Seasoning and timber moisture content
This advisory information includes:
- The implications of timber moisture on product performance
- Acclimatisation before installation
- Moisture content at time of sale
- Moisture content at the time of use or installation
‘Seasoning’ is the process of drying moisture from timber. It is undertaken for two important reasons:
- To improve structural performance—a prerequisite to the allocation of seasoned timber strength (SD) and joint groups (JD) is that the average moisture content in the piece be no more than 15%
- To improve stability—appearance-grade, milled products (e.g. strip flooring, parquetry, decking, panelling, chamferboard cladding, mouldings, furniture and joinery) should be seasoned to ‘pre-shrink’ them prior to use.
When these specifications are followed, the timber will have satisfactory performance with respect to appearance and stability, and the seasoning recommendations of the Australian Standards have been considered in this context. Australian Standards set different moisture content ranges for different products (but usually 9% to 14% for interior milled products) reflecting the type of timber and product application.
Research has shown that in-service moisture contents will vary seasonally and may differ from the target manufacturing range. In addition, other factors relating to house design, heating and cooling systems, and the micro-climate of the particular locality can have a significant influence on in-service moisture contents. Installation and finishing practices need to accommodate both the adjustment to climatic conditions associated with the in-service environment and the seasonal movement that will occur in that climate.
Relative humidity is the major influence determining whether seasoned products will absorb moisture from the air and swell or lose moisture to air and shrink. If the moisture content of timber products is close to the average in-service moisture content, seasonal changes in humidity will result in small dimensional changes. The average equilibrium moisture content (EMC) of timber used indoors is often 1% to 3% below that of timber articles, components and assemblies used in outdoor applications.
Acclimatisation is appropriate for ‘feature’ interior applications, such as flooring, where information obtained from timber moisture-content testing indicates that the moisture content of the timber to be used varies from the average EMC for the in-service environment by more than 2%.
At the time of sale, moisture content for timber supplied for feature products, such as floors, must meet the following requirements:
|Hardwoods||9–14% moisture content with the average of 11% (AS 2796.1-1999 Timber—hardwood—sawn and milled products—part 1: product specification, Standards Australia 1999)|
|Softwoods||9–14% moisture content with the average 11% (AS 4785.1-2002 Timber—softwood—sawn and milled products, Standards Australia 2002)|
|White cypress||10–15% moisture content with the average 12% (AS 1810-1995 Timber—seasoned cypress pine–milled products, Standards Australia 1995)|
The range nominated by the relevant standard may not be appropriate for all in-service environments. Prior to installation the installer should ascertain whether the timber moisture content is appropriate for the in-service environment. Moisture-content testing is done in accordance with AS/NZS 1080.1-1997 Timber—methods of test—moisture content (Standards Australia 1997b).
If timber is installed at an average moisture content that is higher than the average in-service moisture content, greater shrinkage can be expected after installation. Similarly, timber installed at an average moisture content lower than the average in-service moisture content will swell after installation and allowance needs to be made during installation to accommodate this potential for expansion.
Where appropriate, acclimatisation can be used to raise or lower the average moisture content of the timber supplied, to bring it closer to its average in-service moisture content. Acclimatising should therefore be considered when the average in-service moisture content is high (e.g. 14% in the tropics) or low (e.g. 9% in inland regions or with air-conditioning used continuously).
The usual method of Acclimatising timber products is to stack boards in such a way as to allow free air movement to all surfaces. With products such as flooring, boards may be loose laid for a period prior to fixing until they have equalised to the average in-service conditions.
At the time of construction, if conditions are not similar to the average in-service conditions, acclimatisation may be detrimental (e.g. acclimatising during dry weather in a normally humid climate).
Acclimatisation is usually only effective in an air-conditioned building if the air-conditioning is operating at the time or, in dry localities during normal weather conditions. The species type, period and method of acclimatisation will also influence how effective the process is. For some higher density species that are slow to lose or take up moisture, acclimatisation may have little effect. Acclimatising products such as flooring and paneling in dry climates does not remove the need to provide for expansion during periods of wet weather.
For more information on best practice for installing flooring, See: Hayward, D. 2005, Timber flooring—version 1, Forest and Wood Products Research and Development Corporation, Victoria, viewed February 2006 .