How to Fill Gaps Between Floorboards: Methods, Gap Sizes and When Not To

To fill gaps between floorboards successfully, you must match the repair technique directly to the gap width and seasonal movement of the timber. Fine cracks under 2 mm are best treated with a resin-and-sawdust slurry during full floor restoration; medium gaps between 2 mm and 4 mm require a flexible acrylic mastic; and wide separations exceeding 4 mm demand glued timber slivers.

Rigid wood putty should never be used across wide expansion voids, as natural moisture shifts will crack it loose. Knowing whether, when, and how to fill gaps prevents costly refinishing errors and keeps your timber stable year-round.

Gap Width	Recommended Method	Flexibility	Best Application Timing
< 2 mm (Hairline)	Resin and sawdust slurry	Rigid / Semi-rigid	Bare wood stage during sanding
2 mm – 4 mm (Medium)	Flexible gap sealant (e.g. Bona Gap Master)	High (elastic)	After final fine sand or spot repairs
> 4 mm (Large)	Reclaimed timber slivers / wedges	Matches timber	Mid-restoration before coarse sanding
Deep voids (> 8 mm deep)	Foam backing rod + flexible sealant	High	Prior to final surface sealing
Seasonal hairline (< 1 mm)	Leave unfilled	Natural movement	Monitor across seasons

Why Gaps Appear Between Floorboards

Timber is a hygroscopic, organic material. It breathes, absorbs environmental humidity, and expels moisture as ambient conditions change. When boards lose moisture, their cellular structure contracts across the grain, pulling adjacent planks apart.

Understanding what causes these separations in Melbourne homes helps determine whether a void needs structural intervention or is simply a harmless seasonal shift.

Seasonally induced moisture fluctuations (Winter Shrinkage, Summer Closing)
There is distinct seasonal change in Melbourne’s humidity. In the summer when it is humid outside, the floor boards swell up and push against one another tightly.

The drier the timber, the more its prone to splitting and cracks and holes

Board width decreases, gaps increase. When the timber is wet, sealing out large voids can cause the wood to crush the filler or buckle when it expands the next time.

Heating, air-conditioning and direct sunlight.
Timber shrinkage is sped up by modern climate control. With the ducted gas heating and reverse cycle air conditioning systems, ambient relative humidity is removed from the buildings, and the moisture is extracted from the timber flooring system.

Likewise, windows that face north and extend all the way up to the ceiling heat certain areas of the floor. This stress is concentrated and results in unequal contraction of boards in the same room.

Proper sub floor ventilation and moisture control.
Older homes in Australia are usually on timber stumps or brick piers, and have suspended sub-floors. When sub-floor vents become blocked by garden beds and paths, ground moisture accumulates under the floor boards.

If the bottom of a plank is wet and the top is dry, due to the conditions of living indoors, then there is a differential expansion. This results in cupped boards, edge-lifting, uneven and irregular gaps around the edge.

Aged 50 and over, Plain-Edge Boards, and Settling in Older Melbourne Homes.
Original Baltic pine floors or Oregon pine floors are common in Victorian and Edwardian homes from Fitzroy to Hawthorn and all points in between. Many of these are square-edged (no tongue and groove) heritage floors.

It’s only natural that when settlement, motion and thousands of footfalls occur over the course of a century, plain-edged boards tend to part ways, creating long, straight voids, straight down to the sub-floor cavity.

Should You Fill Gaps Between Floorboards?

Not every floorboard separation needs to be filled. Before applying any compound, evaluate whether filling will stabilise the floor or simply create a recurring maintenance headache.

If it feels right, fill it
When there are gaps in the home that result in functional issues, it is a practical solution:

• Draught reduction: Heat from the floor above the suspended floor flows through gaps and into the living areas bringing in cold sub-floor air and musty smells.
• Debris traps: Large channels catch dust, crumbs and pet dander that a vacuum cleaner cannot remove.
• Aesthetic uniformity: Where a planned timber floor restoration is required to produce a uniform aesthetic surface of the building, filling may be considered as a justifiable procedure.

