Gordon Ewbank, Director of the Wood Protection Association, gives a masterclass on why wood needs preservative protection.
All mature trees have an inner core known as heartwood. This is surrounded by an outer layer of younger sapwood where the tree stores nutrients essential for its growth. These food reserves remain after the tree is logged and sawn into components.
The heartwood of some species contains naturally occurring chemicals that make it relatively durable and provide some resistance to decay and insect attack. The level of natural durability varies between species.
Sapwood, on the other hand, is a source of food for many species of fungi and insects and is always vulnerable to attack. This risk increases significantly if the wood’s moisture content rises above 20% – for example through poor installation practice or maintenance, or persistent condensation and damp.
A significant proportion of sapwood is likely to be present in the plantation-grown softwood timber we use for building, hence the need to treat most softwood timbers before use to enhance durability.
© Hoppings Softwood Products. Decking must be treated to Use Class 4 standards.
Technical standards
It is a mistake to assume that all pressure-treated wood is the same. While one piece of treated wood may look like any other, the level of preservative protection could be very different. That’s because national technical standards for wood preservation require that the loading and penetration of preservative is tailored to the desired end use.
Minimum wood treatment standards are set out in British Standard 8417 and the WPA Code of Practice – Industrial Wood Preservation. In these standards the level of treatment is tailored to the application Use Class of a wood product as defined in BS EN 335 (Durability of wood and wood-based products).
This means a designer can specify a structural timber component and expect it to perform for a certain end-use for a defined Desired Service Life (DSL).
Controlled industrial preservatives
Different timber products require different preservative treatments depending on their usage and the environments in which they will be installed.
The level of protection conferred by a wood preservative depends on its method of application. Brush, dip or spray-applied products will afford a degree of protection but, for extended service lives of 15, 30 or 60 years, typically only wood pre-treated by an industrial penetrating process can give the required level of protection.
The most widely used process for applying preservative treatments to solid wood components and some panel products is vacuum-pressure impregnation. This is carried out by specialist companies in large pressure autoclaves under factory controlled conditions.
Certain timber species are more difficult to treat due to their cell structure. In these cases, pressure treatment is the best option to ensure penetration of the treatment reaches the greatest depth possible. Additional techniques such as incision may be required to help resistant species achieve the penetration requirements, particularly for ground-contact applications.
Know your Use Classes
The most commonly used Use Classes for preservative treatment is UC2 for interior applications and UC3 and UC4 for exterior uses.
Use Class 2 is for end uses where wood is used indoors, above ground and within the building envelope, but with an occasional risk of wetting (for example, from condensation in or around the insulation in a cold roof space).
While the risk of failure resulting from decay or insect attack in Use Class 2 applications may be relatively low, the cost and difficulty of remedial work on such structural timbers may be significant, so preservative treatment is still recommended. The National Housebuilding Council, for example, specifies that all external structural walls in timber frame structures must be treated for this reason.
Common Use Class 2-treated components have a minimum preservative retention requirement to offer a 60-year DSL.
Use Class 3 is for end uses where wood is used outdoors but is not in contact with the ground.
Use Class 3u is for uncoated timber components such as deck boards, cladding and fence rails, while Use Class 3c is aimed at the specialist coated joinery product sector, for applications such as painted timber windows and doors.
Preservative penetration and retention requirements for common Use Class 3u components offer a minimum 15-year DSL, with the option of a 30-year extended life. Preservative requirements for common Use Class 3c components offer a minimum 30-year DSL.
Use Class 4 is for end uses where wood is in contact with or very close to the ground, is frequently wet and/or provides exterior structural support. Such applications include timber components such as fence posts and decking or boardwalk sub-structure components, such as joists, beams and posts, plus playground equipment.
Preservative penetration and retention requirements for common Use Class 4 components are set to offer a minimum 15 year desired service life (DSL) with an option to extend to 30 years.
TDUK and the Wood Protection Association work closely together to promote best practice and raise awareness of the importance of correct preservative wood treatments. You can find more information at www.thewpa.org.uk and www.timberdevelopment.uk
- This article was originally published in Issue #01 of Supplying Timber magazine.