Wood floors move in June. What is normal and what is a warning

The gaps between boards that were obvious in February have closed. Some boards feel tight underfoot and one doorway threshold has started to bind. This is June behaving normally.

Wood is hygroscopic. It takes up moisture from the air and gives it back, and it changes dimension when it does, mostly across the width of a board rather than along its length. Indoor humidity in most of the country swings substantially between winter heating and summer weather, and a wood floor follows it.

What normal seasonal movement looks like

• Winter gaps that close in summer. Thin lines between boards in February, invisible in July. This is the floor's designed behavior and the expansion space at the walls exists for exactly this reason.
• Slight height differences at board edges that come and go with the season.
• A door that swings freely in January and rubs in August. Usually the door, not the floor.
• Sound changes. A floor is quieter when boards are tight and squeakier when they are dry and shrunken.

Wider boards move more than narrow ones, solid moves more than engineered, and a floor installed at an unusual moisture content moves more than one that was properly acclimated. None of that is a defect.

What is not normal

Four symptoms that indicate moisture arriving from somewhere other than the season.

1. Cupping, where board edges sit higher than their centers. This means the underside is wetter than the top, which points to a crawlspace, a slab, or a leak below rather than to summer air.
2. Buckling or tenting, where a board lifts off the subfloor entirely. This is expansion with nowhere to go, usually because the perimeter expansion gap was omitted or has been blocked by trim, cabinets or a later addition.
3. A localized problem. Seasonal movement affects the whole floor. A patch of cupping in front of a dishwasher, under a window, or along one wall is a leak.
4. Movement that does not reverse. A floor that gapped last winter, closed this summer, and gapped wider the next winter is losing ground, and the cause is usually a persistent moisture source.

Why summer is when the failures appear

The mechanism is simple compression. A floor with adequate expansion space absorbs summer growth into that space. A floor without it has nowhere to put the growth, so the boards press against each other, and the pressure has to go somewhere. It goes up.

Two things commonly remove the space that was originally there. Cabinets or a kitchen island installed on top of the floor after the fact, which pins it. And baseboards or shoe molding nailed through the flooring instead of into the wall, which does the same thing at the perimeter.

What actually helps

• Keep indoor humidity in a reasonable band year round. Flooring manufacturers publish a target range in their care instructions, and staying inside it is the single most effective thing a homeowner can do. In practice this means air conditioning or a dehumidifier in summer and often a humidifier in winter.
• Check the crawlspace. A ground vapor barrier and a dry crawlspace remove the most common source of cupping.
• Look at the outside. Gutters discharging at the foundation and grade sloping toward the house put water where it becomes floor movement.
• Do not sand a cupped floor while it is cupped. Correct the moisture, let it flatten over weeks or months, and only then assess whether sanding is needed. Sanding a cupped floor flat produces a crowned floor when it dries.

The summer jobs worth doing

If the floor is going to be refinished, summer is often a good window in a house with air conditioning, because indoor conditions can be held steady. If the house has no cooling, the humidity swings during cure are a real consideration and fall is usually better.

Two smaller tasks that suit the season: replacing felt pads under furniture, which are the main defense against the scratches that accumulate through a year, and resetting any shoe molding that was nailed through the floor rather than into the baseboard.

A floor that moves with the season and comes back is a floor doing its job. The ones worth acting on are the ones that move in one direction only, and June is when they announce themselves clearly enough to find.