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Managing moisture
Moisture management is used to minimize shrinkage and movement in the timber. For instance, when wood comes from a newly felled tree it is virtually saturated with water. As moisture leaves the wood it influences strength, dimensional stability, stiffness, hardness, abrasion resistance, machineability, insulation value, resistance to decay and nail holding ability.

Measuring Moisture
In managing moisture there is a need to be able to measure it. Moisture content is described in terms of the weight of water contained in the wood expressed as a percentage of oven-dried wood weight. Key levels of moisture content are linked to simplified terms such as 'seasoned' and 'unseasoned' timber. Seasoned timber relates to low levels of moisture content, less than or equal to 15 percent. This approximates the equilibrium moisture content (EMC) of most timber. EMC is the moisture level that the timber wants to stay at once it has dried out and acclimatised to its environment. Timber with a moisture content higher than 15% is regarded as unseasoned or green.

Figure 7: Effects of shrinkage on common cuts of timber

Shrinkage and Seasoning
Timber needs to be specified as being seasoned or unseasoned for installation purposes. This decision is very important since seasoned timber is generally stronger than unseasoned timber – all other things being equal. Unseasoned timber is also less dimensionally stable due to the effects of potential shrinkage. Shrinkage is where wood fibres move closer together causing distortion of the three dimensional features of timber. It only begins to happen once the timber dries out to fibre saturation point – a point between 25–35% moisture content that varies for each species. Longitudinal shrinkage is very small and generally disregarded; radial shrinkage is about 2 to 7%; while tangential shrinkage isgenerally up to twice this amount, at 4 to 14%. When this amount of shrinkage takes place in an uncontrolled way, the risk of splitting in timber increases significantly. In addition, there is the risk of shape distortion, as shown for typical cuts of timber in Figure 7.

Timber species with high shrinkage (e.g. New England Blackbutt, Messmate, Mountain Ash, Silvertop Ash and Turpentine) require greater care if being used in an unseasoned state.
Applications where any radial and tangential shrinkage may have significant impact such as
flooring, panelling, some cladding, mouldings, furniture and joinery, need timber to be seasoned prior to use. Unseasoned timbers can be used in other applications but this involves a value judgement and creates the need for careful detailing and attention to expected shrinkage rates.

Detailing Unseasoned Timber
Show care where:

• materials with different shrinkage characteristics are combined, e.g. unseasoned timber next to seasoned timber or non-timber products, end grain next to cross grain,
  
• large timber sections are involved,
  
• multiple stacked timber members eg joists on top of bearers
  
• large timber areas are involved e.g. flooring, cladding,
  
• clearance needs to be provided relative to brick veneer walls i.e. at lintels, eaves lining, window sills and floor framing,
  
• multi-storey construction causes combined shrinkage i.e. where each floor may add to the overall effect
  
• shrinkage affects fire resistant construction e.g. at gaps in walls.

Further perspective of the effects of shrinkage on unseasoned timber is shown by the figures
in Table 2.

Seasoning Requirements for Specific Service Environment
Seasoning requirements only approximate the equilibrium moisture content on-site. Each environment is different and so some adjustment may be necessary to get the best result – especially where appearance is important. In such instances there maybe a need to condition the timber before installation to limit the chances of movement. There is also a need to consider different service environments – especially air humidity. Humidity changes for coastal and arid environments, interior and exterior environments, air conditioned or heated environments. Each changes the stability of moisture in the timber. Generally the moisture content of timber products such as flooring should be approximately suited to 'normal' environments. If they are not or conditions onsite are unusual then they should be acclimatised to the expected in-service equilibrium moisture content level before installation. This may mean that timber products are conditioned on-site before fixing. For instance, it is not uncommon for floor fixers to leave timber appropriately stacked onsite for three or more weeks prior to fixing. Another approach is to predict the EMC prior to taking the timber to site (e.g. condition it at the timber manufacturer’s premises). To assist, Table 4 provides the average moisture content for coastal, inland and air-conditioned environments. In order to determine which applies to your situation refer to Figure 9 which shows three climate zones. 'Inland' refers to Zones 1 and 2, while 'coastal' refers to Zone 3. Specifiers must be mindful that the figures derived from Table 4 are still approximate because humidity is constantly changing in the distinct micro-climate of each service environment. Some degree of moisture induced shrinkage (or swelling) is hard to avoid – even if only due to seasonal climate change. The only tool for dealing with such movement is to limit restraint of the timber in-situ – thus allowing freer movement. Movement joints or specialist connectors can be integrated into the construction to achieve this end.

Figure 8: Climate Zones based on relative humidity