BTCV Handbooks Online

Book:	Footpaths
Chapter:	C Timbers and preservatives
Section:	Preservative treatments
Metadata:	Details	Buy this book

There are three factors to be considered:

The permeability of the timber (see timber characteristics).
The method of application.
The type of preservative.

Methods of application

Application of preservative is better done on timber which has already been cut to size.

Brushing and spraying

This is the least effective method, and should only be used on joints and end grain cut after main treatment, when no other method is possible. Apply liberally, using at least four coats.

If this is the only possible method of application, timber should be re-treated every few years to give satisfactory penetration. Creosote penetrates better if heated before application.

Immersion

Usually called 'dipping' for periods up to about 10 minutes, and 'steeping' if immersed for hours or days. This is only effective for permeable timber, as resistant timber absorbs little even after long periods of steeping. Organic solvents or creosote can be used for dipping. Creosote is normally used for steeping and is a convenient method for the treatment of estate timber which is often permeable sapwood, such as round fence posts.

It is impossible to give optimum immersion periods, but as a general rule aim at a minimum of five minutes for dipping, and 24 hours for steeping. Dipping is the best method where timber, for example newly cut end grain, has to be treated on site.

Double vacuum

This is a commercial process for applying organic solvents. It gives long term protection against decay and insect attack for timber not in contact with the ground.

Pressure

This is a commercial process, and the most effective method of applying preservative. Even resistant timbers will absorb sufficient preservative. All wood merchants should be able to supply pressure-treated wood, and may also treat wood supplied and cut by the customer.

TYPES OF PRESERVATIVE

Coal tar creosote to BS 144

This is best applied under pressure, but can also be applied by brush or spray. Creosote helps prevent checking (cracking) of timber exposed to the weather. Highly toxic to wood-destroying fungi and insects. Non-corrosive to metals. Variety of shades of brown. Gives some repellance to water which helps retard dimensional movement. Strong smell, making treated timber unpleasant to handle. Unsuitable for handrails or handrail supports. Stains. Cannot be painted over. Inflammability increased for short period after treatment, but once thoroughly dried, is slightly more fire-resistant that untreated timber.

Water-borne salts to BS 4072

These are mostly copper/chrome/arsenic, such as Tanalith or Celcure, and need to be applied under pressure. They have no smell. Forms insoluble compound in treated wood which cannot be leached out. Gives greenish-grey colour to timber. Can be painted over and glued. Not moisture repelling. Timber swells during treatment and should be re-dried to restore to original size before use as joinery. Non-corrosive to metals, non-staining and non-flammable.

Organic solvent

These are organic fungicides/insecticides in an organic solvent, such as Cuprinol or Solignum. Double vacuum is the most effective method of application, followed by immersion and then brush or spray as the least effective method. Preservative resistant to leaching but some lost by evaporation. Non-corrosive and non-staining. Can be painted and glued. Treatment does not cause swelling of the timber and therefore can be used on accurately cut wood. No fire hazard once solvent has evaporated. Not water-repellent, but additives can be included to resist moisture changes in use. Many colours available. Not suitable for timber in contact with the ground. Suitable for signboards, handrails, gates.