Filling gaps between old timber floorboards is a satisfying DIY job that most homeowners can tackle with the right preparation and materials. The key is choosing a filler that can flex with the natural movement of timber — because in New Zealand's variable climate, timber floors expand and contract with changes in temperature and humidity throughout the year. Rush the job with the wrong product and you'll be back on your knees within a season.
Why Timber Floors Develop Gaps
Old timber floors — particularly those laid in villas and bungalows common across NZ — were often installed with solid native timbers like rimu, kauri, or matai. Over decades, these boards dry out, shrink, and pull apart. Gaps are also more pronounced in homes with underfloor ventilation (which is actually a good thing for moisture control under NZ Building Code clause E3), as drier air accelerates timber shrinkage. Small gaps (under about 6 mm) are generally a DIY-friendly fix. Larger or uneven gaps may need professional attention.
Choosing the Right Filler
This is the most important decision. Using a rigid filler like standard wall-patching compound will crack and pop out as the boards move. You need a product with some flexibility.
| Filler Type | Best For | Flexibility | Sandable/Paintable |
|---|---|---|---|
| Timber putty / wood filler | Small gaps, nail holes | Low | Yes |
| Flexible gap filler (acrylic) | Gaps up to ~8 mm | Medium | Yes |
| Rope/cord (jute or cotton) | Wide, consistent gaps | High (natural) | No |
| Sawdust & PVA mix | Matching colour, small gaps | Low–Medium | Yes |
| Epoxy wood filler | High-traffic areas | Low | Yes |
Flexible acrylic gap fillers are the most popular choice for DIYers. Look for products specifically labelled for timber floors at stores like Bunnings or Mitre 10. Many come in timber tones (pine, rimu, oak) to blend with your floor.
A sawdust and PVA glue mix is a traditional favourite — collect fine sawdust from sanding your own floor for the closest colour match, then mix with PVA to a stiff paste. It's economical and blends well, though it can crack in wider gaps.
For wide, consistent gaps, some tradespeople use twisted jute rope or cotton cord pressed firmly into the gap with a putty knife. This allows natural movement and can be stained to match.
What You'll Need
- Flexible floor gap filler or your chosen product
- Putty knife or filling knife
- Fine-grit sandpaper (120–180 grit)
- Vacuum cleaner and stiff brush
- Painter's tape (optional, for neat edges)
- Matching floor stain or finish (if needed)
Step-by-Step: Filling Timber Floor Gaps
1. Clean the Gaps Thoroughly
Vacuum out all dust, grit, and debris from between the boards. Use a stiff brush or old screwdriver to dislodge compacted dirt. Any contamination will stop the filler bonding properly.
2. Check for Movement or Damage
Before filling, press down on the boards around each gap. If boards are springy, squeaky, or lifting, the subfloor fixings may have failed — filling over this won't last. Check for signs of moisture damage or rot while you're down there.
3. Apply the Filler
Apply your chosen filler along the gap, pressing it in firmly with a putty knife held at a low angle. Slightly overfill — most products shrink a little as they dry. For acrylic fillers, smooth the surface with a damp finger or cloth before it skins over.
4. Allow to Cure Fully
Follow the manufacturer's drying time — typically 2–24 hours depending on the product and gap depth. Don't rush this step in cooler or humid conditions, which are common in many NZ regions.
5. Sand Flush
Once fully cured, sand the filled areas flush with the surrounding boards using 120–180 grit sandpaper, working with the grain. Wipe away dust with a slightly damp cloth.
6. Finish to Match
If your floor is oiled, stained, or varnished, apply a matching finish coat over the filled areas. Products from Resene or Wattyl include floor-specific oils and varnishes suitable for NZ conditions.
Tips for a Better Result
- Work in mild conditions — avoid filling on very hot days when boards are at maximum expansion, or in winter when they're at maximum contraction. Aim for a moderate day.
- Don't fill every gap — some small gaps are normal and allow for seasonal movement. Filling every crack can cause boards to buckle in summer.
- Test your filler colour in an inconspicuous spot first.
- For heritage homes, check with BRANZ guidance on appropriate repair methods for older timber species.
When to Call a Professional
Some situations are beyond a DIY fix. Call a qualified flooring installer (you can find local professionals on tradedirectory.co.nz) if:
- Gaps are wider than 10 mm or very uneven
- Boards are cupping, warping, or lifting — this often signals a moisture problem requiring investigation under MBIE Building Performance guidelines
- You suspect subfloor rot or structural damage
- The floor needs a full sand and refinish to look its best after filling
- You're in a rental property — landlords have obligations under Tenancy Services Healthy Homes standards, and a professional can document the work
A licensed flooring installer can also advise whether your floor is worth repairing or whether replacement is more cost-effective in the long run.
