Understanding the timber treatment process

Timber Treatment

Timber Types

Naturally grown wood can be divided into two categories – hardwood and softwood.  Contrary to popular belief, these terms have no bearing on the physical strength of the wood.

According to the NORDIC WOOD COUNCIL, ‘Hardwood and Softwood are broad biological terms used to describe the species of wood. The terms have nothing to do with the physical hardness of the wood. Hardwoods come from broad-leaved trees and softwoods species from coniferous evergreen trees’

Examples of hardwood include mahogany, birch or oak.  Examples of softwood include pine or fir.  The fundamental major difference between the two species is that Hardwoods are slow growing and Softwoods faster growing.  Each type of wood also boasts a different physiological make-up.

All timber is made up of heartwood and sapwood. Heartwood is the inner part of the timber, it tends to have a denser structure and is more resistant to decay.  Sapwood is the younger, outermost wood on the tree which has a more porous structure and is therefore more susceptible to damage. Typically hardwoods tend to contain more heartwood.

Further factors influencing successful treatment

In addition to identifying the correct species of timber, the success of the treatment is also determined by further factors:

Moisture content: preservative can be applied to the wood by dipping or brushing the surface of the wood which will treat only the surface.  However, a far more effective means of treating the whole timber is to drive the preservative through the sapwood into the heartwood using a pressure system. 

The drier the timber, the greater its capability to absorb preservative but just checking the outside of the wood is not an effective measure of the level of moisture content. Air-drying alone can lead to inconsistencies and the outside of the timber will invariably be drier than the inside. Kiln drying which forces air through the timber at controlled temperatures and humidity conditions is the only effective means of achieving the desired moisture content of less than 28%, the point at which the timber is able to absorb the required level of preservative.

Loading of chemicals: when the timber is dry enough to take on the preservative, it is important to ensure that the correct amount is actually forced into the timber.  Jacksons uses a pressure treatment system  which ensures the preservative penetrates deeply into the cell structure of the timber.  According to the choice of preservative system, the amount of treatment per cubic meter might change.  Jacksons’ Jakcure process uses over 22kg of chemical to every cubic metre of treatment timber intended for contact with the ground.  In contrast some treatment systems use only a fraction of that amount and therefore are unable to offer the same level of protection, others use as much as 36kg of chemicals to attain the same degree of efficiency.  Trying to pump higher levels of chemical into the wood will result in problems in achieving permeation, but also clearly represents a high cost to the environment. 

Jacksons will adapt the loading of chemicals to accommodate the desired usage of the timber. Although this is more labour intensive it demonstrates a responsible approach to sustainability.

Use of end grain preservative: cutting or drilling timber after treatment may jeopardize the overall efficacy of the preservation system.  Any cut end will need to be coated with END GRAIN preservative to maintain the high level of protection.

Responsible sourcing

No article relating to the usage of timber would be complete without a reference to the requirement to adopt a responsible approach to the sourcing of this raw material. Always seek out a timber which has been produced under an environmental chain-of-custody scheme such as the FSC (Forest Stewardship Council) or PEFC (Programme for the Endorsement of Forest Certification).


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What is timber treatment 

In short, timber treatment is a form of wood preservation. The aim of the treatment process is to penetrate deep into the timber and coat the structure with a preservative to help protect against from decay caused by wood-destroying fungi and insects.

Effectively, the preservative is penetrated deep into the timber structure through pressure and a vacuum, this area of the timber structure is known as the heartwood. Initially the vacuum removes any air from cavities within the timber and effectively creates space for the preservative solution to be forced into the wood using extreme pressure.

Why is timber treated?

Any timber which is to employed in an outside application, and especially timber which is destined to come into contact with the ground, freshwater or simply be exposed to the elements, requires some form of preservation to protect it from the challenges of an external environment.

The amount of preservative required to deliver optimum protection will be influenced by the intended end use of the timber. Use classes for timber are defined in the standard BS EN 3335-1 and are based on the level of threat posed by decay or insect attack which can vary from one environment to another. 

Any timber which is sited in a ‘Class 4’ application which means it is used in the ground or has contact with fresh water or a Class 5 (a marine or salt water environment) will fail unless it has been successfully and adequately protected.

Which timber responds better to timber treatment?

In general, softwoods are more porous and as a result, capable of absorbing greater quantities of any timber preservative through the sapwood and into the heartwood. In contrast, the cell structure of a hardwood dictates the requirement for a much higher loading of any preservative which is simply not practical. Hardwoods are also more expensive and difficult to dry - a factor which will greatly influence the efficiency of the treatment process.