Shrinking and Swelling

By "movement of wood” we meant the shrinkage and swelling of air-dried wood as a result of normal changes in relative humidity. When the wood loses even more moisture from the fiber saturation point, it must therefore come out of the cell walls, causing them to deform. The wood will then shrink. When wood absorbs moisture below the fiber saturation point, it will swell. The speed and strength of these shape changes depend on the type of wood.

The Fiber Saturation Point of Wood

Shrinkage and swelling only occur in wood below the fiber saturation point. Above the fiber saturation point, the size of the piece of wood remains the same with varying moisture levels. The fiber saturation point varies for the different wood species between 20 and 36%. Often, the  average value of 27% is taken. To avoid excessive shrinkage, the wood must be dried to a moisture content that is the same as the average relative humidity of the environment in which it will be found.

French Oak Floor Van Gofh White by Uipkes

Direction of Shrinking and Swelling

The shrinking and swelling of wood does not occur to the same extent in all directions of the wood. The longitudinal shrinkage (direction of the fibers) is usually negligible in practice. Perpendicular to the fiber direction the shrinkage is greater, whereby in the direction parallel to the growth rings (tangential direction) the wood shrinks and swells about twice as much as in the direction of the rays (radial direction). This ratio can vary widely for different types of wood.

In general, heavier wood types shrink and swell more than lighter ones. The movement of the wood is also highly dependent on growth stresses and irregularities in the wood. The result can be; warping, twisting or tearing. As a result of the weather and the seasons, wood is usually exposed to changes in relative humidity and thus to shrinkage and swelling. A typical example is the door that gets stuck in a humid season and functions normally again in a drier period.

Wooden Floor Parts and Humidity

When you are going to install wooden floorboards, you should therefore take the humidity of the room into account. The ideal humidity is between 45 and 65%. You can measure this with a hygrometer. When installing a solid wooden floor, it is always taken into account that the wood continues to move. That is why a space is always left along the sides (expansion space) so that the floor can always adapt to the humidity in the room. When the humidity in the room where your solid planks are located becomes too high, the floor can expand to such an extent that the solid plank floor gets stuck between the walls, causing the floor to bulge.

If the humidity in your room is too high, you must ensure that there is good ventilation and that the temperature in the room where the floor is located is higher than the temperature outside. If the humidity in the room where the solid wooden floor is located is too low, you can keep it up to standard by purchasing a humidifier. A characteristic of a too low humidity is that the floor will shrink in the working width. At low humidity, your floor can also make noise when people step on the floor.