Preservation of Timber - Code of Practice
2001 Edition

IS 401 outlines three main categories of preservatives tailored to timber species and their uses: 1) Water-borne preservatives such as Copper Chrome Arsenate (CCA), Copper Napthenate, and Borates, ideal for structural and outdoor timber susceptible to fungal and insect damage; 2) Oil-based preservatives including Creosote and Coal Tar Oil, used predominantly for heavy-duty applications like railway sleepers and poles; and 3) Non-pressure preservatives involving surface coatings and brush-on types, suitable for prophylactic treatments during storage or less critical uses. Selection depends on timber durability, environmental exposure, and treatment technique. While these treatments enhance timber’s resistance and longevity, they do not alter mechanical properties like hardness.

For green timber, which has high moisture content (typically above 60%), IS 401 recommends steaming combined with vacuum cycles to reduce moisture before applying preservative treatments such as pressure or Boucherie methods. This is essential as green timber is more vulnerable to fungal decay during air seasoning. Seasoned timber with moisture content between 20-30% is suitable for surface treatments like brushing, spraying, or dipping, including paste or injection methods for difficult species. Timber with moisture below 20%, especially oil-type treated, primarily undergoes surface application. Hence, green timber requires moisture reduction and intensive treatment, while seasoned timber can be treated by less invasive surface methods.

IS 401 prescribes visual and chemical techniques for evaluating preservative uptake. Penetration is determined by applying chemical reagents (e.g., copper trichlorophenate) to cross-sections or wood borings, revealing color changes that indicate preservative distribution. Absorption measurement involves chemical analysis of treated timber samples, comparing results with service tank readings or weight differences before and after treatment, as outlined in IS 2753 Part 1. Factors influencing penetration include preservative concentration, diffusion time, timber species, moisture content, and temperature. These methods ensure accurate assessment of treatment effectiveness and timber protection levels.

IS 401 mandates pressure treatment for all timber employed in marine settings to ensure durability. For refractory species, incising to depths of 12 to 19 mm is necessary to promote preservative penetration, followed by full-cell pressure treatment. Common preservatives include copper-based formulations such as CCA, CCB, and BCCA, with typical total salt retention around 8 kg/m³ (e.g., 4 kg copper). Surface treatments with zinc naphthenate, copper abietate, or TCP are also employed. Penetration requirements specify a minimum of 30 mm sapwood penetration for embedded piles and marine timbers, ensuring protection against fungi, termites, borers, and marine organisms for extended service life.

IS 401 recommends a multi-tiered strategy to prevent biological degradation. For timber already affected, it advises exposing the damaged sections and applying organic solvent-based preservatives through brushing or spraying, and flooding boreholes with suitable chemicals. Sterilization methods like boiling or steaming logs are used to arrest light fungal or insect attacks, followed by prophylactic preservative applications to prevent recurrence. Additionally, mechanical protections such as metal straps and specific end-holding methods per IS 10753 are recommended for sleepers and valuable timber. The use of insecticides like deltamethrin (IS 11996) and other approved preservatives (IS 12016) further contributes to effective control of borers and fungi.