Wood Works No.1 – Moisture content

What is moisture content in timber?

One of the most important and yet least understood characteristics of timber is that of moisture content. Indeed, over 80% of the investigations into timber problems that we have conducted over the years relate to moisture content.

Timber contains a large amount of water when it is freshly felled, the amount of which is referred to as its moisture content (“MC”). This is expressed as a percentage of the dry weight of the piece, not of the total weight; hence it is possible to have MC’s of more than 100%.

Moisture content in timber cells is like water in a drinking straw
Cell structure of wood acts like a collection of straws

Using the analogy of a paper straw, when the timber is at or above around 30% MC, the wood cells are full of water, as when drinking water through a straw. As timber dries below this point (known as ‘fibre saturation point’) the free water drains away (removing the straw from the drink) and the bound water begins to be lost from the wood cells (the straw itself) which causes it to shrink across the grain; longitudinal movement in timber is negligible.

Timber is a hygroscopic material, which means that the amount of water it contains rises and falls in response to changes in temperature and relative humidity. Although it is not possible to prevent wood from expanding and contracting in use, this movement can be minimised by drying, manufacturing and installing the wood products at a level as close as possible to that which it is likely to achieve in service, referred to as its equilibrium moisture content (“EMC”).

How is timber dried?

Timber was traditionally dried by stacking it and allowing it to air-dry over many months, even years. This method is still used for larger section timbers (above 75mm thick) but is only able to dry timber down to around 17% – 20%. For smaller sections, and to achieve a lower moisture content that is more suited for construction and internal use, timber is normally dried commercially in kilns which can dry the timber to the desired level more quickly by closely controlling temperature and humidity to produce ‘kiln-dried’ timber.

What are the implications of moisture content in use?

It is usually desirable to seek to minimise timber movement in use, so the aim is to ensure that the timber has an EMC that is appropriate for the service conditions. For example, furniture and wood flooring in a modern heated home could be expected to attain an EMC of around 8-10%. External joinery, such as windows, around 14% MC and a more exposed timber deck or garden building of around 16%. These external levels would be subject to seasonal variations, becoming higher in winter and lower in the summer.

Given the importance of MC, it is surprising how often the condition of the timber is left to chance and not checked before installation. For instance, by quickly checking the condition of your wood flooring with a moisture meter before installing, you will know whether it is likely to expand or shrink in service and can accommodate the movement accordingly.

The amount of movement that can be expected across the grain in timber varies with species but as a general guide it can be considered as 1% for every 3% to 5% change in moisture content below 30%, which equates to 1.5mm across a 150mm wide solid floorboard for example. This may not seem significant, but it represents 50mm across a 5m wide floor.

The effect on shape as wood dries
Nature of distortion varies depending on where it is cut from log

Movement also varies depending on how the timber has been cut from the log, being almost twice as much across the width of a flat sawn board (parallel with the growth rings) as it does across a quarter sawn board (at right angles to the growth rings). As a result, distortion can occur in the timber or finished component. Also, timber which is cut from near the centre of the log or which is fast grown is more likely to distort than that cut from slower grown (tighter growth rings) timber towards the outer part of the log.

What can I do to minimise the risk of excessive movement and distortion?

Specification of a higher quality timber at an appropriate moisture content is the best way to minimise movement and distortion. By choosing high quality finished products, the careful selection of timber at an appropriate condition should already have taken place prior to manufacture. However, we would recommend that the moisture content of the timber or product should always be checked on delivery to ensure it is at an appropriate level as well as providing useful information when it comes to installation as discussed above.

Moisture content testing in timber
Electrical resistance type moisture meter

There are various meters available on the market and we recommend ones which are specifically calibrated for timber. The most common type are hand-held electrical resistance meters which measure the conductivity between pins that are pushed into the timber. Capacitance meters are also available which are well suited to decorative timbers as they cause no damage to the surface of the wood. In all cases, it is important to follow the manufacturer’s instructions. For instance, some readings may need to be calibrated to account for a slight variation in readings between different species. Similarly, some products, such as plywood or treated timber may give spurious results due to adhesives or chemicals present in the products.

Delivery should be ‘just-in-time’ to avoid the need for wood products to be stored on site. Where this is unavoidable, provision should be made for careful storage of timber products to minimise the risk of moisture pick-up. This could involve a temporary storage facility, or the introduction of temporary heating and dehumidification and monitoring of the environment.

A final consideration is that the conditions within the building at the time of installation are as near to the service conditions as possible, particularly with new-build. Further, once the joinery is installed, avoid baking it by testing the new heating system on full for a prolonged period during commissioning!

Moisture content is of course just one factor when considering using timber. There is a wide range of industry standards and guidelines relating to timber specification to ensure that the correct timber is used in respect of durability, strength, appearance, movement characteristics etc. and these will be explored in future Wood Works blogs.

Useful references:
BS EN 942: 2007 Timber in joinery. General requirements
BS 8201: 2011 Code of practice for installation of flooring of wood and wood-based panels

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