3.3 Timber preservation (natural solid timber)
This chapter gives guidance on meeting the Technical Requirements for the protection of natural solid timber against fungal decay and insect attack, dependent upon its use.
Definitions for this chapter#
For the purposes of this chapter, the following definitions apply:
| Coated/coating | Application of a suitable water-repellent decorative surface coating, such as a paint or stain, which must be maintained over the life of the component. The application of a surface coating should be in addition to the use of factory-applied preservative treatments or timber with adequate natural durability appropriate for the intended application |
| Desired Service Life | The period over which a component can function as it was intended. A Desired Service Life is indicative rather than a guarantee of performance and is based on an assumption of good design and maintenance in service |
| Durability Class | Durability Class are systems of determining and classifying durability of wood species or wood-based materials to various wood destroying organisms based on test methods described in relevant EN Standards. Durability Class are defined in BS EN 350 along with durability ratings of commonly available wood species. The following grade scales are used depending on the destroying organisms:
• A five-grade scale (Durability Class 1-5) for decay basidiomycete fungi and soft rotting micro-fungi (Table 1 of BS EN 350 & expanded in Table 2 of this chapter) • A two-grade scale (Durability Class D & S) for wood-boring beetles - Hylotrupes bajulus, Anobium punctatum etc (Table 2 of BS EN 350) • A three-grade scale (Durability Class D, M & S) for termites and marine organisms (Tables 3 & 4 of BS EN 350) |
| Durability to biological agents | Inherent resistance of a wood species or a wood-based material against wood decay organisms |
| Engineered wood products | Engineered wood products such as glue-laminated timber, laminated veneer lumber, I section and metal web studs, joists and rafters etc, should either have adequate natural durability or be preservative treated following the guidance in this chapter |
| Heartwood | Inner zone of wood that, in the growing tree, has ceased to contain living cells or to conduct sap |
| Modified wood | Wood modification (not to be confused with preservative treatment) is the industrial processing of timber using thermal or chemical processes to enhance the natural durability of a timber species. Examples of wood modification include thermal treatment (not kiln drying) and acetylation
The modified wood manufacturer should confirm the Durability Class of the product and provide information on its handling, processing and use Modified wood and modified wood-based materials should assessed in accordance with Technical Requirement R3 |
| Preservative treatment | Treatment with a chemical substance to improve resistance to fungal decay and/or wood destroying insects. Application of preservative treatments is an industrial process normally carried out using a combination of pressure and vacuum in a treatment vessel (industrial spray and dip treatments may also be suitable for certain applications). The pressure, duration, formulation, and timber species used will offer varying levels of protection. Further guidance on treatment specifications can be found in BS 8417. Treatment specifications should be suitable for the Use Class and Desired Service Life combinations set out in Table 1 below |
| Sapwood | Outer zone of wood that, in the growing tree, contains living cells and conducts sap. The sapwood of all wood species is considered non-resistant against decay fungi unless other data are available |
| Service Class | The Service Class system is intended for use when assigning strength values and calculating deformations
under defined environmental conditions for timber components and wood-based materials
A structural engineer will need to specify and consider the Service Class of a timber component when undertaking structural designs and calculations. Further guidance on Service Classes can be found in BS EN 1995-1-1 |
| Use Class | Is based on differences in environment exposures that can make the wood or wood-based products susceptible to biological deterioration. Table 1 of BS 8417 summarises the Use Class system and gives examples of typical service situations. BS EN 335 contains definitions of Use Classes which have been
summarised below:
• Use Class 1: Situations in which the wood or wood-based product is inside a construction, not exposed to the weather and wetting • Use Class 2: Situations in which the wood or wood-based product is under cover and not exposed to the weather (particularly rain and driven rain) but where occasional, but not persistent, wetting can occur. In this Use Class, condensation of water on the surface of wood and wood-based products may occur • Use Class 3: Situations in which the wood or wood-based product is above ground and exposed to the weather (particularly rain). A large variety of in-use situations exist and, when relevant, Use Class 3 may be divided into two sub-classes 3.1/3(c) and 3.2/3(uc) • Use Class 3.1/3(c) coated: In this situation the wood and wood-based products will not remain wet for long periods. Water will not accumulate. This may be achieved by, for example, a maintained and suitable coating • Use Class 3.2/3(uc) uncoated: in this situation the wood and wood-based products will remain wet for long periods. Water may accumulate • Use Class 4: A situation in which the wood or wood-based product is in direct contact with ground and/or fresh water |
| Wood species | Trade name according to BS EN 13556 which can on occasion include more than one botanical tree species |
Introduction#
This chapter sets out Use Classes, Desired Service Life, preservative treatment, and natural durability of solid wood components. It does not cover:
- products such as plywood and wood-based particle boards
- the condition before the treatment
- treatment schedules, formulation, and process techniques, which is the responsibility of the organisation carrying out the operation.
Clauses for 3.3 Timber preservation (natural solid timber)
- 3.3.1 Compliance
- 3.3.2 Durability
- 3.3.3 Sitework
- 3.3.4 Protection and storage
- 3.3.5 Treatment of cut surfaces
- 3.3.6 Compatibility with metal
- 3.3.7 Further information
- Figure reference table
Last updated: 2nd January 2024