Healthy Homes Moisture & Drainage

Healthy Homes Standard 4 covers moisture ingress and drainage on rental properties. The property must have efficient drainage for surface water, stormwater and groundwater, and — where there’s an enclosed subfloor — a ground moisture barrier if it’s reasonably practicable to install one. The goal is simple: keep liquid water away from the building and stop ground moisture rising into the floor.

Drainage

You need working gutters, downpipes and drains that carry roof and surface water to an appropriate outfall — a stormwater connection, soak pit or kerb — clear of blockages, with no overflow running against walls or foundations. The system has to remove stormwater, surface water and groundwater so it doesn’t pool under or around the house.

• Keep gutters, downpipes and drains clear and flowing to an appropriate outfall.
• No overflow against walls or foundations.
• Discharge downpipes away from the foundation, not straight onto the ground beside the wall where it soaks back under the floor.

Ground moisture barrier

A ground moisture barrier is required on rentals with an enclosed or suspended subfloor — a floor over a crawl space — where it can reasonably be installed. It’s a polythene sheet laid over the ground in the subfloor, typically 250 µm (0.25 mm) black DPM polythene, that stops moisture evaporating up from the soil into the floor structure and the rooms above.

• Lay it across the whole ground surface.
• Lap the sheets and turn them up at the perimeter.
• Combine it with subfloor ventilation so the space can still breathe.

Why it matters

Rising ground moisture is one of the biggest drivers of damp, mould and rotting framing in older NZ homes. A barrier plus good drainage dramatically cuts subfloor humidity. If the subfloor genuinely can’t be accessed, document why a barrier isn’t reasonably practicable.