Timber is very sensitive to moisture, and it is vital to understand all aspects of the interaction between timber and moisture in order to be able to use timber wisely.
Moisture in wood cells
The wood cells of a living tree are very porous and contain a great deal of water. In fact, the moisture content of wood in a tree can often exceed 100%. Moisture content as defined in all aspects of timber production uses the weight of the moisture in the wood divided by the dry weight of wood material. (This is an unusual definition compared with many other
materials that use the wet weight as the denominator.) In a live tree, there is actually a much bigger mass of water than wood!
Water is stored in wood in two main forms:
- As free water in the vessels and/or cells, used to move nutrients within the tree.
- As cell (or bound) water, which is an integral part of the cell walls.
As soon as the timber is cut, the wood starts to lose moisture. The initial reduction in moisture content is a result of free water loss. This usually occurs without any significant dimensional changes to the timber as the loss of moisture represents the drainage of voids or vessels in the timber. If the environmental conditions are favorable, the moisture loss continues until all the free water is released to the atmosphere. This point is known as the fiber saturation point (fsp). The fiber saturation point varies a little with each piece of timber, but it is generally taken to be at a moisture content of between 25% and 30%.
The loss of free water will occur relatively quickly in small cross-sections of timber, even if the timber is exposed to rain. However, in larger cross sections, it can take many decades for all of the free water to be lost. Initial drying of the outside forms a hard “case” which can act as a barrier to further moisture loss.
After all of the free water has been lost, the timber will still contain moisture, but this moisture is bound into the cell walls. Much more energy is required to remove this moisture as it is held in the wood structure by weak chemical bonds. This loss
of moisture occurs more slowly than the loss of free water. It also results in a reduction in the size of the cell walls, which causes the timber to shrink in size.
Moisture in timber
Timber can be classified according to its moisture content:
- Unseasoned or “green” timber has a moisture content higher than the fiber saturation point (~25% mc). In unseasoned timber, all of the bound water is present, and at least some of the free water is still in the wood. Unseasoned timber can “feel” wet to touch, and if very green will ooze out water as a nail is driven in.
- Seasoned timber has had the moisture content reduced to 15%mc or less, and will generally lose very little further moisture if used in a protected environment, such as under cover or indoors. Some shrinkage has taken place in the transition from unseasoned to seasoned timber.
Partially seasoned timber has a moisture content of between 25% and 15%. Some shrinkage has taken place, but further shrinkage will result from additional moisture loss. Partially seasoned timber can come from unseasoned timber that has partly dried, or from seasoned timber used in a moist environment so that it has taken up moisture and increased in moisture content above 15%mc.
Partially seasoned timber is often unseasoned timber that has partly dried since processing and is also sold as “unseasoned”. The process of removing moisture from timber or drying timber is known as “seasoning ”.
Changes in moisture content
Moisture can move between the atmosphere and the timber where an appropriate moisture gradient exists between the wood and the environment in which it is placed.
Moisture movement out of the wood into the atmosphere happens where the atmosphere is relatively dry and/or the wood contains a lot of moisture. It is the process of continued seasoning. Typically, this may happen if unseasoned timber is used in framing that is protected from the weather (such as in a house frame), or if seasoned timber is used in an air-conditioned environment.
Where the wood has already been dried (seasoned timber) and it is used in an environment that has a lot of atmospheric moisture, then the moisture will follow the reverse path the cells will take up moisture from the atmosphere. This movement is the reverse of seasoning. Examples of its use may occur when the timber is used unprotected in a temperate climate, or if used as part of an indoor swimming pool enclosure.
Equilibrium moisture content
Timber loses or gains moisture to be in equilibrium with the atmospheric moisture in its immediate environment. When the timber and its environment have moisture contents that are in equilibrium, then the moisture content in the timber in this state is known as the equilibrium moisture content, or emc. No moisture will move in or out of the timber where the moisture in the timber is in equilibrium with the moisture in the atmosphere.
Shrinkage
As wood dries below its fiber saturation point, it shrinks. Shrinkage is the reduction in dimensions of timber due to the movement of moisture out of cell walls of the wood. Below the fiber saturation point, all of the moisture remaining is bound water and is an integral part of the cell walls. Removing this water makes small changes to the thickness of the cell walls. Aggregation of this reduction over thousands of cells causes reductions in the thickness of the timber.
The loss in dimension is not the same in all directions:
There is little change in the longitudinal dimension. There is virtually no shrinkage parallel to the length of a piece of timber. Radial shrinkage is perpendicular to the growth rings. It is shrinkage in the direction towards the centre of the tree.
Tangential shrinkage is in the direction parallel to the growth rings. It is always a little larger than the shrinkage in the radial direction because radial shrinkage is partly restrained by rays (fibers that run perpendicular to the growth rings).
