1. Are Wider Gaps in a Gym Floor in Winter a Sign of Damage?
If you crouch down and look closely at a gym floor, you may see long boards lined up with thin lines between them. In some gyms, these lines widen in winter. The Forest Products Research Institute investigated four gymnasiums. In Facility A, the width of the seams between the long sides changed by an average of 0.6 mm and a maximum of 2.6 mm between summer and winter. The winter average reached about 3.0 mm at its maximum, while in summer, many seams were so tight that the boundary was hard to see [2].
It is important to note that 3.0 mm is the largest average value for that specific facility in winter. It is not the same for every gymnasium. So, why is wood used for gym floors in the first place?
2. Why Wood Is Said to Be Suitable for Running and Jumping
Every time you jump and land, impact returns to your feet from the floor. A slightly softer floor seems gentler on the body than hard concrete. It is explained that wooden floors have the power to soften this impact. A 1985 document from the Forest Products Research Institute states that wood is suitable for flooring because it has high insulation, appropriate elasticity, shock-absorbing power, and resistance to wear [1]. If you look at individual performance only, there are materials better than wood. However, having several of these properties at the same time is considered an advantage.
Unlike flooring glued directly onto concrete, wooden flooring is laid on top of a framework called neda (joists). Because of this, the floorboards also bear the load. The elasticity of the wood becomes the elasticity of the floor, making it suitable for gyms and stages [1]. However, this is a 1985 document, and it does not say that wood is the best material, only that it is suitable.
3. How Wood Releases Water and Shrinks in Dry Winters
Wood absorbs and releases moisture from the air, changing its water content (moisture content). It swells in humid summers and shrinks in dry winters. In Facility A, the average moisture content of the flooring changed by about 3 to 5% between summer and winter, dropping to around 8% in winter. Gym humidity varies by facility depending on heating, cooling, and usage. In Facility A, ceiling-blown heating lowered the relative humidity to about 20% in winter. Looking at monthly averages, there was a difference of 40 to 50% between summer and winter [2].
On the other hand, in Facilities B and D, where humidity changes were small, the change in moisture content was less than 1% [2]. The size of the gaps seems to be connected to how the air in the gymnasium is managed. By the way, is it only the visible boards that move? There are also boards under the floor.
4. Why the Plywood Under the Floor Is Laid in a Staggered Pattern
Under the floor, plywood is laid. In the gyms investigated, this plywood was installed using a "staggered" pattern. Staggered laying means shifting the seams of adjacent boards by half, similar to the arrangement of a brick wall [2].
The research team noticed a tendency for the change in floorboard seam width to be larger near the seams of the subfloor plywood. In Facility A, the plywood was installed with a 450 mm shift. In winter, there were places where the seams opened significantly at positions close to that interval [2]. However, this is not a confirmed fact that "the plywood seams are the cause." The report states that further detailed investigation is needed to understand how the movement of the plywood affects the floor [2].
Also, plywood and flooring wood shrink by different amounts. The shrinkage rate of lauan plywood is smaller than that of the flooring wood, and its seasonal change in seam width was also smaller than that of the flooring [3].
5. How Gaps Give Pressed Floor Boards Room to Move
The short seams between board ends are much narrower than the seams along the long sides. In Facility D, which uses composite flooring and is different from Facility A, the summer average was 0.02 mm, and even in winter, the average was 0.12 mm. Many seams remained tight even in winter. In summer, boards that had changed shape by pressing against each other were also seen [2].
Gaps seem to serve as an escape route for wood when it expands. The Maple Flooring Manufacturers Association (MFMA) in the United States specifies an "expansion void" near walls and columns to allow the floor to move. They explain that if this void is blocked by trash, the floorboards may press against each other and warp upward [4]. However, this is a document from a US organization and may not match the specifications of Japanese gymnasiums.
In recent years, injuries from splintered floorboards have occurred frequently in gyms. According to the introduction of the Forest Products Research Institute report, the Consumer Safety Investigation Commission published an investigation report in 2017, noting that aging and excessive absorption and release of moisture were listed as factors for cracking [2]. Because of this background, managing the temperature and humidity above and below the floor is considered important.
Safety Note: The Consumer Safety Investigation Commission reported in 2017 that aging and excessive moisture changes were factors in cracking. This context is introduced by the Forest Products Research Institute report; the commission report itself has not been directly verified here.
6. How to Investigate Your Own Gym Floor
Try crouching down and looking at the gym floor from the side. Are the boards lined up in "irregular lengths" (not all the same length)? Are the end seams staggered here and there? Where are the places where the lines are thin, and where are they wide?
If you see gaps, it might be interesting to record their location, such as near walls, windows, or where heating wind blows. The movement of the floor may be related to differences in humidity.
Gymnasium management is handled by schools or local governments. If you want to know more about the floor, ask the teacher in charge.