Preserving Confidence

David Sulman, of the Scottish Timber Trade Association, provides an update on timber preservatives...

Timber is the only truly sustainable material commonly used in construction. It is a versatile material and a highly durable material - just think of the number of old timber buildings and other structures that have stood the test of time.

Vital Factors in specifying

In specifying and using timber, we must take into account strength, appearance, cost and availability. Where the timber is to be used externally and for certain other uses in construction, we also need to consider its durability.

As trees grow, some produce a natural preservative, protecting against fungal decay and sometimes, insect attack. This natural resistance, known as natural durability, varies between species. All commercial species have been assessed and assigned a natural durability category, described in BS EN 350-1 as follows;

Natural Durability

  • Class 1, very durable, i.e. greenheart, teak, opepe
  • Class 2, durable, i.e. sweet chestnut, European oak
  • Class 3, moderately durable, i.e. cherry, sapele
  • Class 4, slightly durable, i.e. European redwood, European whitewood
  • Class 5, not durable, i.e. ash, beech, poplar, sycamore

Importantly, these classifications refer to the heartwood of the species, as most sapwood is considered "not durable" or "slightly durable" and should not be used in exposed situations without preservative treatment.

Where durability is key, we have a choice:

  • use a naturally durable timber, or
  • use modern timber preservatives to enhance the natural durability of less durable species.

By enabling us to make better use of less durable species, modern timber preservatives help us to maximise our wood resource.

Construction Timber

Pergola and deckIn construction, the commonly used softwood species are rated as Class 4 to Class 5. For external uses, or where there may be a risk of wetting, preservative treatment by impregnation is required. Hardwoods are usually specified for their decorative appeal, but some hardwood species have outstanding natural durability and require no preservative treatment when used externally.

The first issue to consider when considering preservative treatment is the risk that timber may be exposed to, particularly exposure to wet conditions. If this happens for prolonged periods, timber may be susceptible to fungal decay.

Good detailing, especially good water-shedding and the avoidance of water traps, is also essential in prolonging the life of timber used externally.

To Use or Not to Use?

For some uses, preservative treatment is obligatory, whilst in others, it may be an "insurance policy". Hazard Categories exist to assist in determining when preservative treatment is necessary. They are:

  • Internal, dry, insect risk, i.e. trussed rafters, first floor joists etc.
  • Internal, risk of wetting, i.e. ground floor joists, plywood sheathing.
  • External, above ground, frequent wetting, i.e. external cladding/joinery.
  • Direct soil or freshwater contact, i.e. fence posts, gravel boards etc.
  • Marine use, i.e. marine structures, including sea defence works.

Specifying preserved timber must address three key points; Hazard Class, timber species and desired service life.

Guidance on the need to preservative-treat timber is given in a number of British Standards, including BS 5268-5 and BS 5589.

Preservation Processes

For the best long-term performance, applying preservatives by controlled impregnation processes using pressure/vacuum cycles is required. Brushing, spraying or dipping provides only limited penetration of preservative and therefore, limited protection. Any surfaces exposed by cross-cutting, notching, drilling etc after treatment should be re-coated with preservative to maintain the envelope of protection.

Traditionally, wood preservatives applied by impregnation methods were classified into three groups;

  • water based preservatives, typically used for ground contact uses
  • solvent based preservatives, for above ground uses, including joinery, cladding and roof trusses
  • tar oils, used for telegraph poles, railway sleepers etc.

Timber frame detailingToday, with advances in technology and growing environmental awareness, new formulations are used. Many traditional formulations have been replaced or modified. There have been developments in traditional water-based preservatives and many traditional solvent-based preservatives are now aqueous solutions. Tar oil type preservatives (creosote), are still available, but like other preservatives, are for professional use only.

Insect Attack

Insect attack is generally of less significance in the UK than fungal decay. BS EN 350-2 provides information on the resistance of some timbers to insect and marine borer attack .


In addition to solid wood, plywood can also be treated by impregnation processes to enhance its durability.

Specialist information on timber preservative treatments can be obtained from STTA Members, from the Scottish Timber Trade Association, tel: 01786 451623, e-mail: and from the British Wood Preserving and Damp-proofing Association (BWPDA), tel: 01332 225100, e-mail

Photos show a Naturewood treated Gazebo and a Protim treated timber frame house, courtesy of Osmose UK.

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