Timber framing moisture targets under NZS 3604 (and why 18% matters)

The 18% rule

NZS 3604:2011 — the primary standard for timber-framed buildings in New Zealand — sets the maximum moisture content of framing at the time linings are fixed. For sawn radiata pine that's 18% MC. For engineered timber such as LVL it's tighter, at 16% MC.

It applies to wall framing, bottom plates, floor joists and roof framing alike. If timber has been wetted — by weather on an open build, a wrap failure, or a leak — it must be dried back below 18% before any plasterboard goes on.

Why closing up wet framing is a problem

Lining over framing above 18% MC is a Building Code breach. It also traps moisture against the timber and the back of the plasterboard, where it can drive mould, corrosion of fixings and fungal decay — the classic ingredients of a weathertightness claim years later.

Because durability (NZBC clause B2) requires structural elements to last a minimum of 15 years, moisture closed into the assembly directly undermines the performance you're signing off.

How it's measured

A calibrated pin moisture meter is pressed into the timber at marked stud locations to read MC directly at depth. Pinless meters and thermal imaging are used first to scan and find the wet areas quickly, then the pin meter confirms the number that gets signed off.

Readings are taken over the drying period, dated and recorded, so there's a documented trace from wet to spec — not just a single reading on the day.

What we hand over

Before your lining trade starts we issue a signed report: marked reading locations, the moisture content at each, the target threshold, and a pass/fail against 18% MC. It satisfies the BCA inspector and protects you against future moisture claims.