Thin lines open between the boards every winter and close again by summer. When that is the floor doing what wood does, when it points to something else, and how humidity changes it.
Every year, a few weeks after the furnace comes on, we start getting the same call: there are gaps between the boards that were not there in August. Most of the time that is a hardwood floor doing exactly what wood does in a central Indiana winter, and the gaps will close on their own when spring comes. Sometimes it is a sign of something else. This guide explains why wood floors move with the seasons, how to tell normal movement from a real problem, and what you can do to keep it small.
Wood holds water, and it trades that water with the air around it. When the air is humid the boards take on moisture and swell slightly across their width. When the air is dry they give it up and shrink. A single board only moves a tiny amount, but across a room full of boards, that movement shows up at the seams.
Central Indiana swings hard between the two. Summers are humid, often with the windows open or the air conditioning working against sticky outdoor air. Winters are dry outside, and heating that cold air makes it drier still indoors. So a solid wood floor is slightly fuller in summer and slightly smaller in winter, every year, for as long as it is down.
Boards move across their width far more than along their length, which is why the gaps run along the long edges of the planks rather than at the ends.
Normal seasonal gaps are thin, fairly even, and spread across the floor rather than concentrated in one spot. They tend to show up a few weeks into the heating season, are most noticeable in the driest part of the winter, and close again as the house becomes humid in late spring and summer.
Wider boards show bigger gaps, because each board has more wood to shrink. A wide-plank solid floor will usually open more visibly than a narrow strip floor in the same house. Rooms nearest the furnace, heat registers, a fireplace or a wood stove often show it first.
If that describes your floor, nothing is wrong with it. Wood floors in homes all over the state do the same thing.
A few patterns are worth a closer look, because they are not just the season.
Gaps that never close, even in the middle of summer, can mean the floor was installed when the wood was wetter than the house it went into, so it shrank once and stayed shrunk. A single wide gap in one place, with tight boards around it, can mean boards have shifted or a fastener has let go. Gaps with boards that move, flex or squeak underfoot suggest a fastening or subfloor issue rather than humidity.
Cupping is the other thing to watch. When the edges of each board sit higher than the middle, the board is taking on moisture from below or from the seams. In summer that is often humidity; in winter, when the air is dry, cupping usually means moisture is coming from somewhere it should not, such as a damp crawl space, a slab, a leak or a cleaning habit. How to clean hardwood floors covers the cleaning side. Crowning, where the middle of the board is higher than the edges, is often what is left when a cupped floor was sanded flat before it had dried out.
Buckling, where boards lift off the subfloor, is a moisture problem that needs attention quickly.
The amount a floor moves depends on how far the indoor humidity swings, so the most useful thing you can do is narrow that swing. In winter that means adding moisture: a whole-house humidifier on the furnace, or room humidifiers in the rooms with wood floors. In summer it means air conditioning or a dehumidifier, especially in a humid basement.
Most hardwood manufacturers publish a relative humidity range they want the house kept within, often somewhere in the 30s to 50s percent, and some make it a condition of the warranty. Check the range for your specific floor rather than assuming. An inexpensive hygrometer, the small humidity gauge sold at hardware stores, shows what your house is actually doing.
Do not overcorrect. Very high indoor humidity in winter can cause condensation on cold windows and in walls. The goal is steady and moderate, not humid. If you are unsure what your house can handle, your heating and cooling contractor can advise on a furnace humidifier.
It is tempting to fill the gaps with wood filler or caulk. On a floor that moves with the seasons, that usually makes things worse. The filler fills the gap while the boards are small, and when the boards swell again in summer there is nowhere for them to go. The filler cracks and pops out, or the boards push against each other hard enough to cup.
If a floor has gaps that stay open all year and the house is kept at a reasonable humidity, there are repair options, but they should be decided by someone who has looked at the floor in more than one season.
Engineered hardwood has a real wood top layer on a core of cross-laid plies, and that construction resists the seasonal swelling and shrinking much better than a solid board. It can still show slight seasonal change, but far less, which is one reason it is a good choice for wide planks and for rooms over a slab. Engineered hardwood in Indiana homes explains how it is built.
Luxury vinyl plank does not hold water the way wood does, so humidity does not move it in the same way. It does move with temperature, which is why a floating vinyl floor still needs its expansion gap at the walls. Hardwood vs LVP for Indiana homes compares the two in more detail.
A solid hardwood floor installed in winter goes down when the boards are at their smallest, and one installed in summer when they are at their fullest. Both are fine when the wood has been allowed to adjust to the house before it is installed and the installer leaves room for movement at the walls. That is why solid wood is acclimated and moisture-checked before it goes down, and why the subfloor is checked too. Subfloor prep covers what we look at.
If you are planning a new wood floor now, it is worth getting the humidity in the house steady first, so the floor is installed into the conditions it will live in. What happens during a flooring installation walks through the rest of the process.
If the gaps are thin and even and showed up with the heating season, give the floor until late spring before worrying about it. If you are seeing cupping in winter, boards lifting, gaps that never close, or one board pulling away from the rest, it is worth having someone look before it gets worse.
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