How to Fill Gaps in Hardwood Floors: Causes, Methods, and When to Call a Pro

Gaps between hardwood floor planks are one of the most common complaints from wood floor owners, and they almost always prompt the same questions: is this normal, is it serious, and what’s the best way to fix it? The answers depend on the size of the gap, whether it’s seasonal or permanent, and what caused it in the first place. Getting that diagnosis right before reaching for any filler saves a lot of wasted effort.

This guide covers the full picture – why gaps form, which situations actually need filling, and every repair method matched to the right gap size.

What We’ll Cover

• Why gaps form in hardwood floors and what’s considered normal
• How to tell a seasonal gap from a permanent one
• When filling is the right call and when it isn’t
• Every gap filling method organised by gap size
• How to fill large gaps in hardwood floors properly
• Mistakes that make hardwood floor gaps worse
• How to prevent gaps in new flooring installations
• When professional repair is the better option
• Answers to the most common questions about gaps in hardwood floors

Why Gaps Form in Hardwood Floors

Understanding the cause matters because a gap caused by seasonal humidity change behaves completely differently from one caused by structural movement or installation error. Applying filler to a seasonal gap that will close in summer can cause the boards to buckle as they push against each other.

Seasonal Humidity Movement

This is the most common cause of gaps between hardwood floor planks, and it’s the one most often mistaken for a problem when it’s actually normal. Wood is hygroscopic – it absorbs and releases moisture from the surrounding air throughout the year. In dry conditions, typically winter in both Vancouver and Edmonton, wood loses moisture and contracts slightly. The boards pull away from each other and small gaps open. When humidity rises in spring and summer, the boards absorb moisture and swell back toward their original position. Gaps that open in winter and close in summer are seasonal gaps. They don’t need to be filled.

Permanent Gaps from Drying or Shrinkage

Some gaps are permanent. These occur when the wood has dried out significantly over years of low humidity exposure, or when the original installation used wood that hadn’t been properly acclimated to the room’s conditions. Flooring installed at a higher moisture content than the room will sustain will shrink as it dries, and the resulting gaps don’t close seasonally because the wood has reached its stable, dry state. These are the gaps worth addressing.

Poor Original Installation

Flooring laid over an uneven subfloor, installed without adequate fasteners, or installed without proper acclimation often develops gaps at a rate beyond normal seasonal movement. If gaps appeared relatively quickly after installation and are widespread rather than isolated, installation conditions are the likely cause.

Old Wood Floors

In older homes, gaps in hardwood or wood plank floors are often simply the result of decades of seasonal cycling. The wood has dried, the original tongue-and-groove fit has loosened, and the boards have moved enough times that they no longer sit snugly against each other. Filling gaps in old wood floors requires choosing methods flexible enough to accommodate whatever remaining seasonal movement the floor still undergoes.

How to Tell a Seasonal Gap from a Permanent One

The test is straightforward. If your gaps are noticeable in winter but noticeably smaller or closed entirely by late spring and summer, they’re seasonal. Leave them alone. Filling seasonal gaps with a rigid filler creates a different problem: when the boards swell back in humid conditions, they have nowhere to expand and the filler either pops out, cracks, or the boards cup.

If your gaps remain open and unchanged through humid summer months, they’re permanent and worth filling.

Mark a few gaps with a pencil line at each edge in late winter. Check back in midsummer. If the marks have moved closer together, the floor is still moving seasonally. If the marks are in the same position, the gap is stable and can be filled without risk of the filler being forced out.

When Filling Is the Right Call and When It Isn’t

Not every gap needs to be filled, and filling the wrong gap with the wrong method creates more work than it solves.

Fill when: gaps are permanent, wide enough to collect debris and drafts, visually prominent, or causing a trip hazard at board edges.

Don’t fill when: gaps are small and clearly seasonal, when the cause hasn’t been identified and may be ongoing, or when the gap is in a location that will be covered by furniture or rugs.

For widespread gaps across a large floor area, filling individual gaps with putty or rope is a temporary measure. If the gapping is extensive, the floor may benefit from being lifted, re-laid tightly, and refastened – a professional repair that produces a far cleaner result than filling dozens of individual gaps one by one.

Filling Gaps in Hardwood Floors by Size

The gap size determines which method is appropriate. Using a rigid filler in a gap that still moves seasonally, or using a soft filler in a gap wide enough to need structural support, both lead to the filler failing within a year.

Small Gaps Under 3mm: Wood Putty or Flexible Filler

For narrow gaps – under about 3mm – wood putty or a flexible acrylic gap filler are the practical options.

Wood putty comes pre-mixed in a range of wood-matching colours and is available at any hardware store. Clean the gap of dust and debris first, apply the putty with a putty knife, press it firmly into the gap, and wipe excess off both sides with a slightly damp cloth. Allow to cure fully before walking on it. The limitation of wood putty is that it’s rigid when cured – it works well for stable, permanent gaps but will crack if the gap opens and closes seasonally.

Flexible acrylic gap filler is the better choice for any gap that may still have minor seasonal movement. Unlike rigid putty, it cures to a flexible consistency that can accommodate slight expansion and contraction without cracking or pulling away from the board edges. Apply it with a caulk gun or spatula, smooth it flush with the surface, and remove excess before it cures. Flexible fillers are available in wood-matching tones and hold up considerably better than putty in real-world conditions.

