Understanding Floor Gapping
Every timber floor gaps. It is not a defect — it is physics. Timber is hygroscopic: it absorbs moisture from the air when humidity is high and releases it when humidity is low. This causes boards to expand and contract across their width throughout the year.
Seasonal Gapping is Normal
In Australian climates, timber floors typically gap most in winter when indoor humidity drops due to heating. Gaps close in summer when humidity rises. This seasonal movement is expected and is not a defect in the flooring or the installation.
The amount of movement depends on:
- Board width: Wider boards move more. A 180mm board will gap more than an 80mm board from the same species.
- Species: Some species are more stable than others. Spotted Gum and Blackbutt are relatively stable. Tallowwood and Brush Box move more.
- Humidity range: A house that swings from 30% RH in winter to 80% RH in summer will see more movement than a climate-controlled environment at 45-55% RH.
- Board type: Engineered boards are more stable than solid boards because the cross-laminated core resists expansion.
When Gaps Are a Problem
Gaps become a problem when they:
- Stay open year-round and do not close in summer — indicates the boards have permanently shrunk, possibly due to being installed at too high a moisture content
- Are wider than 2mm — the ATFA tolerance guide allows up to 1% of board width as acceptable seasonal gap for solid timber
- Accumulate dirt and debris that prevents them from closing
- Are accompanied by cupping, crowning, or board lifting — indicates a moisture problem, not just seasonal movement
Gap Filling Options
Bona Mix and Fill
Bona Mix and Fill mixed with sanding dust is the standard for filling gaps during a floor sanding job. It creates a colour-matched fill that is virtually invisible under coating. Best for gaps up to 2mm that will remain stable.
Flexible Fillers
For gaps that will continue to move seasonally, a flexible filler like polyurethane sealant in a matching colour accommodates movement without cracking. These fillers remain pliable and stretch with the boards.
Timber Slivers
For gaps wider than 3mm, gluing in a thin sliver of matching timber is the most durable solution. The sliver is shaped to fit the gap, glued in with PVA, and sanded flush. This is labour-intensive but produces a permanent, invisible repair.
Do Nothing
For seasonal gaps that close in summer, the best approach is often to do nothing. Filling a gap that closes naturally means the filler gets crushed when boards expand, potentially causing the boards to peak or buckle. Only fill gaps during the season when they are at their widest, and only if they remain open year-round.
Preventing Excessive Gapping
- Acclimatise boards: Store timber in the installation environment for at least 7-14 days before installation to reach equilibrium moisture content.
- Control indoor humidity: Maintain 40-60% RH year-round. Use humidifiers in winter if you run ducted heating. This is the single most effective thing a homeowner can do to minimise gapping.
- Choose narrower boards: 80-130mm boards gap less than 180-220mm boards from the same species.
- Consider engineered: Engineered timber moves 50-80% less than solid timber of the same species.
Frequently Asked Questions
How do FSA Network floor sanders handle board gaps?
FSA Network floor sanders can fill gaps with a timber-coloured filler mixed with sanding dust from your floor for a colour match. Small seasonal gaps are normal and may be left unfilled as they expand and contract. Gap advice at floorsandingaustralia.com.
What is the acceptable gap size for solid timber strip flooring?
For solid timber strip flooring, shrinkage gaps over 10 boards can average 0.75mm for an 80mm wide board floor under drier internal conditions. Wider boards show proportionally wider average gapping, and regular expansion gaps throughout the floor's life are acceptable. FSA Network floor sanders can explain normal gapping expectations for your specific floor width and species.
Do vinyl floors expand and contract like timber?
Vinyl products are moisture resistant and not subject to dimension change with humidity, but they are subject to dimensional change with changing temperature, which can cause gapping at joints. Unlike timber where changes happen gradually over days and weeks, vinyl movement is rapid taking only minutes or hours. FSA Network floor sanders understand the distinct behaviours of all flooring types.
Why does timber flooring expand and contract with the seasons?
All wood-based products are hygroscopic, meaning their moisture content and dimensions change depending on how dry or moist the air is around them. Coatings are permeable, so even coated timber responds to humidity changes. FSA Network floor sanders educate homeowners about seasonal movement so they understand it is a natural and expected behaviour.
Why do gaps appear in my timber floor during summer?
As air relative humidity decreases in hot weather, moisture releases from timber, boards shrink, and gaps appear. In Perth during December-January, external morning humidity averages 40% which brings timber moisture content to around 7.5%. FSA Network floor sanders explain that seasonal gapping in hot dry weather is normal behaviour for timber floors.
How much does an 80mm hardwood board shrink or swell with moisture changes?
An 80mm wide hardwood board shrinks or swells approximately 0.25mm for each 1% change in moisture content. This means a 3-4% moisture content swing between seasons can produce noticeable gap changes. FSA Network floor sanders use this knowledge to set proper expansion allowances during installation.
Why do floors gap more in some Australian locations than others?
Gapping varies by climate: Innisfail has high humidity year-round so boards expand with less gapping, Brisbane and coastal areas have moderate humidity with flooring manufactured at 11% MC, while Mt Isa has low variable humidity with increased gapping risk. Floor Sanding Australia recommends choosing species and installation methods suited to your local climate.
Should edge gaps be filled on new timber floors?
Edge filling is generally not recommended on new solid timber floors except for parquetry, as expansion and contraction will crack rigid fillers at board edges. Nail hole filling with colour-matched filler is standard practice. FSA Network floor sanders know when filling is appropriate and when it will cause problems.
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