Why Floorboards Gap in Winter and Cup in Summer (and When to Worry)

Every winter I get calls about gaps opening up between floorboards. Every summer I get calls about boards lifting at the edges. Most of the time, the floor is just doing what timber does. Sometimes, though, it's the first sign of a leak or a damp problem under the house that will keep getting worse until someone fixes it.

This guide shows you how to tell the two apart. I've worked on Sydney timber floors since 2006, and these are the checks I run when someone asks me whether their floor is "normal". You can do most of them yourself with a torch, a coin and a phone camera.

Your floor moves with the weather

Timber is hygroscopic. That's a technical way of saying it takes in moisture from the air when it's humid and gives it back when the air is dry. As it does, each board swells and shrinks a little across its width.

Sydney makes this easy to see. Summers are humid, so boards swell and the gaps close up. Winters are drier, and with heaters running, boards shrink and fine gaps appear. Air conditioning dries the air too. Rooms with big sunny windows or a heater usually show it most.

Wider boards move more than narrow ones, because there's more timber in each board to swell and shrink. Some species are known for moving more than others, too.

A gap in July isn't a fault. A gap that's wider every July is.

The four shapes a floor can take

1. Seasonal gaps: the normal kind

Seasonal gaps are fine and fairly even, appear in the cooler, drier months and close up again when the weather turns humid. They aren't a defect, and they don't need filling.

One version that looks worse than it is: sometimes a coating glues groups of boards together along their edges. Instead of a hairline gap between every board, you get a wider gap every few boards. It's the same amount of movement, just bunched up. It's more noticeable, but the cause is the same.

2. Gaps that never close

If gaps stay open all year, or get a little wider every winter, something else is going on. Common causes:

• Boards laid before they'd adjusted to the conditions in the house, then dried out and shrunk.
• A house that's become much drier than it used to be, from new heating, air conditioning or more sun.
• Decades of movement in an older floor, with fixings loosening over time.

Permanent gaps can often be fixed. Depending on the floor, that might mean filling, re-fixing loose boards or taking up and re-laying a section. The timing matters. Fill a gap in the driest part of winter with something rigid, and summer swelling can squeeze the filler out or push the boards up.

3. Cupping across a whole room

When a whole floor cups, the cause is almost always below the boards. Raised timber floors in Sydney sit over a crawl space that needs air moving through it. If that space gets damp, the undersides of the boards take in more moisture than the tops, swell more, and the edges curl up.

The usual causes:

• Subfloor vents blocked by garden beds, paving, decks or an extension.
• Not enough vents for the size of the house.
• Stormwater running under the house, or pooling against the walls.
• Downpipes that leak or empty right next to the house.
• Leaking pipes under the floor.

This is common in older homes with raised timber floors, from the Inner West to the North Shore. The fix usually starts with a plumber, a drainage contractor or a builder, not a floor sander.

4. Cupping in one spot

Cupping in one area, like in front of the dishwasher, next to the laundry or beside a shower, nearly always means a leak. It can be slow and quiet. A drip under a sink can wet the same boards for months before anyone notices.

Find and fix the leak first. Then the boards need time to dry. If you've just found the leak, read what to do in the first 48 hours after water damage before you do anything else.

5. Crowning: when a floor was sanded too early

Crowning is the opposite of cupping. The middle of each board sits higher than the edges. It usually happens when a cupped floor is sanded flat while it's still damp. Sanding takes timber off the raised edges. Then, as the boards dry and flatten, the edges end up lower than the middle.

That's why I won't sand a cupped floor until the moisture problem is fixed and the readings show the boards have settled. Sanding too early fixes the look for a few months and makes the real problem harder to fix later.

6. Buckling and lifting: call someone now

If boards are lifting off the floor, pushing up into a peak or pressing hard against the walls, the floor has taken on a lot of moisture or has run out of room to expand. Common causes are a flood, a burst pipe or a floor laid without enough expansion gap around the edges.

This one won't fix itself. Find the water source, take photos and get the floor looked at. Our cupping and gapping repair service covers how we assess and repair it.

What not to do

• Don't fill seasonal gaps with rigid filler in winter. When the boards swell in summer, the filler gets squeezed out or the boards get pushed up.
• Don't sand a cupped floor before the moisture is fixed and the boards have settled.
• Don't try to dry it with heaters. Fast, uneven drying can split and crack boards.
• Don't screw or nail down cupped boards to flatten them. The moisture is still there, and the boards will keep fighting the fixings.

What a proper fix looks like

1. Find the cause. Check moisture in the affected boards against dry boards elsewhere in the same floor, and look at the subfloor.
2. Fix the source. A plumber, drainage contractor or builder may need to do this part.
3. Let the floor settle. Boards often flatten noticeably once the moisture is gone, but it takes time. Moisture readings tell you when it's ready, not the calendar.
4. Repair what's left. That might be nothing, re-fixing loose boards, filling permanent gaps, replacing damaged boards or a full sand and polish.

If the floor has been through a few rounds of this over the years, it's worth checking whether it can take another sand. I've covered that in sand and polish or replace?. Our timber floor repair service covers the full range of repairs.

How to keep movement small

You can't stop timber moving, but you can stop it moving more than it needs to:

• Keep subfloor vents clear and make sure water drains away from the house.
• Fix leaks quickly, even small ones.
• Avoid blasting heaters or air conditioning straight at the floor.
• Use curtains or blinds on big sunny windows.
• Put mats at outside doors so water doesn't get tracked in.

Our guide to caring for your timber floor covers day-to-day care.

Check it yourself, or call someone in?

The coin photos, the walk around the house and a look underneath will tell you a lot, and they cost nothing. If the gaps close in summer and nothing is cupping, you probably don't need anyone.

If you've got cupping, gaps that keep growing, boards lifting or a spot that feels soft, it's worth getting moisture readings from someone who works on these floors every week. Finding the cause is the part people most often skip, and it's why some floors get "fixed" twice.