Timber and exterior walls Part 2

• Various timber preservative qualities


  • Easy to apply
  • Able to penetrate deeply into the timber
  • Highly toxic to fungi and wood boring insects at relatively low concentrations
  • Able to be retained in the timber, even when saturated
  • Harmless to operatives, residents, paint coatings and furnishings
  • Non-corrosive to metals
  • Non-inflammable
  • Available at a reasonable cost
• No one preservative can claim to have all the qualities listed above but the best have most of them
• Careful thought needs to be given to whether timber preservatives really need to be applied, as they can contain hazardous and carcinogenic substances that can be harmful not only to the environment but also workers who are involved in manufacturing and applying them
• Treated timber is classified as hazardous waste and its disposal can threaten the environment
• Types of timber preservatives


  • Tar oil preservatives
  • Waterborne preservatives
  • Light organic solvent preservatives
  • Micro-emulsion preservatives
• Tar oil preservatives


  • Leach resistant (they do not easily drain from the wood when saturated with rain water), particularly suitable for treating timbers used in exterior work, immersed in water or buried
  • Can be applied to timber with a relatively high moisture content
  • Not usually corrosive to metals
  • Difficult to paint over and the preservative tends to creep when placed in contact with plaster or other absorbent materials
  • Have a strong odour
  • Freshly treated timber is more flammable than untreated timber, due to the volatile oils used as a solvent, but the flammability of the treated timber is no greater than that of untreated timber once these volatile oils have evaporated
• Waterborne preservatives


  • Cheaper and easier to transport than other preservatives as they are in powder form and can be mixed with water on site
  • Water is not absorbed quickly into timber
• Timber preservatives


  • Easy to apply
  • Able to penetrate deeply into the timber
  • Highly toxic to fungi and wood boring insects at relatively low concentrations
• Timber preservatives application


  1. Overcome the timber's natural resistance to penetration

  2. Typically vacuum impregnation under high pressure
• Copper


  Main ingredient for many preservatives, effective against wood-destroying fungi
• Compounds mixed with copper

  • Arsenic
  • Boron
  • Phosphates
• Chromium compounds


  Added to provide a fixative to improve the resistance of the preservative to leaching
• Copper chromium arsenic (CCA)


  Most widely used waterborne preservative, use restricted since 2004
• Chromated copper boron (CCB)


  Replaced CCA, but safety and environmental impacts also causing concern
• Alternative treatments using copper

  • Azoles
  • Quaternary ammonium compounds
• Borates


  Most environmentally friendly formulation, easily diffused into timber
• Waterborne preservatives

  • Odourless
  • Non-inflammable
  • Non-creeping
  • Easily penetrate into most timbers
  • Treated timber has light-green colouration
• Waterborne preservatives used on timber in contact with ground or water

  Tend to leach over time, particularly if chromium salts omitted
• Waterborne preservatives


  Corrosive to metals, re-drying of timber after treatment necessary
• Light organic solvent preservatives

  Most often applied by double vacuum pressure, preservative chemicals dissolved in light organic solvent
• Light organic solvent preservatives


  • Non-corrosive to metals
  • Non-creeping
  • Leach resistant
  • Penetrate well into most timbers
  • Do not swell timber on application
  • Re-drying of timber not required
  • Treated timber colourless, may be painted when dry
  • Non-injurious to plant life after solvent has evaporated
• Light organic solvent preservatives used in exterior environment


  Timber requires protective coating
• Light organic solvent preservatives

  • More flammable than untreated timber until volatile solvent has evaporated
  • Have strong odour that may be picked up by nearby food
• Light organic solvent preservatives are the most expensive both to manufacture and to apply
• Methods of applying preservatives

  • Brushing or spraying
  • Dipping, deluging or steeping
  • Hot and cold open tank
  • Pressure impregnation
• Factors determining the application method


  • Location timber is to be used
  • Type of timber
  • Permeability of timber
  • Type of preservative
  • Cost of application process
• Brushing or spraying


  Penetration into timber is limited, surfaces must be effectively coated and process repeated every 2-3 years
• Deluging, dipping or steeping

  1. Quick dip

  2. Pouring preservative over timber sections

  3. Timber sections in tank of preservative for several weeks
• Hot and cold open tank

  Timber immersed in preservative, heated to 80-90°C then cooled, forces preservative into timber cells, more expensive due to energy use
• Pressure impregnation


  Timber sealed in pressure vessel, air removed under vacuum, preservative forced in under high pressure, full cell or empty cell method
• Pressure impregnation


  • Most expensive application process but gives best depth of penetration, particularly useful for low permeability timbers
• Timber member sizes restricted by size of tree
• Timber sheets/boards


  Developed to overcome size restrictions, using resin adhesives to bond composite materials into larger sheets/boards
• Common timber sheets/boards


  • Plywood
  • Blockboard and laminboard
  • Particleboard
  • Fibreboard
• Plywood manufacture


  Thin timber strips (veneers) stripped from logs, bonded together with alternate plies crossed at 90°
• Plywood has an odd number of plies to balance movement stresses
• Plywood strength


  Determined by adhesive bond performance, classified by BS EN 314-2 for dry, high humidity and exterior uses
• Urea-formaldehyde resin suitable for dry interior uses, melamine urea-formaldehyde for high humidity, phenol-formaldehyde for exterior
• Plywood can be engineered for specific structural strength characteristics, denoted by suffix S
• Bonding quality of the adhesive


  Affects the end use classification of plywood