Approved Document C (Resistance to Moisture): What Homeowners Need to Know

Part C addresses two distinct problems:

• Contaminants — some sites contain hazardous substances (radon gas, methane, heavy metals, hydrocarbons) that must be identified and mitigated before building.
• Moisture — buildings must resist moisture from the ground, from rain and from condensation. This includes damp-proof courses (DPCs), damp-proof membranes (DPMs), cavity walls and roof coverings.

The document requires that the site is investigated before construction to identify any contaminants, and that appropriate remediation is carried out where needed.

The most common Part C requirements for domestic projects relate to moisture control:

• Damp-proof course (DPC) — a horizontal barrier in a wall, typically 150 mm above external ground level, that stops ground moisture rising through masonry. New extensions must have a DPC that links to the existing DPC.
• Damp-proof membrane (DPM) — a sheet of heavy-duty polythene under the ground floor slab, preventing moisture rising through the concrete. The DPM must be continuous and link to the DPC in the surrounding walls.
• Cavity walls — the cavity in a cavity wall must not be bridged by mortar droppings or insulation that allows moisture to cross from the outer to the inner leaf.

Radon is a naturally occurring radioactive gas that seeps from certain rocks (particularly granite and limestone). It is a significant concern in parts of Cornwall, Devon, Derbyshire, Northamptonshire and other regions. In high-radon areas, Part C requires:

• A radon-resistant membrane under ground floors
• In very high-radon areas, a sub-floor depressurisation sump and pipework (radon sump) ready to be activated if needed

Check the UK Health Security Agency radon map to see if your area is affected. Your building control officer will advise on what level of protection is needed.

For sites with other contamination (former garages, dry cleaners, industrial land), a Phase 1 desk study and possibly a Phase 2 intrusive investigation may be required before building control will approve your project.

External walls must resist the penetration of rain. For masonry cavity walls, the cavity (typically minimum 50 mm clear) provides the main protection. For solid walls, renders, cladding or special brickwork details are required. Roofs must:

• Use an appropriate underlay beneath tiles or slates to resist wind-driven rain
• Have adequate fall (minimum 1:80 for flat roofs using warm deck construction)
• Include effective flashings at all junctions with walls, chimneys and rooflights

For extensions, the main inspection points for Part C are:

• Foundation/oversite inspection — the BCO will check that the DPM is in place and continuous before the floor slab is poured.
• Wall construction — checks that the DPC is at the correct height and that the cavity is clean.
• Roof completion — checks that flashings and roof coverings are complete before the final certificate is issued.