The short answer
In New Zealand, timber flooring can be laid over a concrete slab once the slab surface tests at or below 75% relative humidity (the BRANZ benchmark), the slab is flat and sound, and a compatible adhesive and barrier system is used. New slabs dry slowly – a 100mm slab can take four months or more – so moisture testing, not the calendar, decides when you lay (BRANZ BU644, PlaceMakers).
Why slabs matter so much in NZ
Most new New Zealand houses sit on a concrete slab – about 92%, according to BRANZ figures reported by ArchiPro (ArchiPro NZ). That makes slab moisture the single biggest technical issue for wood flooring in new builds and extensions.
New Zealand's climate adds to it. Much of the country is mild and humid, building programmes are tight, and slabs are often closed in before they've had time to dry. Timber laid on a damp slab takes up moisture from below and can cup, swell or lose its bond.
What BRANZ BU644 says
BRANZ Bulletin 644 covers solid timber strip flooring on a concrete slab and sets 75% relative humidity or less at the slab surface as the benchmark before laying. BRANZ is New Zealand's independent building research organisation, and its bulletins are widely used by designers, builders and flooring installers.
Practical points for any glued timber floor on an NZ slab (read the bulletin itself for its full method and detailing):
- Measure, don't guess. Slab moisture is measured as relative humidity with a calibrated hygrometer, following the method in the guidance you're working to.
- Account for the whole slab. Readings vary across a slab – test in several locations, including near external walls and in cooler rooms.
- Keep the building closed and conditioned while the slab dries and before timber is delivered.
- Record the results as part of the job documentation.
Damp-proofing and the Building Code
A damp-proof membrane under the slab keeps ground moisture out, but it does not dry the construction water already in the concrete. Ground moisture protection for floors is part of the NZ Building Code's external moisture clause, E2, which has acceptable solutions and verification methods including E2/AS1 (MBIE). Your designer and building consent documents set out how the slab is damp-proofed – confirm this before specifying a glued timber floor, particularly on older slabs where a membrane may be missing or damaged.
How long does a slab take to dry?
A 100mm slab can take four months or more to dry enough for timber, and longer in cold, damp or unventilated conditions (PlaceMakers).
| Factor | Effect on drying |
|---|---|
| Thicker slab | Slower |
| Winter, cool temperatures | Slower |
| Closed, unheated building | Slower |
| Ventilation, gentle heating (once enclosed) | Faster |
| Wet trades (plastering, painting, tiling) still under way | Adds moisture |
| Curing compounds or sealers on the surface | Can slow drying and affect adhesion |
Build the drying time into the programme and test early so there are no surprises.
Laying solid oak parquet on an NZ slab
Solid oak parquet is glued directly to the slab with a flooring adhesive – often a moisture-barrier adhesive or a separate barrier plus adhesive – once moisture and flatness are within limits. The adhesive manufacturer's limits and instructions govern.
- Test the slab against BRANZ guidance and the adhesive maker's limit.
- Check flatness and prepare the surface – grind high spots, fill low spots, remove contamination.
- Choose one compatible system – barrier/primer, adhesive and levelling compound from the same manufacturer. See adhesives and moisture barriers.
- Acclimatise the blocks to the in-service conditions – see acclimatising oak flooring.
- Set out, glue and lay, leaving the expansion allowance the installation guidance requires.
- Sand and finish once the adhesive has cured.
Our Australian concrete slab installation guide covers each step in more depth – the method is the same, with BRANZ as the NZ reference.
Programming an NZ build for a timber floor
Plan the floor into the build programme from the start, so the slab has time to dry and the timber isn't rushed in at the end.
- At design stage, confirm the flooring type, damp-proofing and any underfloor heating, and allow for floor build-up at doors and stairs.
- After the pour, record the pour date and slab thickness; estimate drying time (four months or more for 100mm is a useful planning figure).
- Close in early. Get windows and doors in so the slab can dry in a protected environment.
- Avoid sealing the slab with products that slow drying or interfere with adhesion unless the flooring system allows it.
- Test early – a first round of slab readings well before the planned flooring date shows whether the programme is realistic.
- Finish wet trades before timber is delivered.
- Deliver timber when the site is ready, then acclimatise and lay.
Your Timbo partner can advise on timing for delivery from Melbourne; freight to New Zealand is quoted per order.
Engineered vs solid on NZ slabs
Engineered timber is often preferred on slabs because it is more dimensionally stable, but solid oak parquet is a good choice when the slab is dry and the client wants maximum sanding life. If there's underfloor heating, engineered is generally preferred – read underfloor heating and oak flooring and talk to Timbo before specifying solid.
NZ costs and supply
NZ retailers list engineered oak at about NZD $99–$320+/m² incl. GST, with installation about NZD $40–$150/m² (Marchand, Vienna Woods). NZ GST is 15%. Slab testing, grinding, levelling and barriers are extra.
Timbo supplies Prime grade European solid oak parquet to trade partners in New Zealand from Melbourne stock, with freight quoted per order. See wood flooring in NZ and partnering with Timbo in New Zealand.
Sources
- BRANZ BU644 – Solid timber strip flooring on a concrete slab
- PlaceMakers – Live and let dry
- ArchiPro NZ – Solid wood vs engineered wood flooring
- MBIE Building Performance – E2 External moisture
- Marchand – Engineered oak flooring cost NZ 2026
- Vienna Woods – Timber flooring cost NZ
Frequently asked questions
What relative humidity should a slab be before laying wood flooring in NZ?
BRANZ Bulletin 644 uses 75% relative humidity or less at the slab surface as the benchmark for solid timber flooring. Your adhesive or moisture-barrier manufacturer may set its own limit and test method – the stricter requirement applies.
How long does a new slab take to dry in New Zealand?
A 100mm slab can take four months or more, and longer in winter or in a closed, unheated building. Drying time depends on slab thickness, mix, weather and ventilation, so test rather than count days.
Does an older NZ slab need moisture testing before new flooring?
Yes. Older slabs are usually dry of construction water, but some were built without an effective damp-proof membrane, or have been affected by leaks or poor drainage. Previous coverings such as vinyl can also trap moisture. Test before laying, as for a new slab.
Does a damp-proof membrane under the slab mean I can skip testing?
No. A membrane under the slab stops ground moisture rising, but the slab still contains construction water that must dry out. Test the slab surface before laying.
Who should programme slab drying time on an NZ build?
The builder or project manager, at the planning stage. The flooring installer then tests before laying. Agreeing the drying period and test requirements in the contract avoids pressure to lay timber on a slab that isn't ready.