The short version
- Gaps that open in winter and close in summer are the floor working normally.
- A gap that stays open through a humid August is a different problem and worth looking at.
- Cupping, where board edges rise above the middle, means moisture is coming from below.
- Crowning, the opposite, usually means a cupped floor was sanded flat before it dried.
- Our data at /data/ carries the age of the housing in 635 places, and older stock means solid boards that move across their full thickness.
What is actually happening in the board
Wood exchanges moisture with the air around it continuously, and a solid board does that across its whole thickness, so it is measurably wider in a humid August than in a dry February.
Heating season is the driver. Indoor air in a Philadelphia winter is dry, the boards give up moisture, and each one narrows by a small amount. Multiply a small amount by the number of boards across a room and you get a visible line at every seam.
In summer the process reverses. That is why the honest advice for a seasonal gap is usually to wait and look again in August, which is not the advice people expect from a contractor.
It is also why humidity control matters more than any repair. A house kept at a stable humidity moves less, and a floor that moves less gaps less.
Before paying to fill gaps, mark one with a pencil and look at it in August. If it closed, nothing needs fixing.
When a gap is not seasonal
A gap that stays open through a humid month, or one that is much wider than its neighbours, is not the floor breathing and is worth looking at.
Isolated wide gaps often mean a board has come loose from its fastening, or that a section was installed at a much different moisture content than the rest of the room.
Gaps that appear along one wall, or only in one room, point at a moisture source rather than at the season: a crawlspace, a slab, a bathroom that vents into the ceiling instead of outside.
Filling a seasonal gap in February is the most common wasted repair in this trade. The filler is rigid, the boards swell in July, and the floor pushes the filler out or crushes it.
Cupping and crowning are the two that matter
Cupping means the edges of the boards are higher than their centers, which points at moisture from below; crowning is the opposite and usually means a cupped floor was sanded flat before it dried.
Cupping is a gradient: the underside of the board is wetter than the top, so the bottom swells more and the edges rise. A leak, a damp crawlspace or a slab without a vapour barrier will do it.
The important thing about cupping is that it often reverses once the assembly dries. That is why sanding a cupped floor flat while it is still wet is the most expensive mistake available: when the middle comes back up, the board is thinner at the edges than the center and the floor crowns.
So the sequence is find the source, dry the assembly, measure, and only then decide. It takes weeks and it is worth every one of them.
How much movement is normal
Movement is proportional to the width of the board, so a narrow strip floor shows hairline gaps and a wide plank floor shows gaps about three times as large under the same conditions.
A two and a quarter inch strip moves a small amount across its width from summer to winter. Across a fifteen foot room there are around eighty seams to share that movement, so each one opens very little.
A seven inch plank moves roughly three times as much per board, with a third as many seams. The same room shows fewer gaps, and each one is wide enough to notice from standing height.
Neither floor is defective. The second one is simply showing what wood does at that width, and it is worth knowing before choosing wide plank for a house with dry winters.
| Floor | Typical winter behavior | What to do |
|---|---|---|
| Solid strip, narrow | Hairline gaps at most seams | Nothing. They close in summer. |
| Solid plank, wide | Visible gaps at every seam | Raise indoor humidity. Do not fill. |
| Engineered | Little or no gapping | Keep humidity within the maker's range. |
| Any floor, one wide gap | A single gap far larger than its neighbors | Have it looked at. Something is loose. |
Keeping humidity steady in an old house
A humidifier through the heating season narrows the swing the floor goes through, and a simple hygrometer shows whether it is working.
Houses with forced air heating can take a humidifier fitted to the furnace, which treats the whole house. Houses with radiators, common in older Philadelphia housing, have no ducts, so the answer is one or more room humidifiers in the rooms with wood floors.
A hygrometer costs very little and removes the guesswork. It shows the relative humidity in the room, and watching it through a winter tells you how far the air swings between a cold dry week and a mild one.
In summer the problem reverses. Air conditioning or a dehumidifier keeps the boards from swelling against each other, which matters most on the ground floor and over a damp basement.
Old floors and gaps that never close
In very old floors some gaps are permanent, because the boards were once swollen tight against each other and the wood at their edges was crushed.
When boards take on a great deal of moisture, from a leak or a damp season in a closed house, they swell and press hard against their neighbors. The wood fibers at the edges are compressed. When the boards dry, they shrink back narrower than they started, and the gap that opens does not close again.
A century gives a floor many chances for that to happen. Add several sandings, each of which removes the tightest part of the joint at the surface, and an old floor ends up with gaps that are simply part of it.
The dataset we publish at /data/ shows how much of this housing there is: 138 of 635 towns and neighborhoods have 35 percent or more of their housing built before 1940. In those floors a stable gap is character, and it can be filled when the floor is refinished if the owner prefers.
What actually fixes a gap that needs fixing
A loose board is refastened, a shrunken or damaged board is replaced, and a stable gap is filled with a sliver of matching wood or with filler during refinishing.
The repair depends on the cause. A board that has come away from its fastening moves when stepped on and opens a gap beside it. Fastening it into the joist or subfloor closes the gap and stops the movement.
A wide, stable gap in an old floor can be closed with a thin strip of the same species, glued along one edge only, so the boards on either side can still move. Sanded and finished with the floor, it reads as part of the board.
Filler troweled across the floor during refinishing handles small gaps and nail holes. It is the right tool for gaps that have stopped moving and the wrong one for seasonal gaps, which will crack it by the following summer.
New floors that gap in their first winter
A new floor that opens wide gaps in its first heating season was usually installed with more moisture in it than the house holds in winter.
Flooring arrives from the mill at one moisture content and lives in the house at another. If it is nailed down before the two have come together, it finishes drying in place, and every board shrinks.
The installation guideline sets the target: flooring and subfloor within four percentage points of each other for narrow strip, and within two for plank three inches and wider. Meeting it takes a meter and some patience, since boxes left in a house for a few days have not necessarily reached it.
Gaps from that cause do not fully close in summer. If a new floor shows them, the first step is to get the humidity under control and see how much returns over a full year. What remains is then assessed board by board.
