Improving the Durability and Service Life of Wooden Components in Outdoor Applications

The context

Because of its inherent chemical structure, wood is susceptible to biological decay. Its natural durability to biological agents varies depending on the timber species, geographical origin, age and growth conditions of the tree, and the presence of heartwood or sapwood and their relative proportions.

The service life of wooden commodities, which means how long a product is expected to perform under specific environmental conditions, depends on many factors, which include both the material's inherent characteristics and environmental factors. Exposing wooden commodities to harsh outdoor conditions such as rain, wind and sun highly increases the risks of the material being damaged by biological organisms such as xylophageous insects and decay fungi. Therefore, proper design and protection of wooden products for outdoor use is crucial to ensure the best service life for them as expected by the market and final users.

The choice of a particular wood protection technology should thus be dictated by the wood's natural durability (= the durability class), its susceptibility to preservative treatments (= the impregnability) and its exposure to environmental parameters (= the use class). However, assigning a wooden commodity to a specific Use Class is often difficult and controversial. Additionally, current knowledge about what the reference lifespan (or “life in service”) of outdoor timber structures should be is still limited. As a consequence, biological damage (mainly fungal decay) is frequently reported, chiefly caused by inappropriate use of building materials, poor design and bad maintenance generating water traps and increasing the moisture content of the wood. This is particularly true about such common wooden commodities used outdoors as decking, cladding, log houses, and exterior carpentry, which are at constant risk of being prematurely damaged.

Deficiencies in current practices used in designing outdoor commodities made of wood often result in the wood's excessive or abnormal moistening of wood. Wood products can contain zones where rainwater may accumulate, stagnate in a quasi-permanent way, and generate so-called “spots of insalubrity," which are generally places where biological (particularly fungal) attacks occur. Mistakes made at the conception stage often lead to a switch of the in-service situation of the wood: wooden elements initially meant for Use Class 3 are finally exposed to a level of biological risk that is higher than expected in this Use Class, such as soft rot fungi which develop more frequently on wood in Use Class 4. As a result, in situations where wooden components are not in ground contact but may permanently accumulate water due to their design or surface deposits, it may be necessary to consider that these situations are equivalent to contact with the ground or fresh water and thus require a higher level of natural or preservative-based protection.