Gaps opening up between floorboards is one of the most common things we get asked about, but the answer really depends. Some gaps should be filled. Some are better left alone. And some are telling you about a problem that filling them will only hide.
Filler in the wrong gap at the wrong time won't stay there. It'll crack, lift or get pushed proud of the boards, and you'll be looking at the same job again in twelve months.
Here's how to work out which situation you're in.
Why Gaps Appear in the First Place
Timber is a natural material, and it never really stops responding to the air around it. Through humid months, boards take on moisture and swell slightly. Through drier months, they release it and shrink back. On a solid timber floor, that movement shows up as gaps between the boards.
It's not a sign of poor installation, and on most solid floors it happens to some degree every year. What varies is how noticeable it gets, which depends on your climate, your heating and cooling, the species, and how the floor was laid.
Do Your Gaps Move?
Before you buy anything, work out whether the gaps you're looking at are seasonal or permanent.
The simplest test is time. Pick a gap that bothers you, measure it, and note the date. Check it again six months later. If it's noticeably narrower, you've got seasonal movement, and the gaps will keep opening and closing every year regardless of what you do. If it measures the same, the floor has settled and the gap is permanent.
When You're Better Off Not Filling
Seasonal gaps, with a rigid filler
If you fill a moving gap when it's at its widest with something that can't compress, you've left the boards nowhere to go when they swell again. The filler gets crushed and pushed up above the surface, or the boards themselves start to lift at the edges.
The expansion gap around the perimeter
The gap between your floor and the wall is what allows the whole floor to expand without buckling, and it should never be filled. That's what skirting boards and scotia are there to cover.
Gaps that are a symptom of something else
If the boards are cupping, if gaps have opened up in one section of the room but nowhere else, if boards are loose underfoot or the floor has started to squeak, filling is the wrong first move. Those are usually signs of a moisture issue, a subfloor problem or fixings that have let go, and filler will only cover it up while it gets worse. Sort out the cause first.
Gaps too wide to fill
Once you're past roughly 15mm, you're outside what a floor filler is designed to handle.
When Filling Is Worth It
As part of a sand and refinish
This is the ideal time. The floor is bare, the filler can be trowelled across the whole surface, sanded back flush and locked in under the coating. You get a far better result than filling a finished floor, and it barely adds to the job.
On parquet and mosaic floors
These have far more joints per square metre than a plank floor, and a whole-floor fill is standard practice. Our guide to sanding a parquet floor covers where this fits in the process.
One-off gaps in obvious places
A single wide gap in the middle of a living room doesn't need a full floor treatment. It just needs the right product.
Floors that don't move much
Engineered timber, direct-stick installations and floors in climate-controlled homes tend to be far more stable than a traditional solid strip floor, so gaps that appear are more likely to be permanent and worth filling properly.
Practical reasons
Dirt and crumbs collecting in the gaps, draughts coming up through the floor, or a pet that keeps catching a claw are all perfectly good reasons to fill, even if the gaps aren't bothering you visually.
Matching the Filler to the Gap
Once you've decided to fill, the product depends on the size of the gap and whether you're doing a few spots or the whole floor.
One-off gaps: Berger-Seidle Aqua-Seal FlexFill
An elastic, silicone-free joint filler that comes in a cartridge and can be used on both finished and unfinished floors. Because it stays flexible, it bridges natural timber movement rather than cracking out of the gap the first time the floor shifts. It also sands and coats over, so it works just as well during a refinish. Available in a range of colours to match different timbers.
Small gaps across the whole floor: Berger-Seidle Pafuki
Designed for joints up to about 2mm, and used during a sand and refinish. The liquid gets mixed with the fine sanding dust from your own floor, which is what gives it an exact colour match, then trowelled across the whole surface.
Wide gaps across the whole floor: Berger-Seidle Pak Stop
Built for gaps in the 3mm to 15mm range. It stays permanently elastic once cured, so it holds up on older floors that still have real seasonal movement in them, and it's the product that saves plenty of historic floors from being pulled up altogether.
You'll find all three, along with the trowels and colour options, in our Timber Gap Fillers collection.
Still Not Sure?
If you're weighing up whether to fill, sand or leave your floor alone, get in touch with our team.
Tell us what you're seeing and we'll point you toward the right product or let you know if it's a job better handled by our floor sanding team at Quicksand Flooring.
FAQs
Should I fill gaps between floorboards or leave them?
If the gaps open and close with the seasons, they're normal and are best either left alone or filled with a flexible product that can move with the boards. If they've stayed the same size year-round, the floor has settled and filling them permanently makes sense.
Can you fill gaps without sanding the whole floor?
Yes, for one-off gaps. Filling every gap across a whole floor is a different job, and that's best done as part of a full sand and refinish.
What size gap can you fill in a timber floor?
Small joints up to around 2mm are handled by a trowel filler like Pafuki. Gaps from roughly 3mm to 15mm need an elastic filler such as Pak Stop.
Why do gaps in my floor keep coming back after filling?
Almost always because the gap is still moving and the filler used couldn't move with it. Either the floor has significant seasonal movement and needs a flexible product, or there's an underlying moisture issue that needs addressing before any filling is worth doing.