Gapping in timber floors during winter is one of the most common complaints NZ homeowners have after a new floor is laid — and the good news is that it's almost always preventable. The root cause is low indoor humidity: as your home dries out in winter (especially with heating running), timber loses moisture and the boards shrink across their width, opening gaps between them. Getting on top of this before and during installation makes a huge difference.
Why Timber Floors Gap in Winter
Timber is a hygroscopic material, meaning it constantly exchanges moisture with the surrounding air. When indoor relative humidity (RH) drops below about 40% — which is common in NZ homes during winter, particularly in drier regions like Canterbury or Central Otago — boards lose moisture and contract. The reverse happens in summer, when boards can swell and cup if humidity climbs too high.
This movement is perfectly natural, but excessive gapping usually points to one of three problems:
- The timber wasn't properly acclimatised before installation
- The subfloor or site conditions were too wet or too dry at the time of laying
- The home's indoor humidity swings too dramatically between seasons
Acclimatise Your Timber Properly
Acclimatisation is the single most important step you can take. Before installation, the timber flooring must be stored inside the room where it will be laid — not in a garage or outside — so it can reach equilibrium moisture content (EMC) with its environment.
- Stack boards with spacers (stickers) between layers to allow airflow on all sides
- Run the home's heating and ventilation as you normally would during winter
- Allow a minimum of 5–7 days for most solid timber; some species or thicker boards may need longer — always follow the manufacturer's guidelines
- Use a timber moisture meter to check that the boards have stabilised before laying begins
Your flooring installer should be checking moisture content at delivery and again just before installation. If they're not, ask them to.
Control Subfloor Moisture
Moisture rising from below is just as problematic as dry air above. In NZ, many homes — particularly older villas and bungalows — have suspended timber subfloors over a crawl space. If that space is damp, moisture will migrate upward into your new floor.
- Ensure the underfloor space is ventilated in line with NZ Building Code requirements (check the relevant clause with your council or a building professional)
- Lay a polythene vapour barrier (minimum 0.25 mm) over bare earth under the house
- Check subfloor moisture content — it should generally be within 4% of the flooring timber's moisture content at the time of installation
- For concrete subfloors, test for residual moisture using a calcium chloride test or in-situ RH probe before laying any timber or engineered flooring
BRANZ publishes excellent guidance on moisture management in NZ buildings if you want to dig deeper into the science.
Maintain Indoor Humidity Year-Round
Once your floor is laid, keeping indoor RH between 45% and 65% will minimise seasonal movement. Here's how:
| Season | Typical Risk | Solution |
|---|---|---|
| Winter | Low humidity, boards shrink | Use a humidifier; limit excessive heating |
| Summer | High humidity, boards swell | Use ventilation or a dehumidifier |
| Year-round | Rapid swings | Aim for stable, consistent conditions |
- A plug-in hygrometer (available from hardware stores) lets you monitor RH cheaply and easily
- Heat pump heating tends to dry air out more than flued gas heating — consider a humidifier if you rely heavily on a heat pump
- Avoid drying laundry indoors without ventilation, as this can spike humidity and cause swelling
The Energywise website has useful advice on balancing home heating and ventilation in NZ conditions.
Choose the Right Timber and Profile
Not all timber floors are equal when it comes to movement. Some tips:
- Engineered timber flooring (a hardwood veneer over a stable core) moves significantly less than solid timber and is often a better choice for homes with concrete slabs or underfloor heating
- Narrower boards gap less visibly than wide boards — a 65 mm board will show less of a gap than a 130 mm board with the same amount of shrinkage
- Quarter-sawn or rift-sawn timber is more dimensionally stable than flat-sawn
- Always choose a species and grade suited to NZ conditions — ask your supplier for guidance
You can find quality timber flooring products and advice at stores like Bunnings, Mitre 10, or PlaceMakers.
Allow for Expansion Gaps
A correctly installed floor must have expansion gaps around the perimeter (typically 10–12 mm, covered by skirting or scotia) and at any fixed obstacles like columns or doorframes. If these gaps are missing or too small, the floor has nowhere to expand in summer — which can cause boards to buckle, and then gap excessively when they contract again in winter.
When to Call a Professional
Timber floor installation isn't restricted work under NZ law, so homeowners can lay their own floors. However, getting acclimatisation, moisture testing, and expansion gaps right requires real skill and the right tools. If you're seeing large or uneven gaps, cupping, or squeaking, it's worth having a qualified flooring installer assess the situation — especially if the floor is still under warranty, as improper installation can void it.
A professional installer will have calibrated moisture meters, know the specific requirements for your timber species, and can identify subfloor issues before they become expensive problems. You can find experienced local flooring installers on tradedirectory.co.nz.
If your gapping issue is linked to underfloor ventilation or subfloor drainage problems, you may also need a builder or drainlayer — check whether any work requires a building consent with your local council, as requirements vary.
