Direction of installation determines the size of gap

Wood is a natural product and hygroscopic to that, which means it will absorbs moist from its surroundings when the air is more humid than the moist content in the wood itself and will release moist to the air when the air is dryer - like in Winter when the central heating is on. Because of this absorbing and releasing wood expands and shrinks - more moist is more mass is more material and vice versa.

A plank/board/block of wood expands/shrinks across the width and hardly length ways.

The total width of your floor determines the size of the expansion gap you have to keep all around to give your Solid wooden floor room to "move". We're basing the following examples on a room 6 by 4 meter:

The total width of your floor here is 4 meter, which means with a Solid Oak floor the expansion gap should be 4 x 4 = 16mm wide.

Although the width of the room stays the same (4 meter) the direction of the installation makes the width of the floor 6 meters wide, meaning an expansion gap of 6 x 4mm = 24 mm.

And 6 meters wide is in fact the widest you can safely install a Solid Oak floor without the need of an extra divider, so if you are planning to install the same floor in a connecting room, like a dining or study, use a threshold and create in fact two separate areas.

Thresholds or dividers come in many types and materials. Most unobtrusive in the above situation would be to use a Solid Oak strip or T-bar wide enough to cover the expansion gap in both rooms. If you have a coloured Oak floor you can apply the same colour to your thresholds but do remember every board and threshold has its own character.

No matter how narrow your room is, the minimum expansion gap should be 10mm. A hallway of 1 meter wide, 4 meter long and the wood is installed parallel to the longest wall: 10mm gap. Same hall, but now installed parallel the shortest wall: 4 x 4mm = 16mm gap.

And it is always easier to keep the same expansion gap all around the floor, even on those sides where the boards end perpendicular to the walls. You only have to have one type of spacer and it just works simpler.

If you install your Solid floor during the season where the air is dryer (Winter) it is better to add 2 - 3mm extra width to the expansion gap because your floor (acclimitised properly in your home) will be dryer too for starters and could expand more once the air humidity increases again during Spring and Summer.

The above examples are based on Solid Oak floors, other species react differently to changes in humidity.

Solid Beech: 7mm per meter wide
Solid Pine: 5mm per meter wide
Most tropical species can be treated as solid Oak floors: 4mm per meter wide

For Wood-Engineered floors, because of their stablising construction: 10mm expansion gap all around.

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