Module: Construction principles Unit: Designing for durability | Part 1: preservative

Date Published

15 August 2022

Document Type

Author

TRADA
Summary

Timber will not decay if it is kept dry, to a moisture content below 20%. This may not be possible if timber is used outside, so either a non-durable softwood that has been treated with a suitable preservative or a naturally durable timber can be specified.

As a renewable material, wood is a valuable resource for the manufacture of sustainable building products. Extending service lives of these products can avoid costs associated with their replacement over the lifetime of a building and reduce environmental impact.

Wood preservatives are used to protect susceptible timbers and wood-based products against wood-destroying organisms such as fungi and insects.

The ‘design life’ of a building or component is the period of use intended by the designer and the client. The ‘service life’ (‘working life’ in European documents) is the period of time during which no excessive expenditure is required on maintenance or repair.

The ‘durability limit’ is the point at which loss of performance leads to the end of service life. The choice between specifying high-quality durable components, undertaking scheduled maintenance or replacing accessible components is part of the designer’s responsibility in risk assessment and life-cycle costing.

Key Information

Wood preservatives are used to protect susceptible timbers and wood-based products against wood-destroying organisms such as fungi and insects.

The need for preservative treatment should be assessed on the following: in-service conditions, effect of moisture, insect attack, service factors and desired service life.

Timber species vary in their resistance to attack by wood-destroying fungi and insects. However, a timber’s natural durability may not be the only consideration, for example it may not be certified from a sustainable source.

The attraction to wood for both fungi and insects can be reduced by a controlled process of drying wood and then heating it to a very high temperature.

Case studies

Cowan Court was needed to house Churchill College’s growing undergraduate numbers, many of whom had to live offsite. It was also important that the building would centre the students in the campus, would be environmentally sound and fit within its historic surroundings.

Streatham & Clapham High School, a successful independent day school for girls, has been transformed and upgraded by Cottrell and Vermeulen Architecture.