When Australia was first colonised, our forefathers found an abundant building material in the ancient timbers of local forests. The mature trees provided a relatively stable timber, due primarily to their age and stage of development and also due to being native to the area. Today the demands on our timber resources are such that much of the timber used around the country is derived from plantation stocks, juvenile forests or lumber imported from overseas – all of which are factors that render the resulting timber more vulnerable to dimensional change.
A critical factor in timber flooring installation and problem solving is an understanding of the structure of timber and its relationship with moisture. This knowledge is fundamental to diagnosing the cause and nature of dimensional change in milled timber and is essential when evaluating potential jobsites or addressing timber related floor failures.
Essentially, wood is made up of two things, cellulose and water. The fibrous cellulose cells – which look much like drinking straws with the ends pinched together – are bonded together in overlapping strings by lignin, a polymer in plant cell walls that gives the plant rigidity and strength. The water embodied in wood exists in both the cells and cell wall structure. A living tree can contain between 95 to 200 percent water, depending on species and the stage of development.
Newly cut logs also contain a high percentage of water. As the wood cut from these logs starts to dry out, the free water contained in the cell cavities evaporates until only the water bound in the cell walls remains. This is known as fibre saturation point. Water loss will stop when the moisture in the wood is in balance with the humidity of the surrounding area – this is known as Equilibrium Moisture content (EMC). Moisture content is determined by the ratio of the amount of water in the unseasoned wood compared to the weight of the kiln dried timber.
Timber flooring is seasoned – either air-dried or kiln-dried to a moisture content of between nine and 14 percent in accordance with AS 2796. The seasoning of timber is a delicate process, as the ends and exposed outer surfaces of a piece of wood loose moisture more rapidly than the material at its core. This disparity between exterior and internal moisture loss can cause stress within the timber resulting in end splits, surface checks and internal checks, or honeycombing.
The problem of dimensional change in timber is directly related to changes in moisture content during the life cycle. The degree to which a section of timber will expand or contract when subject to moisture change will also vary according to the method of conversion. The large majority of timber offered for decorative flooring is plain sawn or flat sawn; the boards are cut on a tangent to the growth rings to reveal the distinctive figuring of the grain; but this is not without its disadvantages as illustrated in Australian Standard AS 1684.2-1999 Appendix H Timber Species and Properties which states:
H2.8 Column 8 – Tangential shrinkage
Average percentage shrinkage for tangential direction only is given as these are normally about double that of the radial shrinkage. Shrinkage is the measure of the percentage reduction in dimension from the unseasoned to 12% moisture content condition.
H2.9 Column 9 – Unit Tangential Movement (%)
The unit tangential movement is the percentage dimensional change for each 1% moisture content change between about 3% moisture content and the fibre saturation point for the particular species.
For example the percentage unit tangential movement of Blackbutt is 0.37 percent per one percent change in the moisture content of the timber. Therefore a three percent change in moisture from nine to 12 percent will create approximately 1mm change in the width of an 85mm wide board. While it doesn’t sound like much over 10 boards it can add up to become a problem.
Given timber’s capacity to easily absorb moisture from its surrounding environment, the moisture content of timber may change, increase or decrease while in storage or when shipped from one location to another. Therefore, an initial conditioning period for the timber flooring to equilibrate to its new environment and intended application is essential. For this reason the final acclimatisation of the timber flooring must be carried out in the proposed in-service area.
Dimensional changes in timber flooring have always been at the forefront of consumer issues. The main cause of this is incorrect acclimatisation of the timber (Australian Standard AS/NZS 1080.1 states that at the time of installation the moisture content of the flooring shall be within +/- two percent of the expected EMC for the area and proposed conditions during service), and consumer ignorance regarding the need to maintain a stable environment for the floor in service. The latter falls within the head of contract’s duty of care to advise and educate the end user regarding the long-term care and maintenance of the floor.
When we understand that moisture is an inherent and integral element in the structure and development of the living tree, it becomes easier to accept the cause and impact of the dimensional change that occurs in the process of milling and seasoning timber for commercial use. We should therefore not be surprised by the movement we often witness in timber flooring installations, given the potential for variations in ambient humidity in service post-installation. This knowledge should also provide greater insight and understanding that not all job sites are suitable for the installation of timber flooring.
Ray and Sharon Brice specialise in mediation training and project management. They have four decades of experience in the flooring industry.
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