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Preservatives are applied by one of two general methods: pressure or non-pressure. Figure 3.4 gives some indication as to how the method used affects the depth of penetration.

3.4.1 Pressure methods

Timber is put into a sealable chamber into which preservative is introduced under pressure as described below:

High Pressure Methods:

● _{Full-cell process} ● Empty-cell process

Low Pressure Method:

● Double-vacuum process

a Full-cell process – After sealing the chamber, the air is removed by using a vacuum pump. With the chamber still under a vacuum and after a prescribed period, preservative is introduced, filling the chamber. Vacuum is then released and pressure applied. The chamber will remain filled until the timber has absorbed sufficient preservative, which will vary according to the permeability of the wood. Pressure is then released on completion, surplus preservative is pumped out of the chamber back into its storage tank. Finally, a further vacuum is set up in the chamber, but only of sufficient strength to remove any surface preservative – cell cavities will remain full. This process may use creosote or water-borne

preservatives.

b Empty-cell (Lowry) process – In this case, the preservative is pumped into the chamber subjected to normal air pressure. After the wood has absorbed sufficient preservative, the pressure is released and surplus preservative is driven out of the wood by the expanding air in the wood cells. A vacuum is then used to draw off any residue. Although the wood cells are emptied, their walls remain fully treated. This process is used for both creosote and water borne preservatives.

c Double-vacuum process – (A typical treatment plant is shown in fig. 3.5). The chamber is sealed and a partial vacuum is created. The chamber is then filled with preservative and pressurised to atmospheric pressure, or above, depending on the process. After a prescribed period, it is then drained and a final vacuum is created to remove excess preservative from the timber. This process is mainly used to apply organic- solvent preservatives into timber for exterior joinery – required depths of penetration are therefore less than for the full-cell and empty-cell processes but better than for immersion. It does not cause timber to swell or distort, and all machining should be done before treatment.

3.4.2 Non-Pressure process

With the exception of the ‘hot and cold open- tank treatment’ and ‘diffusion’ methods described below. Depth of preservative penetra- tion with non-pressure methods is often limited to just below the surface of the wood – as shown in figure 3.4 (a & b).

The following methods of application should be read in conjunction with figure 3.6.

a Brushing – Can be used for applying creosote, organic solvent, and some water- borne types of preservative but because of low penetration, it is not a suitable method for timbers that come into contact with the

Methods of applying preservatives 79

Table 3.3 General use of water-borne (WB) preservative treated timber Methods of treatment/application Examples of general use Low pressure (double vacuum) – Pitched roofs:

example, ‘Vacsol Aqua’ Wall plates, trussed rafters, ceiling joists, rafters, ridge boards, purlins, binders, sarking (sheet roof covering - prior to weather proofing e.g., tiles / slates etc.).

Note: Barge boards, fascia boards, and soffits will require a

maintained weatherproof protective coating.

Flat roofs:

Wall plates, Joists, outriggers, strutting, decking.

Note: Fascia boards and soffits will also require a maintained weather proof protective coating.

Floors:

Ground floor joist

External walls:

Studs, noggings, head binders, sheathing, external battens and counter battens.

Note: External-cladding boards will also require a maintained weatherproof protective coating.

External joinery:

Doors, windows, porches (excluding sole plates), canopies (inc., roof menbers and trims).

Note: Exposed surfaces will require a maintained protective coating High pressure (empty cell ‘Lowry’ process) As above, also, primarily associated with timbers prone to dampness and – for example: ‘Tanalised’ frequent wetting:

Structural building members

Agricultural buildings, boundaries and gates Horticultural timbers

Structural items associated with garden features, playground equipment, and landscape design.

General fencing posts (stakes) and rails. Railway sleepers

Marine work

Cooling tower timbers

*Non-pressure:

Brushing Localised treatment:

Interior woodwork, and exterior woodwork before the application of an exterior protective finish.

Remedial work:

Interior woodwork, or exterior woodwork before the application of an exterior protective finish.

Spraying Remedial work:

Interior woodwork

Immersion (dipping) Small items of joinery and short lengths of timber *Special formulation

Note: Preservative formulations will vary to suit the method of application and degree of protection against fungi and/insect attack

ground. As a rule, re-treatment is advisable every three to four years.

b Spraying – Similar penetration and

conditions apply as for brushing. Because of the health risk associated with applying preservatives, precautions should always be taken – particularly when spraying – to ensure that:

● only coarse sprays are used, to avoid atomisation;

● work areas are well ventilated; ● operatives are suitably clothed; ● hands are protected by gloves; ● mouth and nose are protected by an

approved face-mask;

● eyes are protected with snug-fitting goggles – not glasses;

● Manufacturer’s instructions are followed. c Deluging – In deluging, the timber is

passed through a tunnel of jets that spray it with preservative.

d Immersion

● Dipping – the timber is submerged in a tank of preservative (coal-tar oils or organic-solvent types) for a short period, then allowed to drain.

● Steeping – The timber is submerged for periods ranging from a few hours to weeks, depending on the wood species, the sectional size of the timber, and its end use. Steeping is a suitable method for preserving fence posts etc

e Hot & Cold (open tank treatment) – The timber is submerged in a tank of preservative (coal-tar oil) which is heated. It is then allowed to cool in the tank or is transferred to a tank of cold preservative. This treatment is suitable only for permeable timbers and sapwood. Coal-tar oils are flammable, therefore extra care is necessary with regard to the heat source.

Table 3.4 General use of tar-oil (TO) type (Creosote) preservative treated timber Methods of treatment/application Examples of general use

High pressure – full cell process, or Fencing posts (stakes), and rails:

empty cell process where bleeding of Farms, estates, domestic gardens, railways and motor-way

preservative is to be avoided. embankments, etc.

Poles in the round:

Transmission and telegraph poles.

Piling:

Retaining walls, sea defences, jetties etc.

Timber sleepers:

Railways

Non-pressure:

Hot and Cold tank open-tank Fencing posts (stakes), and rails:

treatment Farms, estates, domestic gardens, etc.

Immersion(steeping) Items above ground:

Fence panels, rails etc.

Immersion(dipping) Items above ground:

The amount of protection offered by all Fence panels, rails etc.

the above treatments will to a greater extent depend on:

● the duration of submersion ● species of timber

● treatability (see Section 1.11(f)) ● amount of sapwood / heartwood

Brushing Remedial work:

Fence panels, rails etc.

Note: Preservative formulations will vary to suit the method of application and degree of protection against fungi and/insect attack.

f Diffusion – This method of treatment is only associated with freshly felled green timber which, at the saw mills, is immersed in a water-borne preservative (usually boron salts) and then close-piled and placed under cover until the preservative has diffused into the wood. The type of water-borne

preservative used in this process is liable to leach out from the wood, which makes the timber unsuitable in wet locations unless an impervious surface treatment is given, i.e. paint or varnish.