Gaps in Hardwood Floors: Seasonal Movement and When Filling Is Worth It
Thin gaps that open between boards in winter and close again in summer are usually normal for a hardwood floor. Knowing which gaps are seasonal and which ones signal a real problem is what tells you whether filling is worth the effort.
Why hardwood gaps come and go
Wood is hygroscopic, meaning it constantly takes on and gives off moisture to match the air around it. In Kansas City, that air changes dramatically through the year. Summer brings humid days that swell boards tight against each other, while winter heating dries the indoor air and pulls moisture back out of the wood. As boards lose moisture in winter, they shrink slightly across their width, and thin gaps open up between them. When summer humidity returns, the boards expand and most of those gaps close on their own. This back-and-forth is normal seasonal movement, not a defect.
Normal gaps versus problem gaps
The key question is whether a gap is seasonal or permanent. Seasonal gaps are typically narrow, fairly even from board to board, appear in the dry winter months, and shrink or disappear when humidity rises. A common rule of thumb is that a gap roughly the thickness of a dime in winter is within the range of normal for solid hardwood. Problem gaps behave differently: they stay open year-round, vary widely from one spot to another, keep getting larger over time, or are paired with cupping, crowning, or loose boards. Those wider, persistent gaps can point to a moisture issue under the floor, a subfloor problem, or wood that was never properly acclimated before installation. Telling the two apart matters, because filling a gap that is going to close again in summer can cause more harm than good.
When gap-filling is worth doing
For ordinary seasonal gaps, the best approach is usually to do nothing and instead manage indoor humidity so the swings are less extreme. Running a humidifier in winter to keep relative humidity in a reasonable range will narrow seasonal gaps without any filler at all. Filling becomes worth considering when gaps are permanent, stable, and wide enough to collect dirt or catch a heel, or when you are already refinishing the floor. The best time to fill is during a refinish, in the season when boards are at their widest swelling point, so filler is not squeezed out or cracked when the wood expands. A wood filler or a mix of fine sanding dust and binder can work for hairline permanent gaps, while wider permanent gaps are sometimes best addressed with thin wood slivers fitted between boards.
Why filling alone is not a fix
It is important to understand that filling a gap does not stop the wood from moving. If you fill a seasonal gap in winter, summer expansion can push the filler out or crack it, leaving the floor looking worse than before. That is why professionals first identify the cause and the pattern before reaching for filler. If the gaps trace back to a moisture source or an installation problem, addressing that is the real repair.
When to call in help
If you are not sure whether your gaps are normal seasonal movement or a sign of something deeper, a professional assessment is the safest path. KC Hardwood regularly evaluates gapping in Kansas City homes, separates normal seasonal behavior from real problems, and recommends gap-filling only when it will actually hold. If your floors have you wondering, reach out for an estimate and an honest look before any filler goes down.