Flooring.lol Editor 
Every November the same thing happens in my hallway. The heating comes on, the air dries out, and by the first cold snap there is a dark line between two planks that was not there in September.
It looks like a fault. Most of the time it is the floor doing what the carton said it would: wood and wood-composite cores lose moisture when the air gets dry.
The hard part is deciding whether the gap is weather or a mistake, because the two need opposite responses. The step-by-step repair is in the winter gap fix. This is the part before it.
Why the heating brings gaps with it
Cold air holds very little moisture. Warm it up inside and its relative humidity drops, even though the water in it has not gone anywhere. That is why a heated room reads drier in January than a damp October, and why your skin, your door frames and your floor all shrink at once.
One plank barely moves. A room is twenty planks wide, and the movement adds up along the run. The floor pulls back from the walls, which is what the perimeter gap is for, and it pulls away from itself at the joints.
Heating dries the floor unevenly too. Planks near a radiator lose moisture first, so the gap in front of the sofa is often wider than the one on the cold side of the room.
Normal seasonal movement, or a real problem?
Seasonal movement has a signature. It arrives within a few weeks of the heating going on, it is widest in the coldest part of the year, and it closes in spring on its own. It is hairline, and it shows up at more than one joint.
A real problem looks different:
- One gap much wider than the rest, in the middle of a room.
- A gap that keeps growing after a month of steady heating.
- A peak or a lifted seam beside it. That floor is being pushed, not shrunk.
- Chipped or shiny plank edges where something has been rubbing.
- A gap you can see the underlayment through, or drop a coin into.
- A soft edge or a dark tide mark, which means water.
Then look at the trim. In winter the gap under the shoe moulding should be wider than in summer. If it still pinches the planks while the middle of the room opens up, the floor is being held.
How to measure the gap honestly
Eyeballing is useless: a gap changes with the light, and by February you will have decided it is either nothing or a catastrophe.
A feeler gauge is the right tool and costs less than a takeaway: a little fan of metal leaves in known thicknesses, sold for checking spark plug gaps. Slide leaves in until one drags, and you have a number you can compare in March. The tool list covers what else is worth owning.
No gauge? Use coins. A penny, a nickel and a quarter are three different thicknesses, which makes them a crude but repeatable set.
A scrap of paper is the finest gauge in the house. Printer paper slides into a gap you can barely see; a business card will not. If the paper goes and the card does not, the correct action is probably nothing.
Measure in the same spot every time, with a pencil mark on masking tape to find it again, and at the same time of day, before the heating has run for hours. Photograph the gap with the coin in frame; a reference beats a note saying “about 2 mm”.
End gap, side gap, seam gap
Three positions, three different stories.
An end gap is at the short end of a plank. Against a wall it is the perimeter gap, and it is supposed to be there. Between two plank ends in the same row it is a butt joint that has opened, which is the classic winter gap on laminate.
A side gap runs along the long edge, parallel with the boards, and it is rarely seasonal. The long edge carries the locking profile, and the lock is built to hold that joint closed. If a side gap opens, the tongue never seated, the locking edge is damaged, or something is stopping the floor from closing. Filling it keeps the joint open in spring.
A gap at a seam is where two rows meet and the plank ends line up, usually near a doorway or a hearth. That is where the floor carries the most movement through the smallest space. A clean gap that opens in winter and closes in summer is seasonal. A ragged one with crushed edges is a failed joint, and a jamb that was never undercut or a pinned transition strip will do it too.
You can wait out an end gap. A side gap usually means the row comes up.
What each floor type is telling you
The same gap means different things depending on what is under your feet.
| Floor type | What the gap means | What to do |
|---|---|---|
| Floating laminate | Planks contracted and a butt joint opened. Normal, within reason. | Measure it, keep the humidity up, check in spring. Still open when it is humid means damage. |
| Floating LVP | Less movement than laminate. A gap often means a pinched floor or a damaged click edge. | Check the perimeter and anything fixed through the floor. |
| Glued engineered | A failed glue line or a moving subfloor. It does not shrink as a sheet. | Do not fill it. This is a repair, not a humidity problem. |
| Solid hardwood | Real seasonal movement, and the most visible of the lot. | Restore the humidity and wait. Filler in December is squeezed out in July. |
| Tile | Not an expansion gap. A cracked grout line is movement or deflection. | Find why it cracked before re-grouting. A straight crack wants an expansion joint. |
Grout is rigid: it does not shrink in dry air and it does not stretch when the floor moves, so a cracked line is never the tile version of a winter gap. Refilling it every year hides a movement problem that will come back.
Why wax sticks and coloured putty are a trap
A wax stick looks like the answer. Warm it, press it in, wipe the surface, and the line disappears. For about six weeks.
Then the humidity returns, the planks close, and the filler has nowhere to go. On a floating floor it squeezes out as a proud bead or jams the locking edge so the planks cannot seat. Grit collects, and next winter the gap is wider.
The second problem is that a filled gap cannot be measured, and the method above depends on watching a number change.
Coloured putty is the same story with better marketing. On solid hardwood, a flexible filler in a small seasonal gap is defensible if you accept redoing it. On a floating floor it is a rigid wedge in a movement joint.
If the gap bothers you that much, the honest options are a rug, a better place for the furniture, or fixing the humidity.
Put the humidity back, slowly
The fix for a seasonal gap is not in the floor. It is in the air.
A cheap hygrometer, the little digital kind that sits on a shelf, tells you what you are dealing with. Keep it near the floor rather than on a cabinet, because the air at ankle height is the air the planks sit in.
A common target for indoor winter humidity is 35 to 50 percent relative humidity, but treat that as a range to check against your flooring manufacturer’s guidance rather than a rule. Some engineered products specify a tighter band, and the carton wins as usual.
Getting there is unglamorous: a humidifier sized for the room, run steadily, lids on pans while you cook, laundry on a rack indoors, the bathroom door open after a shower.
Two warnings. Do not chase the top of the range, or you get condensation on the windows and a floor that swells, which is how you trade a winter gap for a spring peak. And do not blast the heating, because a room that swings from cold to hot pulls the floor around more than one held steady.
When to stop watching and re-lay
Set a date and keep it. Measure on the first of the month, write the number on the tape beside the gap, and carry on until the heating has been off for a month.
If the gap has closed by then, it was seasonal. Do nothing, expect it again next winter, and buy the hygrometer before the heating goes on.
If it is still open, or it got wider while the humidity was climbing, stop waiting. The joint is damaged. Work back from the nearest wall, unzip the row to the open joint, and look at the tongue. A crushed locking edge means a new plank; an intact edge that was never seated gets tapped home with a block.
Re-lay also if the perimeter gap has grown past what the trim covers, or if the plank edges are chipped and shiny.
While the row is up, check the perimeter gap with a coin. A floor laid tight to the drywall opens at the seams in winter and peaks in spring; both come from the same missing gap. Why laminate peaks at the seams is the other half of this story.
The gap that closes by itself in May was never damage. The one still open in June is, and no amount of wax will talk it round. Measure it, write the number down, and let the calendar make the decision.