-Proof Courses (DPCs)

1. A DPC must be provided in all external walls at a height of at least 150 mm above external finished ground level
2. DPCs must be continuous and must lap or connect with the damp-proof membrane in the floor
3. Common DPC materials: polyethylene sheet, bitumen felt, slate, engineering bricks
4. DPCs must also be provided above openings (cavity trays) and at any point where moisture could bridge the cavity

Damp-Proof Membranes (DPMs)

1. A DPM must be provided in all ground-bearing concrete floors
2. The DPM must be continuous and must lap with the DPC in the external wall
3. Minimum DPM specification: 1200 gauge (300 micron) polyethylene sheet
4. DPM may be placed above or below the concrete slab; if below, it must be protected from damage during construction
5. All joints in the DPM must be lapped by at least 300 mm and sealed

External Walls

1. External walls must resist the penetration of rain to the interior
2. Cavity walls: The cavity must be at least 50 mm wide (clear of insulation in partial-fill systems) and must be kept clean to prevent moisture bridging
3. Solid walls: Must be at least 328 mm (one and a half bricks) thick, or provided with external render, cladding, or internal waterproof lining
4. Weep holes: Must be provided above all cavity trays (at DPC level, above lintels, at abutments) at maximum 900 mm centres

Below-Ground Moisture Protection

1. Walls below ground level must resist the passage of groundwater
2. In low water table conditions, a coating of bituminous paint or tanking membrane on the external face of the wall may be sufficient
3. In high water table conditions, structural waterproofing (tanking) designed to BS 8102 is required
4. Subsoil drainage may be needed to reduce hydrostatic pressure on below-ground walls

Practical Compliance Tips

1. Ensure the DPC is at least 150 mm above finished ground level; raised flower beds, paths, and driveways built up against the wall can bridge the DPC
2. Lap the DPM with the DPC at the wall/floor junction; a gap allows moisture to rise into the wall above the DPC
3. Keep cavities clean during construction; mortar droppings on wall ties bridge the cavity and transfer moisture to the inner leaf
4. Install cavity trays above all lintels, at roof abutments, and at any point where moisture could enter the inner leaf
5. In existing buildings with rising damp, an injected chemical DPC may be used as a remedial measure, but the source of dampness should be identified first
6. Ventilate suspended timber floors adequately to prevent moisture buildup in the subfloor space
7. Do not render or externally insulate over a DPC without providing a new visible DPC at or above the render level