Designers rarely know which cross-sectional dimension is radial and which is tangential, so shrinkage is often estimated for each cross sectional dimension using the data for the tangential direction.
Shrinkage not only causes a change in cross sectional dimensions, but can also produce unsightly and sometimes dangerous splits and cracks that can be avoided in many cases. In some cases, shrinkage can change load paths that may be potentially dangerous or costly to repair.
The effects of shrinkage vary depending upon the species, thickness of the timber member, the part of the log from which the member was cut, the initial moisture content, the rate of change of moisture and the environment in which the timber is placed.
Shrinkage tends to be more of a problem for hardwoods than for softwoods. However, regardless of the species, appropriate allowances for shrinkage need to be made in the detailing of all timber. The effects of shrinkage are obviously more significant for unseasoned timber that is allowed to dry in service. For some structural applications, it may be necessary to use large cross sections of unseasoned timber. In these cases, careful design is required to limit the effects of shrinkage, particularly around connections where splitting may occur.
To minimise the adverse effects of shrinkage:
- Specify timber with appropriate moisture content for the service environment envisaged. In most cases in Australia, the emc will be lower than the 25% of unseasoned timber and closer to the moisture content of seasoned timber. Detail elements so they are not affected by changes in cross-sectional dimensions. This is especially important where unseasoned timber is used.
- Particular care is needed in connections. Appropriate detailing will minimise additional restraint that will prevent timber from moving as it shrinks. It is usually when shrinkage movements are restrained that the cracks develop.
Effects of moisture in timber
The general rule for effective long-term use of timber is ‘Keep it Dry’. Moisture can have a
number of detrimental effects on timber:
- Change in cross sectional dimensions – At moisture contents less than fiber saturation point (~25%), wood that is taking up moisture swells, and wood that is drying shrinks. The shrinking and swelling takes place at a range of moisture contents that the wood will experience in normal service. However, the loss in dimension is not the same in all directions. There is little change in the longitudinal dimension. Tangential shrinkage is in the direction parallel to the growth rings. It is always a little larger than the shrinkage in the radial direction because radial shrinkage is restrained by rays (fibers that run perpendicular to the growth rings).
- Strength -Water in the cell walls tends to make them a little slippery. It acts as a lubricant and allows the fibers to slide past each other a little easier. There is a small reduction in strength of wood fibers as moisture content increases. This effect is seen in the relationship between strength and moisture for clear wood specimens.
- Stiffness -Water lubrication within the cells causes a small increase in elastic deflection under load (this is a decrease in stiffness). However, moisture has a marked effect on creep. With only loose bonds between the cells, as load is applied, the fibers rely on friction to stop them sliding over each other. Under long term loading, some sliding will occur. This is creep. Water in the cell walls increases the creep markedly by lubricating the slip interface. Creep is accelerated while water is moving into or out of the wood.
- Durability – fungi and termites need to have moisture to thrive. Moist wood is more vulnerable to biological degradation. Most paints and glues are only really effective if applied to dry wood. Moisture therefore can compromise the durability of timber by making conditions more favorable for biological attack and by reducing the effectiveness of protective coatings.
- Coatings – unless the protective coatings are flexible, the shrinkage and swelling of timber as moisture moves in and out causes deterioration of the coatings. Once a coating has been broken, water can move into the timber. The undamaged portion of the coating can trap moisture in the timber. The swelling and shrinkage of the timber in response to changes in moisture content will cause rapid deterioration of the rest of the coating. Paintwork must be kept in good condition, otherwise rapid deterioration of both the paintwork and the timber will result.
Specification of moisture content
Designers must match the moisture content of the timber with the environment in which the timber will be placed. For example,
Unseasoned timber
Useasoned timber is any timber with a moisture content > 25%. However, for practical reasons, most timber sold as unseasoned has a moisture content > 15% rather than the more strict definition of unseasoned timber (> 25%).
Seasoned timber
Seasoned timber is timber with a moisture content between 10 and 15%. Timber in this condition will be in equilibrium with internal environments in many parts Australia. (Little moisture will move into or out of the timber while it is in service.) It will, therefore, not shrink significantly while used internally. Seasoned timber should be chosen for indoor use where it is particularly important not to have shrinkage associated with drying out in service.
The use of seasoned timber offers some advantages:
- Cross sectional dimensions remain almost constant
- Reduced weight
- Improved strength
- Increased electrical resistance
- Improved gluing and nail holding properties
- Increased joint strength
Some strategies to minimise moisture movement:
- Select timbers with low moisture movement characteristics
- Protect against excessive drying or wetting
- Apply a coating system that will reduce rapid moisture absorption or loss
- Use smaller rather than larger cross sections if possible
