Moisture and timber in construction an introduction
Lewis Taylor and Andrew Pitman introduce timber’s relationship with water, equilibrium moisture content, the fungal decay threshold, variation in the movement of timbers and the ways in which we must account for these factors in building design and construction.
It’s vital to understand the relationship between timber and water. Understanding the implications of too much moisture in construction timber is probably the issue that BM TRADA experts see most difficulties with, and mistakes made, during building site visits.
Wood loves water
Timber is a hygroscopic material. This means wood is water-loving and absorbs water when exposed to rain or leaks. It is also important to understand that wood also absorbs and loses water in the vapour form; water that occurs naturally in the air.
While things like oxygen and nitrogen content are constant in the air around us, water vapour content – or relative humidity – so varies with the time of day. Over time, this inconsistency in relative humidity impacts the moisture content at the surface of exposed wood.
When constructing with wood, it’s very important to consider all potential moisture sources. These include less obvious sources such as humidity within the building, as well as rainwater outside. For example, the humidity in bathrooms and wet areas may differ from that in other regions of buildings.
Moisture in wood
Wood grows in a series of structures which, when magnified using a microscope, look like stacks of drinking straws. In a standing tree, the wood cell walls surround vertical hollows, called lumina, that are filled with water. However, when a tree is felled and turned into logs, moisture will be found in both the lumina and the cell walls of the wood. Whether air-dried or kiln-dried, the water is then removed from the lumina and from most of the cell walls, although even dry wood will contain some moisture.
A cubic metre of freshly felled and dried oak, for example, will retain around 12% moisture content, which is around 77 litres of water. So, even in a heated room at 20°C, under 65% relative humidity, the same oak used in the construction of that room will retain 12% moisture content.
Too much moisture in wood
Problems with construction timber arise when additional moisture is allowed to penetrate the wood. If allowed to stay wet for a period of time, construction timbers can become distorted, wood panels can warp and fungal decay can set in. This can occur during storage and installation, or once construction has taken place, and can result in structural weakness.
Timber is an organic material that will expand and contract with moisture content. Over time, construction timbers move. A well-designed building must account for this expected movement.
For example, if a tongued and grooved system of external cladding boards is installed without space for the wood to swell in wet conditions and shrink as it dries, the cladding can lift away from its supporting frame.
Predicting timber movement
Happily, after many years of research and experience within the industry, we are able to understand the relationship between water and wood and predict how different timbers will behave when exposed to differences in relative humidity. We know that, as relative humidity increases, the equilibrium moisture content of wood also increases. Different timber species are now classified into three classes relative to how much they will shrink and expand under different moisture levels.
The equilibrium moisture content (EMC) of wood is the moisture content that wood will have when exposed to the relative humidity at a set temperature. With this information, we can predict how the wood will behave under any set of conditions.
For example, in an air-conditioned building, the typical moisture content of wood can be as low as 7% EMC. Whereas in a building where the heating is only turned on for short periods, the EMC of wood will be nearer 12–16%. Outside in the UK, however, during wet, winter months, the EMC can rise to around 20%.
Fungal wood decay only occurs when the level of moisture in timber rises above the levels needed by the fungi to colonise and begin breaking down the wood. This needs to happen over a sustained period. This threshold is typically around 23% moisture content and is known as the ‘fungal decay threshold’.
Drying wood
When we first start drying wood, we’re taking all the water out of the hollow lumina. This will get us down to around 30% moisture. The point where all the space in the cell walls of a piece of wood is full of water, but the lumina are dry, is referred to as the ‘fibre saturation point’. Critically, when the moisture content in timber is less than the fibre saturation point, any changes in moisture content will cause the wood to shrink as the moisture levels drop or expand as they increase.
The long, vertical cell walls of wood mean that when those cells fill with water, they expand outwards, getting thicker rather than longer. This means that timber doesn’t naturally move much longitudinally, along the length of what was the tree trunk. So, while movement in the radial and tangential directions can be as much as 3% and up to 8% respectively, movement along the length of the same timber will be less than 1%. This means it can be used for fence posts and freshwater pilings, even in wet conditions.
Classification of timber
When defining the movement of timbers in the 1960s and 70s the Forest Products Research Lab classified timbers into three separate classes based on an average of how much they change dimension with moisture content within the radial and tangential directions. So, classifications are divided into three:
- Large movement timbers will change by 1% in dimension for every 3% in moisture content.
- Medium movement timbers change by 1% in dimension for every 4% in moisture content.
- Small movement timbers change by 1% in dimension for every 5% in moisture content.
This only refers to shrinking and expansion below the fibre saturation point, above which the timber dimensions remain unchanged.
Designing for timber movement
Understanding these movement classes, and knowing the anticipated change in moisture within which construction timbers must function, allows designers to select timbers for their durability under those conditions, predict the movement of individual components and design the construction to allow for any shrinkage or swelling. For some applications, such as exposed decking with narrow gaps between boards, small movement timbers such as teak might be used, for example. Whereas most structural timbers used in timber frame construction are medium movement softwood timbers, to suit an indoor environment that will be subject to cycles of heating and cooling.
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