Subfloor setup: keeping a timber floor dry, ventilated & connected

The subfloor is the foundation → pile → bearer → joist → floor stack that lifts a timber floor up off the ground (NZS 3604 §6 and §7). Two things make or break it: keeping the timber dry and ventilated so it doesn’t rot, and connecting the stack correctly so loads pass down and bracing and uplift hold.

Get the clearances and ventilation right and the structure lasts decades; skimp and you grow rot and borer in the dark.

Critical clearances

• 150mm from finished floor to adjacent ground (NZS 3604 §7.5.2.1) — prevents splash and decay.
• 450mm minimum crawl-space clearance for inspection access.
• No cuts within 150mm of finished ground level on H5 piles (§6.4.3.3).
• End-grain seal ALL cuts on H3.2/H4/H5 with copper naphthenate.

Subfloor ventilation & ground moisture

Air has to move under the floor, and bare earth has to be covered, or moisture builds up and the timber decays.

• 3500 mm²/m² of floor area is the minimum subfloor vent area.
• Cross-flow ventilation — put vents on opposite sides.
• Cover air vents with mesh ≤6mm to keep rodents out.
• Lay a ground vapour barrier (250µm DPM polythene) over bare earth — required by the Healthy Homes Standards 2019.

Pile → bearer → joist connections

Each connection in the stack has a code-compliant way to fix it. The headline: when joists sit on top of bearers, skew nails do the job — not hangers.

• Pile to bearer: M12 holding-down bolt cast in the concrete pile (or coach-bolted to a timber pile).
• Bearer to bearer (splice): over a pile, with 100×3.75 nails or a proprietary connector.
• Joist to bearer: the joist sits ON TOP of the bearer with 4× 100×3.75 skew nails (NZS 3604 §6.5.2(b)), OR 2 nails plus 2 wire dogs (Z-nails). Do NOT use joist hangers here — they waste money and add nothing structurally.
• Joist to joist (end-to-end): over a bearer, with 4× 100×3.75 skew nails OR 2× 75mm coach screws.

The common mistake

Putting joist hangers on a bearer-to-joist connection when the joists sit on top of the bearers. Hangers cost around $4 each and add a weak point. Skew nails are stronger, code-compliant, and free if you’ve already bought the nails.