Certification in the wood preservation sector

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Have a clear understanding of wood use classes

Wood preservation : the key points…

 

In Europe, and France in particular, the durability and preservation of wood are areas well covered by standardisation and regulations. The standards define the specifications to which the wood must comply, so that resistance to wood-destroying insects, fungi and marine organisms is ensured in service. End performances are defined by the fitness for purpose within a determined use class, that corresponds to the area of use of the wood.

THIS OBJECTIVE CAN BE REACHED :

  • – either by sufficient natural durability,
  • – or by using a preservative treatment,
  • – or by a combination of both of these requirements

The role of the advisor is to assess the risks to which the wood structures are subjected. The standards to which advisors should refer are essentially EN 335 and NF B 50-105-3, as well as FDP 20-651.

 

 

Misuse of terms

“Class 3 or class 4 treated wood” is a misuse of terms, this denomination is not proof of performance and fitness for purpose. Wood “treated for fitness for purpose in use class 3 or use class 4” is correct.

The required steps for a durable wood structure

steps fort the definition of a treatment for wood
 

Use class 1 INTÉRIEUR wood

Wood uses

Examples of structures in this class: furniture, parquet flooring, panelling, woodwork and interior fittings, etc.

Biological hazards

The risks are limited to wooddestroying insects and termites.

Choice of wood species

In these essentially non-structural uses, there are no regulatory requirements for the choice of wood species. Certain species that have been stripped of sapwood do not require treatment and are durable in terms of resistance to attack from insects (except termites).

emploi du bois en classe 1
 

Use class 2 INTERIOR wood or under cover

Wood uses

Examples of structures in this class
• Roof structures,
• Frames, etc.

Exemple d'emploi du bois en classe 2
 

Biological hazards

Risks of attack are mainly related to wood-destroying insects. The risk of the appearance of fungi is
not totally excluded, but it is limited to the surface of pieces of wood at risk of occasional wetting (typically through condensation).

Choice of wood species

In these mainly structural uses, the wood should (in accordance with
Decree 2006-591 and 2014-1427) :

  • – be either naturally durable with respect to the risk of attack from insects: for example, some species without sapwood, heartwood of oak, chestnut, larch, Douglas fir,
  • – or made durable by means of the application of a treatment.
    NB: for obligations related to the risk of attack from termites, please refer to the Decree in force (2014-1427).

Use class 3 EXTERIOR wood nit in contact with the ground, exposed to the waether

Use class 3.1 -EXTERIOR wood that dries quickly

Use class 3.2 – EXTERIOR wood exposed to prolonged wetting

Exemple d'emploi du bois en classe 2

The level of durability conferred can be chosen according to the severity of exposure, the amount of time the wood is exposed to wetting, the section of the wood pieces.

In conditions of low exposure, where the wood is not very thick (class 3.1) and is only subject to wetting for short periods (as it dries quickly or because the structure has been designed to promote water drainage), superficial protection may be sufficient (achievable by all the procedures). To guarantee a longer service life, it is strongly recommendet to also apply surface coating products (timber preservative, paint) and to ensure they are maintained.

In highly exposed conditions (class 3.2 ), where the wood is subject to frequent wetting over long periods, but not continuous, water can accumulate and the wood dries more slowly after wetting, deeper protection is required (achievable by autoclave).

Wood uses

The wood is not in contact with the ground and is directly exposed to the weather, especially rain. Examples of structures in this class: windows and other exterior woodwork, external coatings, (cladding in general), parts of frames exposed to the weather (such as certain parts of roof structures), certain elements in poorly ventilated bathrooms, etc.

 

Biological hazards

  • Fungi and mould in one or more parts of the structure where the moisture content exceeds 20% over potentially fairly significant periods of time
  • Wood-destroying insects. In class 3, there is no simple and universal response in terms of how to protect the zones. Rather, in each case, specifications that are specific to the given structure are required. The design of the structure plays
    a big role.

Choice of wood species

In these uses, certain species stripped of sapwood do not requir treatment (except for protection against termites). This is the case, for example, with the duramen, or “heartwood”, of oak, chestnut, pine, larch, Douglas fir and other species of tropical origin. When treatment is required, it is only applied to species that are sufficiently impregnable (cf. EN 350-2 and NF B 50 105-3).

 Use class 4 EXTERIOR wood not in contact with the ground or fresh water

Wood uses

This class includes all wood that is in contact with the ground or fresh water. By extension, structures susceptible to water retention are also assigned to this class. Typical examples of structures in this class are: pillars supporting overhead lines, beams, poles, water body constructions (stilts, pontoons, embankment supports, etc.) With designs that are conducive to
water retention, a majority of exterior joinery applications, outdoor play areas, green area developments, agricultural buildings, decking and duckboards, safety barriers and logs placed horizontally, are assigned to this use class.

Biological hazards

This includes all wood-rotting fungi and wood-destroying insects including termites. The attack the entire wood volume, as it gets wet.

Exemple d'emploi du bois en classe 4
 

Choice of wood species

In these uses, certain species stripped of sapwood do not require treatment (except for protection against termites). This is the case, for example, with the duramen, or “heartwood”, of oak, chestnut, pine, larch, Douglas fir and other species of tropical origin. When treatment is required, it is only applied to species that are sufficiently impregnable (cf. EN 350-2 and NF B 50 105-3).

 Simplified table of use classes (Mainland France)

Tableau simplifié des classes d'emploi du bois