Preservative Treatment For Timber

Under ideal conditions timber should not deteriorate, but when timber is used in exposed (outdoor) conditions, it becomes susceptible to degradation due to a variety of natural causes. It will suffer rot and insect attack unless it is naturally durable or is protected by a preservative. In general, timber with a moisture content of over 20% is susceptible to fungal decay; timber of any species kept in dry conditions will remain sound; however, dry timber may be subjected to insect attack. Timber can be protected from the attacks by fungi, harmful insects or marine borers by applying chemical preservatives. The degree of protection achieved depends on the preservative used and the proper penetration and retention of the chemicals, as treatability varies among the species and also between their heartwood and sapwood. Some preservatives are more effective than others, and some are more adaptable to specific use requirements.

There are a number of widely used methods of application of preservative treatments. Pressure impregnation with a water-borne agent is appropriate for timber in ground contact or high hazard conditions. Double vacuum impregnation with a solvent-based organic preservative is a preferred method for treating joinery timbers. Micro-emulsion treatments, which are water borne, with new more environmentally acceptable products, are now available in the market. Preservatives should be applied under the controlled conditions of an authorised wood treatment plant.

British Standard BS 8417:2003 [25] provides guidance on the treatment of timber for use in the United Kingdom, and includes the requirements of key parts of other relevant BS and EN standards. Issues related to the requirement for preservative treatment include service life, in-service environment, species type and its natural durability as well as the type and form of the preservative treatment.

BS EN 335-1:2006 [26] provides a description of use classes and lists the potential biological organisms and insects that may challenge the timber in a particular service condition. A summary of use classes is given in Table 1.20.

Recommendations for the treatment of softwood timbers are given in BS 8417:2003. Further information on the protection of timber and timber products may be found in the following:

• TRADA publication: Wood Information Sheet WIS 2/3-33 Wood preservation -Chemical and processes, 2005.

• Wood Protection Association: Industrial Wood Preservation, Specification and Practice, Derby, 2006 (www.wood-protection.org).

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