Specifying externally exposed structural timber

Date Published

16 August 2022

Document Type




This Wood Information Sheet (WIS) looks at some of the factors to consider when specifying a desired service life for structural timbers that are to be exposed outdoors but not in contact with the ground.

This WIS addresses general principles of structural design only, giving an overview of the subject with signposts to more detailed sources that are listed at the end.


Types of structures used externally
Designing for exposed structural timber
Specifying for desired service life
Use classes and service classes
Preservative treatment specification
Large cross-sectional solid timber
Design > Engineer;ed timber
Staining and discolouration

Key Information

Timber connections should be protected from wetting and allowed to drain and vent quickly and effectively.

Measures which provide protection from falling and windblown rainwater, and which promote efficient drainage and rapid drying, will prolong the life of the timber. These include overhangs, waterproof cappings and flashings, slopes, drips, gaps at joints between timbers, and ventilations. In situations where these may fail to protect timber, designing for ease of replacement is an option.

Case studies

The Dyson Centre for Neonatal Care (NICU) at the Royal United Hospital in Bath cares for more than 500 premature and sick newborn babies each year; it has been designed as a template to show how good, sustainable healthcare design can be achieved, setting a benchmark for good practice, and creating a therapeutic environment to benefit parents, children and staff.

The sports building houses a 4-court multi-use sports hall, flexible activity/dance mezzanine, changing and store facilities and spectator areas. The music school houses 5 new classrooms, 6 practice rooms, staff offices and staff social space as well as a large new performance space.