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Use of preservative treatments to increase durability
Timbers with low natural durability can be enhanced by the use of preservative treatments. Preservatives are added to the timber to meet the previously described hazard classes.

The use of preservatives involves the introduction of stable chemicals into the cellular structure of the timber. This protects it from wood destroying organisms such as fungi and insects. Preservatives are mainly used in sapwood as heartwood contains resins and other extracts that prevent the uptake of preservative solutions. Timbers such as Cypress cannot be treated.

There are many types of preservatives, each with individual advantages. These include three main groups: Oil-borne, Water-borne and Light Organic Solvent borne.

Oil-borne preservatives
Oil-borne preservatives include creosote or mixtures of creosote, petroleum oil and insecticides. These chemicals are generally limited to heavy engineering structures such as road bridges and power poles. They are sometimes added as a double treatment to marine piles e.g. with CCA treatment.

Water borne preservatives
Water borne preservatives include copper chrome arsenic (CCA), Ammonia Copper Quaternary (ACQ) and Boron compounds.

The copper-chrome-arsenic compounds cover a wide biological spectrum and are not subject to significant leaching. The copper is a fungicide and the arsenic is both an insecticide and back-up fungicide. The chrome acts as a fixing agent to render the copper and arsenic chemicals insoluble. CCA treated timber is odourless and can be readily painted or stained once dry. It has a characteristic light green colouration.

ACQ is a newer form of timber treatment that is similar in performance to CCA. The copper and quaternary ammonium compounds provide protection to timber against decay, fungi, termites and wood boring insects. The timber appears green when freshly treated but mellows to a golden brown with exposure to the weather. Boron is used specifically to protect sapwood against borer attack and is aimed predominately at the Lyctid borer in many hardwoods. It remains water soluble after application and is only suitable for interior use where leaching does not occur.

Light Organic Solvent Preservatives (LOSP)
Light Organic Solvent Preservatives (LOSP) actually describes the carrier of the preservative and not the contents. As a result, LOSP’s vary greatly according to the preservative chemicals added. In general, LOSP’s are solutions of either insecticides or fungicides and insecticides, and may contain additional water repelling agents to develop good weathering characteristics where exposed. They are suitable for interior or exterior, but are limited to above ground situations only. They may also be protected by a coating such as paint or oil based stain in order to extend or enhance the durability.

Irrespective of the type of preservative used the treatment generally forms a protective envelope. As such, cutting and drilling of treated timber should be avoided. Where this isn’t possible the treatment should be patched on-site using paint-on preservatives.

In general, preservative treatments must comply with stringent statutory requirements; whether treated within the state or imported from other states or overseas. State Acts take precedence over all other documents including Australian Standards (e.g. AS 1604). Though State Acts vary, they generally cover:

• The types of preservatives that can be used
  
• The minimum penetration of preservative into the timber
  
• The minimum retention of preservative in the timber
  
• The registered brand that must be applied to the timber

If necessary further detail on these issues should be sought as required.

Adding Coatings to Protect Timber & Connectors
Finishes provide an extended means of protecting against sun, rain, wind and frost.
Without protection, timber colour may be bleached; surface fibres may loosen; boards may cup or warp; cross sectional sizes may slowly erode; continual shrinkage and swelling may cause cracking in the timber. Without protection structure may prematurely deteriorate.

Protective Coatings for Timber
Protection can be provided by the application of coatings such as paints, water repellents, preservatives and pigmented stains. The main objective is to prevent or retard the uptake of moisture and the absorption of ultraviolet light. This applies to external claddings, posts, beams and decks where there are the following options for protective finishes:

• Clear water repelling treatments – these treatments are site applied and are essentially site based versions of the previously discussed preservative treatments. Commonly they contain an organic solvent medium, with additives such as copper naphthanate or zinc naphthanate. In addition wax or resin is often added. Where fully exposed these types of products have a limited service life and re-application in 6–12 months may be required. In addition, solvent or water based finishes with UV inhibitors are also available and give improved service life. Either way these finishes must not be confused with internal clear finishes.
  
• Semi transparent stains – these provide a high degree of protection while enhancing the natural beauty of the timber. Pigments provide a UV screen and light colours are best as they absorb less heat. Stains utilising an oil base are also useful in giving good moisture protection. Prior to applying stains all surfaces including ends should be given a coat of water repellent preservative. Ideally, the first coat of stain should be applied prior to installation, with final coat(s) upon completion. A service life of 3–5 years can be expected depending upon degree of exposure to the weather. Re-application is relatively easy e.g. surfaces generally only require cleaning before additional coats are applied.
  
• Opaque Stains and Paints – when properly applied and maintained these provide the best protection against weathering. Water borne (acrylic) and solvent borne (oil) paints can provide a film which is substantially impervious to water and gives maximum protection against ultra-violet light. These finishes offer a different appearance to the former options – they hide the timbers natural colour and grain. Getting the most out of them is reliant on applying the recommended number of sealing, priming and finishing coats – as recommended by the manufacturer. If done correctly a service life of between 5 and 10 years can be expected from most opaque finishes.

The above options can be used for most applications except decks. They occupy a special category due to the amount of foot traffic, abrasion and exposure to the weather. As a result products specifically manufactured for this purpose should be used. Semitransparent oil stains are generally the main option because the oil penetrates into the surface, providing moisture protection. The pigments also filter the sun’s ultra-violet rays, thus reducing checking and fading.

Protective Coatings for Connectors
Preventing chemical breakdown and corrosion at connections is another important component of dealing with durability. The key issue is to prevent the interaction of moisture, chemicals and metal, from causing corrosion or a breakdown of the wood fibres around metal fasteners. Most use protective coatings to connectors, though some of the more expensive options use a core corrosion resistant material such as stainless steel. Options are shown in Table 16.