SERVICE, USE CLASSES AND TIMBER DURABILITY
During the design process, the service classes for different structural components should be identified. This will then inform the material specification; materials chosen must be suitable for use within the relevant service class, and appropriate durability strategies and details should be provided. Use classes Use classes (UC according to BS EN 335:2013) are conceptually similar to service classes but fulfil a different function; they offer a guide to the vulnerability and decay risk of timber products. BS EN 335:2013 defines five use classes for different environmental exposures for solid wood and wood-based products. For each use class, the standard indicates the biological agents relevant to each instance of exposure. The use class describes the fitness for purpose of a product for a particular environment that can be achieved either through natural durability or through a preservative treatment, or a combination of both. Timber durability Every timber species can be classified based on its natural durability (according to EN 350-2) on a scale from 1 to 5 (1 being very durable and 5 non-durable). Larch, Pine, and other species commonly used for mass timber are all class 4: slightly durable 6 .
When specifying wood components, it is vital to understand their durability characteristics, service, and use classes: key indicators of the correct use and expected service life of engineered wood products. Service classes Service classes (SC) describe the environmental conditions (temperature and relative humidity expected to prevail in service) affecting the moisture content in timber products exposed to them (see relevant table adapted from EN 1995-1-1 Eurocode 5). – SC1 covers timber products situated within the building envelope in spaces that are generally heated, and not exposed to external moisture and temperatures, a space where the timber is kept at a moisture content of 12% or less. – SC2 timber is expected to be exposed to external moisture and temperatures, but not wetting. – SC3 portrays a condition where timber is external, fully exposed to large variations of moisture and temperature.
SERVICE CLASSES INFORMATION
Example from the UK National Annex to Eurocode 5
Service Class Temperature Approx RH* EMC**
Warm roofs, intermediate floors, timber-frame walls - internal and party walls.
1
20 ° C
65%
12%
Cold roofs, ground floors, timber-frame wall - exterior uses where member is practiced from direct wetting
2
20 ° C
85%
20%
Conditions leading to higher moisture contents than service Class 2
3
>20%
Exterior uses - fully exposed
* RH relative humidity: level not to be exceeded for more than a few weeks each year ** EMC: Max equilibrium moisture content for most softwoods. In similar conditions the EMC of panel products will be typically lower
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DURABILITY
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