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Timber structures Timber frame

biological attack of an element and depends on differences in exposure environments. Use classes are further defined by the time wood is exposed to high moisture levels (often defined as levels of 20% or above).

Outer Bark Inner Bark

Similarly, the design of timber components and structures plays an important role in service life. For example, service life increases when avoiding ground contact, covering end grains and avoiding moisture traps.

Cambium Sapwood Heartwood

Standardisation In Europe, CEN/TC 38, Durability of wood and woodbased products, aims to progress standards for wood preservatives and preservative-treated wood, modified wood and untreated wood. It is developing terminology, analytical methods, biological tests, classifications and specifications in accordance with market needs and European regulations. Its objectives include ensuring quality and satisfying consumer expectations, eliminating trade barriers, harmonising methods used in wood protection and promoting sustainable development by delivering reliable wood products. This has helped lead to a gradual shift from traditional wood preservation to wood protection, which is based not only on the inherent characteristics of the wood itself but also on design, maintenance, exposure and moisture risk. In most countries, building regulations and building control systems govern the fire safety aspects of structures. Meanwhile, ten main Eurocodes and subsequent sub-codes provide a common approach to the design of buildings and construction products, with Eurocode 5 being directly related to timber.

Life cycle Life cycle analysis addresses the entire life cycle of a product and a wide range of environmental impacts, avoiding ‘problem shifting’ – for example, from one life cycle stage to another or from one environmental compartment to another. Service life prediction aims to ensure that the service life of a building will equal or exceed its design life, while taking into account its life cycle costs.

Use classes Biological durability is the key factor determining performance for wood in different use classes. The robust laboratory and field test methods that exist make it possible to assign a durability rating (from non-durable to very durable) to timber linked to the intended use class and assuming a worst-case scenario.

The heartwood of timber

EN 335:2013 classifies wood into five durability classes for heartwood of timber species, from “non-durable” to “very durable”, and the natural durability of a wood species can vary widely. The use of heartwood from more durable timber species, wood preservatives or modified wood can also improve the service life of outdoor products. Despite these, better design must also play a part in ensuring their satisfactory service life. While timber indoors (use classes 1 and 2) would normally be dry, care must be taken during construction to ensure that elements are kept dry or allowed to dry. Internal timber should also be protected from interstitial condensation, such as moisture from wet rooms, or from poor design such as moisture ingress through building elements. Larger elements, for example CLT and glulam, are more susceptible to long-term moisture effects. The risk of decay in these larger elements suggests that the use of preservative treatment or monitoring combined with early warning systems can help ensure their integrity. Preservative treatments could be done ‘in laminae’, prior to the manufacture of CLT elements.

Actions and outcomes Over the past four years, a European Commission funded programme or COST Action, FP1303 Performance of bio-based building materials, has engaged more than 260 scientists from various fields in meetings, conferences, training schools and short-term scientific missions on the following themes: • moisture interactions • performance and maintenance of bio-based building materials influencing the life cycle and LCA (life cycle analysis) • performance testing and testing methodologies >> Timber 2019 Industry Yearbook

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Profile for BM TRADA

Timber 2019 Industry Yearbook  

The annual publication of the Timber Research and Development Association (TRADA) includes topical and technical features on all aspects of...

Timber 2019 Industry Yearbook  

The annual publication of the Timber Research and Development Association (TRADA) includes topical and technical features on all aspects of...

Profile for trada