Ambient hollow echoes and sub-floor draft whistles are minimized with acoustic dampening (sealing deep, open board seams).

The Best Times to Leave Gaps Unfilled
It is best to leave alone hairline partings less than 1 mm wide, which occur during dry winter weather. In summer, when the relative humidity rises again above 50%, these hairline cracks will usually close by themselves.

In newer solid vs engineered wood houses with tongue and groove boards, care must also be taken. If the tongue and groove profile is structurally locked, then filling the small flex gaps will limit the natural movement and cause edge crushing.

When rigid filler is used on a moving floor what happens?
One of the most common wood putty errors when installing a timber floor is using a non-flexible type of putty in gaps on the active floor.

Boards expand in high humid conditions and the rigid putty cannot compress with the swelling. As the timber is being crushed, the dried filler is crushed to a powder. The growing timbers push upwards causing edges to splinter and the planks to crown, especially in severe cases.

During the winter months, when the boards shrink, the broken filler comes loose and can fall through cracks or be sucked out gaps in an irregular manner.

How Wide Is the Gap? Match the Method to the Size

Prior to buying materials, carefully measure the largest spacing on your floor. The primary cause of failure of fillers is by using the correct repair technique for the incorrect void spacing.

Choosing a measuring device to gauge the gap.
To get an accurate measurement of your floor:

• For very fine hairline seams less than 1.5 mm, use an automotive feeler gauge.
• Use a steel metric ruler or vernier calliper to measure larger gaps. From end to end of plank, determine the widths at the narrowest and widest points.
• Make baseline measurements in late winter. It is at this stage that Melbourne timber will have the lowest moisture content and will have widest gaps.
• Test depth – insert a thin wooden skewer into the void to see if the tongue is present or if the gap is wide enough to reach the sub-floor space.

5 Ways to Fill Gaps Between Floorboards

Choosing the best filler for gaps between floorboards depends on whether your floor is being fully sanded, how wide the voids are, and how much natural floor deflection exists.

Method	Best For	Typical Gap Size	Pros	Cons	Durability
Resin & Sawdust Mix	Full sand-and-polish jobs	Up to 2 mm	Perfect colour match; seamless finish	Inelastic; will crack in wider gaps	Moderate to High
Flexible Gap Sealant	Finished floors; minor movement	2 mm – 4 mm	Absorbs seasonal expansion; easy DIY	Does not accept wood stain; attract dirt	Moderate (5–8 yrs)
Timber Slivers	Historic/Victorian pine floors	4 mm – 10 mm+	Genuine wood repair; stains identical	Labour-intensive; requires flush sanding	Very High (decades)
Rope / Jute Cord	Heritage cottages; rustic aesthetics	3 mm – 8 mm	Traditional look; stops draughts easily	Collects lint; cannot be sanded flush	Moderate
Backing Rod + Sealant	Deep, open-void boards	Depth > 8 mm	Prevents filler drop-through; saves cost	Must be capped with flexible mastic	High

1. Wood Filler or Sawdust and Resin Mix

This trade method uses a clear solvent or water-based binding resin (such as Bona Mix & Fill) blended with fine, clean wood flour collected from the 80-to-100-grit sanding pass of your actual floor.

• Best for: Bare timber undergoing full mechanical sanding.

• Gap size: Manufacturer technical sheets recommend restricting this to hairline gaps under 2 mm.

• Pros: Blends with the natural grain and undertones of your specific timber species.

• Cons: Strictly rigid once cured. If applied to wide gaps, seasonal shifts will cause it to crack loose within 12 months.

• Durability: High over tight seams; poor over loose, moving boards.

2. Flexible Gap Sealant

Flexible acrylic-polyurethane mastics (such as Bona Gap Master or Timbermate flexible variants) are engineered specifically for timber joints subject to expansion and contraction.

• Best for: Existing finished floors or boards that experience slight seasonal expansion.

• Gap size: Typically effective for gaps between 2 mm and 4 mm.

• Pros: Expands and contracts with timber movement without cracking or debonding from plank edges.

