Timber is a natural material, and solid hardwood floors move with the seasons. A few fine gaps in winter that close up again in summer are normal. Boards with raised edges, a dished centre, or gaps that keep widening year after year are a different story. Cupping floorboards and gaps in floorboards are usually about moisture, and the right repair starts with finding out where that moisture is coming from and whether the floor has stopped moving.
Design Timber Flooring is a mobile, owner-operated business working across Sydney. Jimmy has been working on Sydney timber floors since 2006, and he assesses the floor, explains the cause and carries out the repair himself. Quotes are free and on site, with a fixed-price written quote once the scope is clear. This page goes deeper on gaps, cupping and crowning; for the full range of repairs, see our hardwood floor repair service.
Cupping, crowning and gapping explained
These three problems look different but are closely related:
- Cupping: board edges sit higher than the centre, so each board looks slightly dished. It usually means the underside of the board holds more moisture than the top.
- Crowning: the centre sits higher than the edges. It often follows a cupped floor that was sanded flat while the boards were still wet, then dried out.
- Gapping: spaces open between boards as the timber loses moisture and shrinks.
| What you see | Likely cause | First thing to check |
|---|---|---|
| Even, fine gaps that appear in winter | Seasonal shrinkage, heating, dry air | Whether gaps close again in humid months |
| Wide gaps that never close | Boards laid too wet, long-term drying, old movement | Board moisture and history of the floor |
| Cupping across a whole room | Moisture from below, poor subfloor ventilation | Subfloor conditions, vents, drainage |
| Cupping in one area | Leak from an appliance, shower, pipe or wet entry | Plumbing and the source of water |
| Crowned boards | Cupped floor sanded too early, then dried | Moisture levels and when it was last sanded |
Seasonal gaps versus permanent gaps
With Sydney's humid summers and drier, heated winters, solid hardwood swells slightly in summer and shrinks in winter, so fine gaps that come and go are part of living with a timber floor. They are often more noticeable in heated rooms or near large sunny windows.
Permanent gaps stay open all year or keep widening. They can come from boards laid before they had acclimatised, long-term drying from heating or sun, or decades of movement in an older floor.
Moisture from below
When a whole floor cups, the cause is usually under the boards, not on top. Raised timber floors in Sydney sit over a subfloor space that needs airflow. Common causes of cupping include:
- Blocked or too few subfloor vents, often after garden beds, paving or extensions were added.
- Poor site drainage, with stormwater running under the house or pooling against the walls.
- Leaking downpipes, taps or plumbing under the floor.
- Damp ground and condensation in a closed-in crawl space.
Fixing these may need a plumber, drainage contractor or builder. Once the moisture source is addressed, boards often settle back noticeably, although that can take time. If the cupping started after a leak, our page on water damaged floorboards explains the drying and assessment process in more detail.
Why we don't sand cupped boards too early
It is tempting to sand a cupped floor flat straight away, but a cupped board is still wet underneath. Sand the raised edges off now, and when the board dries and flattens, the centre ends up higher than the edges. That is crowning, and it is far harder to fix because the timber removed cannot be put back.
So the order is: fix the moisture, allow the floor to stabilise, check moisture readings across the floor, and only then sand. Once the boards have settled, our sanding and polishing service can bring the surface back level and refresh the finish.
When to fill gaps, and when not to
Filling can help with fine, stable gaps in a floor that is being sanded, but it is not always the right answer.
Filling may suit small gaps that stay fairly consistent through the year, or gaps around old nail holes and minor defects as part of a sand and finish.
Filling usually doesn't suit seasonal gaps that open and close, because the floor will keep moving and can push filler out or crack it. It also isn't a fix for large gaps caused by ongoing moisture or structural movement.
For wide, permanent gaps, options can include replacing individual boards, or lifting and relaying a section of floor to close the gaps, with new boards laced in at the end. Our page on floorboard replacement and matching explains how that works. We will tell you if gaps are better left alone, which is sometimes the honest answer for a floor that moves with the seasons.
Acclimatisation and new flooring
For new solid hardwood, boards need to be close to the moisture conditions of the home before they are laid, on a dry, suitable subfloor. Boards laid too wet can gap as they dry, while boards laid too dry can swell and cup. This is part of why we check moisture before installing and before repairs, rather than following a fixed waiting period.
What Jimmy checks on site
- The pattern of gaps or cupping across the room and whether it is seasonal or permanent.
- Moisture readings in the affected boards compared with unaffected areas.
- Subfloor ventilation, drainage and signs of damp, wherever there is safe access.
- Any leaks from appliances, wet areas or plumbing near the affected area.
- The floor's sanding history and how much timber remains above the tongue.
- How the boards are fixed and whether any are loose or damaged.
Repair or replace?
Most cupped floors do not need replacing. Once the moisture is fixed and the boards have stabilised, a sand and finish is often enough. Replacement becomes more likely where boards have split, the cupping has not settled after the cause was fixed, the floor has already been sanded too thin, or the boards have crowned badly after an earlier sand. For permanent gaps, a few replaced or relaid boards are often the practical middle ground.
Engineered, laminate and hybrid floors
These repairs apply to solid hardwood floors. Engineered timber, laminate and hybrid floors react to moisture differently, and peaking, swelling joints or gaps in those products relate to the core, the underlay, expansion gaps and the manufacturer's installation requirements. If that is your floor, read about engineered timber flooring and get in touch with the product name so Jimmy can talk through what is realistic.
Gaps and cupping across Sydney
Jimmy works as a mobile service right across Greater Sydney. Cupping linked to subfloor ventilation and drainage is a common conversation in older homes with raised timber floors, such as those through the Inner West, and in homes close to the water on the Northern Beaches, where humidity can change noticeably through the year. For simple habits that reduce seasonal movement, see our guide to caring for timber floors.
Frequently asked questions
Are gaps between floorboards normal?
Some are. Solid hardwood shrinks in drier months and swells in humid weather, so fine gaps that open in winter and close in summer are normal. Gaps that stay open all year or keep widening are worth having assessed.
What causes floorboards to cup?
Cupping usually means the underside of the boards holds more moisture than the top. Common causes include poor subfloor ventilation, drainage problems, damp ground under a raised floor or a leak from plumbing or an appliance.
Will cupped floorboards flatten again?
Often they settle noticeably once the moisture source is fixed and the floor dries, although it can take time. Boards that were wet for a long time may not fully recover, and some may need replacing.
Can I sand my cupped floor flat?
Not until the moisture is fixed and the boards have stabilised. Sanding a cupped floor too early can leave it crowned once the boards dry, with the centre of each board higher than the edges.
Should I fill the gaps in my timber floor?
Stable, fine gaps can sometimes be filled during a sand and finish. Seasonal gaps that open and close are usually best left, because movement can crack or push out the filler. Large, permanent gaps may be better fixed by replacing or relaying boards.
Do you fix cupping in engineered or hybrid floors?
Our repair service is for solid hardwood. Engineered, laminate and hybrid floors behave differently with moisture, and the fix depends on the product and how it was installed. Share the product details and we can talk it through.
Send us photos of the gaps or cupping
Take a few photos of the affected area, including a low-angle shot across the boards to show any cupping, and send them with your suburb through our free quote form. Or call Jimmy on 0404 906 432 to book an on-site assessment.
