American hardwood lumber

Technical Matters: Wood & Water

This article is the second in a series of technical features that deals with different properties of timber and how these properties play an integral role in how timber is processed and used. This feature will describe the significance of moisture content and the relationship between water and wood.

Water plays a very prominent role in the life of wood. A tree needs water in the form of sap and together with the aid of photosynthesis this enables the tree to grow. The wood that is processed from that tree never truly loses all of its water when it is made into something, although we may think that it does. A freshly felled tree can easily have a moisture content of 150%. If you are wondering how it is possible to have a value of more than 100%, it is because the moisture content of wood is expressed as a percentage of the weight of the water it contains compared to the weight of the wood if it contained no water at all.

The reason that wood can contain such high amounts of water is that the structure of the tree is made up of thousands of hollow tube-like cells, like drinking straws, that are full of water in the form of sap. This water is called ‘free’ water. The cell walls also contain water as molecules which is called ‘bound’ water. As soon as a tree is felled it starts to lose water, sometimes you can see water/sap literally dripping from the end of the trunk. The liquid or ‘free’ water is the moisture that is lost first by evaporation which is speeded up at the sawmill when the bark of the tree is removed, and the log is sawn into boards. If left to evaporate naturally then the drying process may take some time. We will explore drying methods in another article.

It is not until the wood has dried all the way down to a moisture content of around 28% that all of the ‘free’ water in the hollow part of the cell has gone. But this still leaves the remaining 28% as ‘bound’ water in the cell wall structure. This figure of 28% is known as the Fibre Saturation Point or ‘FSP’. At this point the timber will be a lot lighter, as it has lost a lot of water, but no dimensional change will have happened. But as the ‘bound’ water is lost from the cell wall, the cells bunch closer together, filling the molecular void that remains. And so, it is at this point in the drying process that the timber will begin to get physically smaller in the cross section. This initial reduction in size is called shrinkage.

We need to dry wood to an acceptable moisture content to be able to use it for commercial applications such as flooring or furniture. But when does timber stop being ‘wet’ and when does timber become dry? What is an acceptable moisture content for a particular end use? Is it 18%, 15% or 8% moisture content (MC)? The answer to these is that all of these moisture content levels are acceptable depending on where you are planning to use the timber when it is in service. For example, timber inside in permanently heated conditions would need to be at a moisture content of about 10%, whereas timber in an external joinery scenario, ideally, would be nearer 16-18%.

It is not just the moisture content of the timber at the initial installation phase that needs to be considered but the long-term moisture content that the timber will achieve once it has been exposed to the environment that it is in for a period of time. This is important because wood is hygroscopic; a property that means that it can interact with the moisture that is in the atmosphere around it, so that it has the potential to either gain or lose moisture.

‘Wet’ timber will lose moisture in a drier atmosphere and, conversely, ‘dry’ timber will gain moisture if exposed to a more humid environment. The wood needs to find its moisture content balance and this is called the Equilibrium Moisture Content or EMC. This is why, ideally, it is good practice to try and place timber in an environment that it is intended to be in for a while, so that it can adjust to its surroundings. This is called conditioning.

The interaction between moisture and timber when it is installed or when it is in service is called movement. Movement such as shrinkage happens across the cross-section of the wood and it will continue to take place throughout the lifespan of the timber. Wood will always have the potential to move, especially if exposed to a sudden and extreme change of atmospheric conditions caused by heat or water, and it never stops moving, although you may think that it has. The next time we will investigate how timber is dried.

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Neil Summers

CONSULTANT TO AHEC FOR TECHNICAL ISSUES

Neil Summers works with AHEC on [...]