• Cons: These products do not take timber stain like raw wood. You must choose an exact pre-tinted manufacturer shade (e.g., Jarrah, Pine, Oak).

• Durability: Moderate to long-term; requires occasional spot touch-ups over high-traffic walk lanes.

3. Timber Slivers

Timber slivers (often called wood wedges or Dutchmen) are thin, wedge-shaped strips cut from aged, reclaimed timber matching your floor species—such as old Baltic pine or Tasmanian oak.

• Best for: Wide, unsightly gaps in heritage, plain-edged, or extensively settled architectural floors.

• Gap size: Ideal for gaps 4 mm and wider.

• Pros: You are filling wood with wood. Slivers absorb stain, sealers, and finish coatings identically to the adjacent boards.

• Cons: High labour requirement. Slivers must be individually planed, glued with cross-linking PVA, hammered into place, and planed or sanded level.

• Durability: Permanent. A properly fitted timber sliver lasts as long as the floorboard itself.

4. Rope or Cord Filling

A traditional European method for rustic cottages, this involves pressing natural jute, hemp, or cotton marine rope into the gap before painting or staining it with an archival timber dye.

• Best for: Historic properties where boards move excessively and modern chemical mastics would look out of place.

• Gap size: Typically 3 mm to 8 mm.

• Pros: Highly economical draught-proofing method that yields to timber movement without failing.

• Cons: Cannot be sanded smooth. Fibres can snag on vacuum brushes and mop heads over time.

• Durability: Moderate; purely functional rather than architectural.

5. Foam Backing Rod with Filler

A closed-cell polyethylene foam backing rod acts as a physical depth stop. When plain-edge boards have lost their lower backing or lack tongue-and-groove joints, wet filler will slump through into the sub-floor space.

• Best for: Deep crevices where the ground or joist space is visible between planks.

• Gap size: Any gap deeper than 8 mm.

• Pros: Establishes a two-point adhesion surface for flexible sealant, preventing three-sided adhesion failure and dramatically reducing product consumption.

• Cons: Must always be capped with an appropriate sealant; cannot remain exposed.

• Durability: High; provides foundational support for flexible surface sealants.

Step-by-Step: How to Fill Floorboard Gaps

Proper preparation directly determines whether a gap repair lasts for years or flakes out within weeks. Follow these steps when filling gaps between timber floorboards.

What You Need

• Industrial vacuum cleaner with a narrow crevice nozzle

• Stiff putty knife or mini scraper tool

• Closed-cell foam backing rod (sized 20% wider than gap width)

• Selected filling agent (timber slivers, flexible mastic, or resin binder)

• Cross-linking PVA wood glue (if installing timber slivers)

• Timber offcut for colour testing

• Random orbital sander or hand sanding block (80-grit, 120-grit, and 180-grit paper)

• Clean cotton rags and mineral spirits / water for cleanup

Step 1: Clean the Gaps

Vacuum the entire floor thoroughly, running the crevice tool slowly along each seam. Use a thin, rigid putty knife to scrape out compacted dirt, pet hair, and old brittle filler.

Safety Note: Homes built in Melbourne before 1990 may contain old bitumen adhesives, toxic waxes, or lead-based sealers deep in the seams. Wear a well-fitted P2 respirator mask and avoid aggressive mechanical scraping that could disperse hazardous particulates.

Step 2: Choose and Colour-Match the Filler

Wet timber appears substantially darker than dry, raw wood. Test your filler on a small, unobtrusive offcut or an inconspicuous corner (such as inside a built-in wardrobe).

Apply a small drop of your proposed clear topcoat over the cured test swatch. Timber fillers routinely dry lighter or shift hue once topcoats cure, making pre-testing essential.

Step 3: Apply and Press into the Gap

• For deep voids: Push the closed-cell foam backing rod down into the channel until it sits roughly 4 mm below the timber surface.

• For flexible mastic: Cut the sealant nozzle at a 45-degree angle matching the seam width. Force the product firmly into the void under consistent pressure to prevent trapped air pockets.

