Filling gaps between floorboards before sanding gives you a smoother, better-looking finish and helps reduce draughts — a real bonus in New Zealand's cooler climates. The best method depends on the size of the gaps, whether the floor still moves seasonally, and the finish you plan to apply. Getting this step right before the sander comes out will save you time and frustration later.
Why Gaps Appear in Timber Floors
Timber is a natural material that expands and contracts with changes in temperature and humidity. In many NZ homes — particularly older villas and bungalows with native timber floors — some seasonal movement is completely normal. Gaps that open in winter and close in summer are a sign of a live floor, and this matters a lot when choosing your filler. Using a rigid filler on a live floor will cause it to crack and pop out after the first seasonal cycle.
Assess the Gaps First
Before reaching for any product, take stock of what you're dealing with:
- Hairline or small gaps (under 3 mm): Common on older floors; usually fine to fill with a flexible product or dust slurry.
- Medium gaps (3–8 mm): May need a thicker flexible filler or a combination approach.
- Large or uneven gaps (over 8 mm): Often best addressed with timber infill strips or by a professional floor sander.
- Structural gaps or springy boards: Could indicate a subfloor or joist issue — worth investigating before sanding.
Method 1: Sanding Dust Slurry (Best for Small Gaps)
This is a popular and cost-effective approach used by many professional floor sanders. The process works like this:
- Do a first rough sand of the floor to generate fine timber dust.
- Mix the collected dust with a compatible water-based timber adhesive or PVA to form a thick paste (slurry).
- Work the slurry into the gaps with a flexible scraper, pressing it in firmly.
- Allow to dry fully — this can take several hours depending on humidity.
- Sand back smooth on your final sanding passes.
The big advantage is that the dust matches your floor's timber species almost perfectly, giving a very natural result. This method works best with water-based finishes applied afterwards. If you plan to use an oil-based finish, check compatibility with the adhesive you use.
Method 2: Flexible Timber Filler (Good for Small to Medium Gaps)
Flexible gap fillers designed for timber floors are widely available at Bunnings and Mitre 10. Look for products specifically labelled for timber floors — these are formulated to flex with seasonal movement rather than crack.
- Choose a colour that closely matches your timber species (most brands offer a range of stains).
- Apply with a flexible scraper or putty knife, slightly overfilling the gap.
- Allow to cure as per the manufacturer's instructions.
- Sand flush once dry.
Important: Always check that the filler is compatible with your intended floor finish (oil, polyurethane, hardwax oil, etc.). Some fillers don't bond well under certain coatings. Read the product data sheet carefully.
Method 3: Timber Infill Strips (Best for Large Gaps)
For gaps wider than about 8 mm, or where you want a truly solid, long-lasting repair, timber splines or strips are the most durable solution. This involves:
- Cleaning out the gap thoroughly — remove any old filler, grit, or debris.
- Cutting thin strips of matching timber (ideally the same species) to fit snugly.
- Gluing the strips in place with a suitable timber adhesive such as a product from Sika or similar.
- Allowing full cure time before sanding flush.
This method is more time-consuming and requires some woodworking skill. If you're not confident, this is a good task to hand over to a professional floor sander.
What to Avoid
| Product | Why to Avoid |
|---|---|
| Standard wall or ceiling filler | Too rigid — will crack and fall out as the floor moves |
| Silicone sealant | Cannot be sanded or stained; looks terrible under a clear finish |
| Expanding foam | Compresses unevenly and is very difficult to sand flush |
| Cheap acrylic gap filler | Often shrinks significantly and discolours under finish |
Preparing the Floor After Filling
Once your filler has fully cured, you're ready to sand. Make sure:
- All filler is fully dry and hard — rushing this step is the most common mistake.
- Any protruding nail heads are punched below the surface (at least 3 mm) to avoid damaging sanding belts.
- The room is well ventilated during both filling and sanding. Fine timber dust is a health hazard — wear an appropriate P2 dust mask and eye protection. WorkSafe NZ has guidance on managing dust exposure safely.
When to Call a Professional
Consider hiring a qualified floor sanding professional if:
- Gaps are large, uneven, or the boards are cupped or badly damaged.
- You suspect subfloor or structural issues beneath the boards.
- The floor is a valuable native timber (rimu, kauri, matai) where mistakes are costly.
- You're unsure which filler is compatible with your chosen finish system.
A professional floor sander will have the right equipment, know the best products for your specific timber, and can advise on whether the floor needs more significant repair before finishing. You can find qualified local floor sanders on tradedirectory.co.nz.
For broader guidance on maintaining healthy, well-finished floors in NZ homes, BRANZ is an excellent technical resource.
