Garage conversion damp proofing: what to check first

A garage conversion should not be lined until the cause of any moisture is understood. Check the external ground level, roof and rainwater goods, wall junctions, existing damp-proof course and floor before choosing a membrane, tanking system or new floor build-up.

Start with diagnosis, not plasterboard

Dark marks, salt deposits, peeling paint and a musty smell do not all point to the same defect. Moisture can arrive through the slab, walls, a failed gutter, a cold bridge or condensation. Take photographs after wet weather, note where the marks begin and look outside at the same wall. A specialist should investigate persistent or spreading moisture before the room is closed up.

Check the outside levels and details

• Compare the external paving or soil with the damp-proof course and internal finished floor level.
• Check that rainwater falls away from the wall and that gutters, downpipes and gullies are sound.
• Look for cracks, defective pointing, bridged damp-proof courses and damaged thresholds.
• Confirm that a new window or door opening will have a suitable sill, cavity tray or weathering detail where required.

Lowering external levels or repairing drainage may be more effective than adding an internal coating. Do not cover a defect simply because the next trade is waiting.

Floor and wall protection need a joined-up detail

The floor solution depends on the existing slab, ground conditions, levels and the required thermal performance. A new floor may need a damp-proof membrane, insulation and screed or board system, but the membrane must connect sensibly with the wall damp-proof course. If the floor is retained, ask how its moisture resistance and insulation will be demonstrated.

Walls need a separate assessment. Cavity walls, solid masonry and single-skin outbuildings behave differently. Internal linings can hide moisture, reduce room width and create cold edges. Any proposed tanking or cavity-drain system should come with a clear design, outlet and maintenance plan.

Before the quote is fixed

Ask the contractor to show the moisture assumptions in writing: what is being repaired, where the DPM or DPC sits, how openings are weathered and what happens if opening-up reveals a defect. Keep photographs and product information with the building-control paperwork. Good damp proofing is a sequence of inspected details, not a single branded product.