• For slivers: Coat both sides of the timber wedge with quality wood glue. Seat the sliver into the void and tap gently with a rubber mallet along its entire length until firmly wedged.

Step 4: Sand Flush

Allow the compound to cure completely according to manufacturer instructions typically 2 to 24 hours depending on ambient temperature and formula.

• If you used timber slivers, remove the excess timber height using a sharp block plane flush with the floor surface before light sanding.

• For resin mixes or standard fillers, sand the surface with an 80-grit abrasive along the direction of the timber grain.

• Progress up through 120-grit and finish with 180-grit for an ultra-smooth transition into the surrounding boards. Never sand across the grain, as cross-grain scratches will show under modern polyurethane coatings.

Step 5: Seal, Stain, or Finish

Vacuum every trace of residual sanding dust from the seams.

Apply your preferred penetrating oil, water-based polyurethane, or solvent-based topcoat across the repaired surface. Keep in mind that synthetic fillers absorb stain at different rates than natural wood cells. If applying a dark timber stain, test the reaction on your sample patch first to confirm color uniformity.

Best Time to Fill Gaps in a Timber Floor

The seasonal timing and sequence of your floor project determine how durable your gap repairs will be.

This is a pre-treatment step before Sanding and Polishing (Recommended).
The ideal time to fill in gaps on the floorboard is in the middle of a professional floor sanding/polishing treatment.

The coarse cut is done by the contractor to smooth out the boards, followed by the application of the resin slurry or fitting of slivers of timber across bare timber and then the medium and fine sanding cuts. It is in this process that the filler is cut down to be level with the timber before the protective sealers and commercial topcoats seal the floor.

Following the work of Polishing (Limitations of Surface Repairs)
When trying to plug holes into a properly sealed and polished floor, you get an incomplete and obvious repair.

Glossy or satin clear coat edges will be left with a bit of an edge film when flexible acrylic mastics are applied. Further, it is impossible to sand the repair flat without scuffing the surrounding polish, revealing the joint in direct sunlight.

What season is the best in Melbourne?
Late winter to early spring (from July to September) is the best time of the year to install a floorboard void filler in Melbourne.

The boards are at their thinnest point of the year at this time, as they have been so contracted by cold temperatures and continuous household heating that the gap is at its widest.

Tree holes filled when perfectly full will leave the holes in place. The higher the humidity in the summer months the more the timber will swell against the flexible filler without spreading any more and tearing the repair off the wood.

Common Mistakes When Filling Floorboard Gaps

• Using rigid plaster or builder’s bog: Standard polyester fillers or interior plaster compounds dry rock-hard and lack the elasticity needed for suspended timber floors.

• Filling during high summer humidity: If you fill gaps when timber is fully expanded, the filler will crack loose as soon as boards shrink in winter.

• Skipping the prep vacuum: Applying expensive sealants directly over compacted dust and grit prevents the compound from bonding to the timber edges, leading to premature delamination.

• Overfilling without backing rods: Pumping wet mastic into deep voids without a foam backing rod causes the compound to sag downward, forming an uneven concave dip along the joint line.

• Ignoring the natural wood grain: Running coarse sandpaper across plank seams while smoothing filler leaves deep cross-grain scratches that are highlighted by clear finishes.

Can You Fill Gaps in Floorboards Without Sanding the Floor?

Yes, you can fill gaps without sanding, but it involves trade-offs.

If your floor finish is in good condition and you want to reduce draughts or clean up debris traps, use a flexible acrylic mastic (like Bona Gap Master) colour-matched to your floor.

To complete this repair cleanly:

1. Apply low-tack painter’s tape along both board edges, leaving only the gap exposed.

2. Gun the flexible mastic smoothly into the void.

3. Smooth the bead flush using a damp profiling spatula or a gloved finger.

4. Peel the painter’s tape away immediately before the sealant skins over.

This technique seals the void and reduces drafts without damaging your clear finish. However, these repairs will remain slightly visible under angled light, as the sheen of the filler rarely matches the polyurethane topcoat.

Draughts, Insulation, and Pests: Do Filled Gaps Help?

