Understanding wood preservation

July 27, 2017

The Stegastein viewpoint in Norway – a wooden deck that has withstood the elements! Image © Tomas EE / Wikimedia Commons.

Wood preservation can be categorised into two types; primary (industrial) and secondary (DIY). Primary wood preservation involves an industrial process whereby wood is impregnated with an industrial biocide-containing wood preservative to render the timber durable and resistant to biological attack, i.e. decay fungi, and wood destroying insects such as wood borers and termites.

The high pressure processes, involving the application of waterborne preservatives such as CCA and Creosote by means of vacuum and pressure cycles in autoclaves, are predominantly used. Other approved methods of primary wood preservation include immersion methods such as the hot and cold open tank process using Creosote; diffusion methods using Borates; and low pressure or double vacuum processes using light organic solvent preservatives (LOSP) such as TBTN-P or Azole Permethrin.

Primary wood preservation is a pre-treatment whereby the timber is treated prior to its intended end use application, and therefore is regarded as a preventative measure. It is not supplemental or remedial (after the fact). Chemical retention (take up), depth of penetration and the processes used are prescribed in SANS standards, and mandatory compliance is controlled through regulations and compulsory specifications.

Secondary wood preservation comprises the hand application of supplemental or remedial preservatives that contain active ingredients (biocides). These preservatives use mainly solvents as carrier and in some cases form part of protective wood finishes, i.e. wood sealers, and therefore used as a dual purpose supplemental or remedial preservative against biological attack as well as a protective finish against weathering factors. Supplemental or remedial preservatives are mainly applied as a corrective measure to stop existing biological attack in untreated timber already in use, but can also be both corrective and preventative.

Protective wood penetrating finishes and sealers that are applied by hand, i.e. by brush, paint, and spray, but which contain no active ingredients (biocides) are not regarded as preservatives but merely wood protective finishes that protect against weathering factors only.

Why preserve timber?
The natural durability of our commercially grown and used plantation species like Pinus and Eucalyptus, is low, rendering it susceptible to insect and fungal attack; therefore it is imperative to preserve the timber. Timber preservation also enhances durability and confidence in using timber and extends its life, as well as providing the added benefit of increasing the carbon sink.

Preservation of timber and the sale and use of preservative treated timber are regulated in South Africa. Regulation A13 1(b) in SANS 10400-A, as well sections in the NHBRC home builder’s manual, requires the use of primary preservative treated timber when used in a permanent structures in specific areas of South Africa. In addition compulsory specifications i.e. VC 9092 regulated by the National Regulator for Compulsory Specifications (NRCS) control the sale of preservative treated timber.

Choosing the correct treated timber
SANS 10005 and the SANS product standards for preservative treated sawn timber (SANS 1288) and poles (SANS 457-2 or SANS 457-3) specify a Hazard Class system (H Classes), which categorises treated timber into different end-use applications based on the following:
• Different exposure conditions
• Potential risk of biological attack
• Preservative retentions/chemical loading

When specifying pressure treated timber be sure to choose the correct H class for your intended application and apply remedial preservative to all cross-cut and exposed areas (except for cross-cut ends exposed in ground, fresh water or marine applications). Apply a suitable penetrating wood sealer and regularly maintain it if a natural non-weathered look of the exterior timber is desired.

When planting a pole or post do not plant them inside an encapsulated concrete base. Instead, use a ‘collar’ or compacted stone and soil with or without a solid (cured) concrete base.

How to plant a pole

Safety Precautions and Warnings
When machining (e.g. sanding and sawing) CCA treated wood, be sure to wear a dust mask and work in a well-ventilated area to avoid inhalation of treated wood particles, and wear safety eyewear to protect your eyes from flying particles. Wear suitable safety gloves when working with freshly treated wood.

Do not make toys or furniture from treated wood that may be chewed on by infants, or make any food utensils from treated wood. Do not use treated wood for firewood, or cooking purposes.

Treated wood shavings or sawdust should not be used for animal litter or where it can become a component of animal feed.

Treated timber waste is not regarded as hazardous waste material; however, treated wood off-cuts and waste should not be allowed to accumulate, but should be disposed of at a registered disposal or landfill site.

For more information on wood preservation and where to find a SAWPA member in South Africa, please contact SAWPA at 0119741061 or sawpa@global.co.za or visit www.sawpa.co.za.

*First published in SA Forestry magazine, June 2017

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