Published: 19/02/25 By: Mike Bekin
Timber is a top choice for the engineering and construction industry. From beams to bridges, it brings lightness, versatility and durability to a wide range of building projects. But before working with timber, it is important to understand how the moisture content of your products can impact their end properties. Intrigued? Take a closer look at the role of moisture in timber in this guide.
Does Timber Contain Moisture?
Timber is a hygroscopic material, which means that it absorbs or loses moisture depending on the humidity of its environment. When first felled, wood cut from trees can contain around 60% moisture, which will decrease over time as the timber begins to dry. This water exists as both free water – which remains in the cell cavities – and bound water – which is stored within the cell walls.
Even when kiln-dried, timber will contain a level of moisture. As it dries, though, the free water will start to disappear first until the moisture content is at around 25-35% (also known as the fibre saturation point (FSP)). If the timber dries further than this, the water will begin to leave the cell walls and trigger property changes in your wood products, namely movement, such as checking, bowing, twisting and warping. This is why kilning needs to be done carefully and slowly.
The amount of moisture timber contains is measured using a ratio of the water weight to the dry weight of the timber.
How is the Moisture Content of Timber Measured?
Typically, moisture metres are used to measure the water content of timber. These work by analysing how the moisture influences electrical resistance, and are calibrated for use on different timber species. You can also oven-dry wood to the right moisture content, giving you plenty of control of the water in your timber products.
At EcoChoice, we supply kiln-dried timber for construction projects such as Oak, Douglas Fir, Pine and Spruce, ensuring your species is always dried to the ideal moisture content,
Effects of Moisture Content on Timber
What happens when your timber moisture content falls below or moves higher than the FSP? Trada, a leading authority in the timber industry, estimates that around 80% of all in-service issues are related to the moisture content of the wood! From rot to splintering, you are leaving yourself open to a plethora of problems.Let’s take a look at some of the most common effects:
- Dimensional Stability: Below the FSP, your timber begins to shrink, causing potential warping and cracking. Above the FSP, you are likely to see your timber swell. These movements are what we call ‘dimensional changes’, and we also refer to timber which is less susceptible to shrinkage and swelling as dimensionally stable.
- Strength and Stiffness: Timber’s mechanical properties also vary with moisture levels. Below the FSP, as the moisture content decreases, strength and stiffness will increase. This is why our sustainable timber used for construction purposes is usually kiln-dried, with the aim of meeting the humidity conditions of your indoor or outdoor environments.
- Decay Resistance: High moisture levels will make your timber more susceptible to fungal attacks and decay. Timber with a moisture content below 20% is ideal for preventing these issues, and it is one of the main reasons why an understanding of how water affects timber is important.
Other Implications of Moisture Content
The moisture content of your timber can have impacts aside from affecting durability. For example, moisture can affect the thermal and acoustic insulation properties of wood, as well as its weight and density.
Measures to Take to Keep Moisture Levels Low
After being kiln dried, wood can still take in moisture. So, how do you keep its percentage from increasing?
The majority of moisture uptake happens at the two cut ends of each board or beam. Sealing them with an appropriate compound (which you can find at any timber merchant) will go a long way to prevent the moisture content from increasing back again. It is also wise to keep timber off of the ground and covered during storage, and always keep it out of the rain. When installing, choose a dry day with low humidity, preferably after the timber has been around to acclimatise for a few days beforehand.
Find Kiln-Dried Timber at EcoChoice
Looking for high-quality timber which has been kiln-dried and is ready for construction? We can help. At EcoChoice, we supply high-quality, sustainable timber products sourced from FSC®- and PEFC-certified forests, ensuring your project has the materials it needs to last a lifetime. Get in touch with us today to learn more and start your order.