Air leakage through suspended timber ground floors can account for up to 20% of domestic winter heat loss in older Melbourne properties. Sealing open board gaps blocks cold sub-floor wind draughts, keeping floor surfaces warmer and lowering space-heating costs.

Gap filling also creates a physical barrier against sub-floor pests such as silverfish, spiders, and ants.

Important Ventilation Caution: While sealing floorboard gaps from above is beneficial, you must never restrict exterior sub-floor wall vents to control drafts. Ground-level sub-floor airflow is vital for clearing rising damp. Sealing sub-floor vents can trap moisture, accelerating structural fungal rot and attracting subterranean termites.

When to Call a Professional Floor Sanding Contractor

While minor spot caulking is a manageable DIY weekend project, comprehensive floorboard gap filling across an entire home is technically demanding.

You should hire a licensed timber floor contractor if you encounter:

• Gaps wider than 5 mm across multiple boards: Wide openings throughout a room generally require custom-milled timber slivers and heavy commercial belt sanding to achieve a flat, unified plane.

• Cupped, crowned, or uneven planks: Filling gaps between warped boards will not correct an unlevel floor. The timber must be mechanically flattened by professional drum sanders.

• Parquetry separations: Block parquetry relies on strict perimeter expansion clearances. Filling parquetry seams incorrectly can cause the blocks to lift from the subfloor.

• Full-home rejuvenation: If your floorboards are worn, stained, or due for a topcoat refresh, gap filling should be incorporated into a complete trade sand-and-seal schedule.

Discover how our specialist floor sanding and polishing services restore heritage and contemporary timber surfaces across Melbourne.

Frequently Asked Questions

Can you fill gaps between floorboards with silicone?

No, never use standard bathroom or building silicone to fill timber floorboard gaps. Silicone cannot be sanded, will not bond with timber topcoats, and leaves an oily residue that prevents stains from adhering to the timber. If an elastic material is required, use a sandable, water-based acrylic timber mastic designed for floors.

Is wood filler or wood putty better for floorboards?

Wood filler is preferable when finishing bare timber prior to sanding, as it dries hard and accepts sealers. Wood putty (often oil-based) stays soft and pliable for longer, making it useful for tiny nail-hole touch-ups on pre-finished timber, but it should not be used across active floorboard seams.

Can you use expanding foam to fill gaps?

No. Standard expanding polyurethane foam expands aggressively and pushes boards out of alignment. It dries into a crumbly, unsightly cellular foam that cannot be sanded smooth or colour-blended to match timber. Use closed-cell foam backing rods instead.

How do you fill large gaps between floorboards?

The most reliable way to fill gaps wider than 4 mm is by fitting wedge-shaped timber slivers cut from the same timber species. Glued in place with cross-linking PVA glue and planed flush before sanding, timber slivers provide an authentic, durable wood repair that moves naturally with your floor.

Should I fill gaps in hardwood floors?

Only fill gaps in hardwood floors if they are permanent structural voids or causing troublesome air draughts. Fine hairline gaps are a natural response to seasonal humidity cycles and should be left alone to avoid crushing the boards when the timber expands during warmer months.

Will gaps in floorboards come back?

Yes, filled gaps can reopen if your home experiences severe seasonal humidity swings or significant sub-floor structural movement. While flexible acrylic sealants and timber slivers minimise this risk, no gap filling method permanently overrides the natural expansion and contraction of timber.

Can you fill gaps with sawdust and PVA glue?

Using standard PVA craft glue mixed with sawdust is not recommended. PVA glue dries into a gummy, rubbery consistency that gums up sanding belts and creates dark, dirty seams under clear finishes. Always use a dedicated, fast-curing commercial resin binder mixed with fine 80-to-100-grit floor sanding flour.

The golden rule for timber floorboards is simple: match the repair method to the width of the gap, use flexible materials or timber slivers for active voids, and complete structural filling during a full sand-and-polish cycle. If you are dealing with extensive gaps, loose boards, or heritage timber across Melbourne, the team at Boss Timber Flooring can inspect your space and restore your floor to a smooth, draught-free finish.