Small Gaps: Sawdust and Resin Mix

For gaps under 5mm, particularly during a refinishing project, mixing fine sawdust from the same species as the floor with wood resin or PVA glue creates a filler that closely matches the floor’s colour and grain. The sawdust takes the stain from the resin and the result blends in better than any commercially coloured putty because it’s made from the actual wood. Apply the paste into the gap, press it down, allow it to cure, then sand flush when doing the broader refinishing work.

Medium Gaps Between 3mm and 8mm: Rope or Jute

Natural fibre rope or jute cord works well for gaps in this range, particularly in older wide-plank floors where some seasonal movement is still expected. The rope compresses into the gap and stays there without becoming rigid, which means it accommodates movement rather than fighting it.

Choose a rope or cord slightly wider than the gap so it sits snugly when pressed in. Stain it to match the floor colour and let it dry before installation. Apply a thin line of wood glue in the gap for extra hold, then press the stained rope in firmly with a putty knife or blunt tool until it sits flush or very slightly below the surface. Trim any excess cleanly with a sharp knife. For a harder surface, a coat of clear polyurethane over the top protects the rope and ties it into the floor’s finish level.

Large Gaps Over 8mm: Wood Strips or Splines

Large gaps between hardwood floor planks need a structural fill rather than a paste or cord. Wood strips cut to the width and depth of the gap are the correct method here, and the result looks far better than any filler approach at this scale.

Cut thin strips of wood from the same species as the floor, or as close a match as possible. The strip should fit snugly – not so tight that it requires forcing, but with no play once inserted. Apply wood glue to both sides of the strip and into the gap, press the strip firmly in place, and tap it gently with a rubber mallet if needed. Wipe excess glue immediately. Once the glue has cured, sand the strip flush with the surrounding floor surface using a hand sander or orbital sander. Finish with a matching stain and sealant coat to blend the repair into the surrounding floor.

For floors where matching strips from the original species aren’t available, splines – thin pieces of wood designed specifically for this purpose – can be sourced from flooring suppliers and cut to length.

Very Wide Gaps: Full Board Replacement

Gaps wider than about 15mm, or gaps where the board edges are visibly damaged, split, or cupped, are generally beyond what any filling method handles well. At that point, replacing the affected boards is the correct approach. A professional can remove the damaged boards cleanly, source matching material, and relay the replacement boards so the repair is nearly invisible.

Filling Gaps in Old Wood Floors: What’s Different

Older floors have usually undergone decades of seasonal cycling, which means the board edges may be slightly rounded from wear and the tongue-and-groove profile may be loose or partially worn away. These floors often have more variation in gap width from board to board, which makes a single filling method inconsistent across the floor.

The most practical approach for widespread gaps in old wood floors is to combine methods by gap size: flexible filler for the smaller gaps, rope or jute for medium ones, and wood strips for anything substantial. Attempting to fill everything with one product either results in overkill for the small gaps or inadequate fill for the large ones.

Old floors that have been painted, stained, or finished multiple times also have residue in the gaps that needs to be cleared out before any filler will bond correctly. A thin tool or screwdriver to clean the gap, followed by a vacuum, is the minimum preparation before applying anything.

Mistakes That Make Hardwood Floor Gaps Worse

Using rigid filler in a seasonal gap is the most common error. When the boards expand back toward each other in humid conditions, rigid filler has no give. It either pops out – leaving a worse-looking gap than before – or it holds and forces the boards to cup or buckle.

Filling without cleaning the gap first. Filler applied over dust and debris doesn’t bond to the wood edges and fails quickly. The gap needs to be vacuumed and the edges wiped down before any product is applied.

Applying filler that’s too light or too dark. Colour matching in bright artificial light rather than in the actual room under natural light leads to visible repairs. Always test a filler colour on a scrap piece before committing to the full floor.

Overfilling and not sanding flush. Filler that sits proud of the surrounding floor surface is a trip hazard and looks worse than the original gap. Sand it flush once cured.

How to Prevent Gaps in New Flooring Installations

Gaps are largely preventable when the installation is done correctly from the start.

Acclimate the flooring in the room where it will be installed for at least three to seven days before laying it. This brings the wood to the equilibrium moisture content it will maintain in that specific environment and prevents the shrinkage that causes gaps to develop in the months after installation.

Maintain indoor humidity between 35% and 55% year-round after installation. In Edmonton’s dry winters especially, this means running a humidifier during the heating season. Consistent humidity is the single most effective long-term protection against gap formation.

Use proper fastener spacing during installation. Under-nailed floors allow boards to drift apart over time. Following manufacturer spacing recommendations for the species and thickness keeps boards where they were placed. VC Floor’s hardwood flooring installation service includes subfloor preparation, moisture testing, and proper acclimation as part of every project – the steps that prevent gap problems from developing in the first place.

When Professional Repair Is the Better Option

Most small to medium gaps are manageable as DIY repairs with the right materials. Call a professional when:

Gaps are widespread across the entire floor rather than isolated to a few boards. Filling dozens of individual gaps is time-consuming and rarely produces a consistent result. Re-laying the affected section produces a cleaner outcome.

The gaps are accompanied by cupping, buckling, or board movement underfoot. These indicate a moisture or structural problem that gap filling won’t solve.

The floor needs refinishing alongside the gap repair. Combining the two means the filler and the floor finish are applied and matched together, which produces the most invisible result.

VC Floor handles hardwood floor repairs across Vancouver and Edmonton, including board-level repairs, subfloor assessment, and refinishing where the damage is beyond surface-level fixes. For more on repair services, visit the floor repair page.

Frequently Asked Questions