The Timber Treatment Challenge

Timber as a substrate has many advantages in Use Class 3 and 4 applications such as decking and fencing with the following characteristics being particularly valued:

Key Features

  • Readily available
  • Adequate strength
  • Ability to work
  • Relatively low permeability to water
  • Environmentally sustainable material
  • Aesthetically pleasing both visually and to touch in comparison to many other materials
  • Relatively cost effective
  • Physical Carbon sink
  • Natural durability (dependent of species)

However, durability over the long term is an issue for many species if protection against fungal and insect attack is not addressed, typically through chemical impregnation. Chemical preservative treatments will of course enhance the low natural durability of any species. But the picture is not straight forward. Just because a species of timber is not naturally durable does not mean that it will be easy to treat.

Treatability is defined into 4 classes as shown in the table below taken from the Standard BS EN 350-2 Natural Durability of Solid Wood – Guide to the durability and treatability of wood species.

Treatability Classes

1 - Easy to treat
Easy to treat; sawn timber can be penetrated completely by pressure treatment without difficulty.

  • Corsican Pine Sapwood (variable)
  • Scots Pine Sapwood

2 - Moderately easy to treat
Fairly easy to treat; usually complete penetration is not possible, but after 2hr or 3hr by pressure treatment more than 6mm lateral penetration can be reached in softwoods and in hardwoods a large proportion of the vessels will be penetrated.

  • Larch Sapwood (variable)
  • Sitka Spruce Sapwood

3 - Difficult to treat
Difficult to treat; 3hr to 4hr by pressure treatment may not result in more than 3mm  to 6mm lateral penetration.

  • Norway Spruce Sapwood (variable) and Heartwood
  • Sitka Spruce Heartwood and Sapwood
  • Scots Pine Heartwood

4 - Extremely difficult to treat
Virtually impervious to treatment; little preservative absorbed even after 3hr to 4hr by pressure treatment; both lateral and longitudinal penetration minimal

  • Douglas Fir Heartwood
  • Larch Heartwood
  • Norway Spruce Heartwood
  • Corsican Pine Heartwood (variable)
  • Scots Pine Heartwood

In order to ensure key commercial species such as those given as examples above are sufficiently protected against fungal and insect attack a clear understanding of the end use of the timber needs to be established.

Waterborne formulations are the most widely used wood preservatives in the world today. They provide protection against wood-attacking organisms in all situations and are applied using vacuum and high pressure process cycles to maximise penetration of the timber. They are odourless and after impregnation combine with the wood becoming resistant to leaching. It is possible to add dyes or water repellents during the treatment process and once dry, timber can be glued, painted or stained. Suitable for all situations but generally found meeting the requirements of Use Classes 3, 4 & 5.

The Use Classes are defined in the Standard BS EN 335-1 Definitions of Use Classes as shown below:

UC 1 - Internal, dry – for example upper floor joists
UC 2 - Internal, risk of wetting – for example tile battens
UC 3.1 - Outdoors, coated, above ground – for example window frames
UC 3.2 - Outdoors, uncoated, above ground – for example fence rails
UC 4 - Direct soil or fresh water contact – for example fence posts
UC 5 - Marine use – for example marine pilings

The definitive Standard for treatment is  BS 8417 Preservation of Wood- Code of Practice with the latest revised edition was launched on the 31st March 2011. Within this standard guidance and parameters are given on key elements when considering treating timber for certain applications such as:

  • Desired service life
  • Chemical retention levels within the analytical zone
  • What the analytical zone is
  • Levels of penetration.

By employing a Porcupine Incisor benefits will be seen in both levels of retention of chemical and levels of penetration of chemical when timber is pressure impregnated after incising. This will help to ensure that the timber products of different species but particularly refractory species do achieve the requirements for specific desired service life for products in different use classes. For example achieving consistent levels of penetration of 6mm or more in the analytical zone of a Spruce fence post. This also provides the solution to heartwood penetration.