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PRESERVATIVE TREATED WOOD – PART 1

Part 1 of this two-part review from Jack Norton, secretary of the Timber Preservers’ Association of Australia, looks at wood structure and natural durability; Part 2 will be addressing timber preservation.

With wood being the primary food source for the wood eating species of termites, having a good understanding of wood structure, durability and wood preservation can be valuable information in predicting termite foraging behaviour, likely levels of termite resistance and making the correct recommendations to customers.

Wood has been used as building material for a very long time. Over the centuries, builders have found many ways of preserving timber to increase its life in service. In this article, we look at:

  • how a tree is built and how it affects timber properties
  • the Australian durability classification system that predicts how long timber lasts
  • levels of treatment that target where the wood product is to be used and
  • the chemicals that are used to extend service life.

How is a tree built?

There are many processes that occur to build a tree. The process occurs in a thin layer just under the bark called the cambial layer. In the cambial layer, carbon dioxide captured by the leaves, and water from the soil, combine to make a glucose molecule. These glucose molecules then combine to form cellulose, which then goes on to make the cell wall (Figure 1). So the basic building blocks of a tree are glucose molecules and because glucose is a sugar, it is a source of energy or food. It is this food source that termites (and insects and decay or rot) are after when they attack wood.

Figure 1: How wood is created

The living tree moves the glucose, water and minerals up and down the stem to the growth sites using a system of pipes (Figure 2). Industrial wood preservation uses these same pipes to move preservative liquid into and around the wood.

The pipes around the outside of a tree trunk are empty and allow water and tree food to travel up and down the stem. The pipes on the inside have been filled up with resins and waxes that stop the flow of liquids. These filled inner pipes stiffen the stem so that the tree can grow to great heights.

Figure 2: Internal plant ‘pipework’

A tree trunk has three main zones: bark, sapwood and heartwood (Figure 3).

The outer bark is a layer of dead tissue that protects the tree from external damage and reduces the loss of water by evaporation.

The sapwood zone is under the bark and is made up of empty cells/pipes. The sapwood carries water and mineral salts from the roots to the leaves, carries tree food and water up and down the stem and stores food materials. This zone varies greatly in width from species to species and even within a species. Sapwood is usually lighter in colour than heartwood and the two layers may be well defined. However, this is not always the case. In some species, the sapwood may be the same colour as the heartwood.

Figure 3: Three different zones of the trunk: outside – bark, middle layer – sapwood (yellow in diagram), inner layer – heartwood (red in diagram)

Heartwood is formed from sapwood when the cells are blocked and stored food materials are converted into waxes, resins and other substances. It is these chemical changes that can make the heartwood more resistant than the sapwood to attack by insects, termites and decay. Wood cells in the heartwood are dead and may also be known as truewood.

Apart from envelope treatments to protect against termites, all specifications in the Australian/New Zealand wood preservation standard AS/NZS 1604 Timber – Preservative-treated, specify complete penetration of the sapwood and a shallow depth of penetration into the heartwood. There are five standards in the AS/NZS 1604 series, dealing with different wood products e.g. sawn and round wood, plywood, particleboard etc.

What is timber durability?

A major consideration when building any project, is a prediction of how long you want the finished structure to last. The durability of a species is a measure of how long the heartwood of that particular species will last in service.

Some species will resist attack by termites, insects and decay better than others. For example, ironbark fence posts last longer than radiata pine posts. Cypress pine heartwood will resist termites and mountain ash heartwood will be attacked.

Australian Standard AS5604 – 2005, Timber – Natural durability ratings, lists the in-ground and above-ground resistance to decay of untreated heartwood. The Standard also gives information on whether or not a particular species is resistant to attack by termites, the Lyctus beetle and marine borers. All untreated sapwood is non-durable. Except for resistance to marine borer attack, the same information is available in a free online resource called Construction Timbers in Queensland (CTIQ).

For long-term protection, you need to use wood species where the heartwood is naturally resistant (durable) to attack by insects and decay, and the sapwood needs to be protected by treatment with wood preservatives. This is discussed in Part 2, in the next issue.