Oak Floors Online explain how the drying of wood affects its size and dimensions

Because moisture content is so important and will always define the size of any piece of timber (oak flooring or other) we at Oak Floors Online feel that it’s important to explain what happens from the time when the tree is cut down and then the process that it goes through before it arrives on your doorstep as beautiful flooring.

Hopefully by providing you with this information, you’ll understand why we advise everyone who owns an oak floor to monitor and control the conditions with some kind of Thermo-Hygrometer, especially the relative humidity, that surrounds their floor.

When a tree is alive and growing, it can be completely ‘full’ of water, sometimes to the extent of having a moisture content of more than 100%, and the moisture within weighing more than the wood itself.

The moisture content of timber is calculated by the weight of the water divided by the weight of the dried timber, and then expressed as a percentage.

Timber ‘holds’ moisture in two ways;

• Moisture called ‘Free Water' is held within the space between the cells of the wood, and is the first moisture that’s evaporated during the drying process.
• Moisture called ‘Bound Water' is attracted to the cell walls by a chemical reaction, making it more difficult to dry, thus making it the moisture source that’s most relative to the size of the timber section, as it either increases or decreases. The bound water has positive hydrogen atoms, which are attracted by the negative charge within the cell walls, and it’s this ‘attraction’ that makes it most difficult to dry.

When you dry any piece of freshly felled timber, it doesn’t start to shrink until all the Free Water has evaporated, which then forces the Bound Water to then start to evaporate as the drying process continues.

When this starts to happen, this stage of the drying process is called the Fibre Saturation Point (FSP), and timber doesn’t start to change dimensionally until it reaches its FSP (different species of timber have different FSP levels but in most it’s between 25-30% moisture content).

This is hugely important to understand and accept because it’s this FSP that will define how your oak flooring ‘behaves’ dimensionally after installation, as it’s forced to either ‘take on’ or ‘lose’ moisture.

Wood used as flooring within the home, timber will sometimes ‘live’ at well below its FSP level (depending on the living conditions like heating, insulation, ventilation etc…), so this is why a period of acclimatisation is necessary to allow it to reach equilibrium with the room it’s to be installed… before installation rather than afterwards.

If your floor is installed before the flooring has reached this level of equilibrium, it will be forced to reach it after installation is complete, thus forcing it to move dimensionally one way or the other.
This is why acclimatisation is so important before installation, and then maintaining the conditions as consistently as possible afterwards, to avoid excess dimensional movement of each floor board within the finished floor.