A matter of life and obsolesce

Designers and builders set out to produce perfect buildings, but experience shows that they do not always succeed. But no building would ever get built if we always assumed the worst. So while we have to proceed with confidence in the design, some knowledge of the risks of early failure is essential.

The introduction of PFI, value engineering and facilities management has also raised the need for, and awareness of, whole life costs. Assessment of the life expectancy of materials and components is the key information required to calculate the whole life costs of buildings.

The assessment of the life expectancies of specific materials and components in particular circumstances is the province of testing laboratories. A huge amount of this specific advice is available from manufacturers, trade associations and research organisations, particularly the Building Research Establishment and the British Board of Agrément.

The basic causes of deterioration in buildings are due to the action of weathering, biological infestation, stress, chemical interactions, physical interactions and normal use.

These factors all result from human endeavours and therefore do not lend themselves to accurate prediction. This is the key problem associated with estimating component life expectancy.

Deterioration will be accelerated by: poor design/detailing, inappropriate selection of material or component for intended use, quality of material or component used, adverse onsite storage and handling, poor workmanship, inadequate maintenance and inappropriate use.

The design and construction process tries to minimise these factors and experience suggests that it is usually a combination of factors that leads to failure. For example, a poor design that is poorly built and poorly maintained may fail, when getting any one of these processes right might allow it to perform adequately.

Physical life is just one aspect of life expectancy and many components are replaced because they are obsolete for other reasons

However, physical life is just one aspect of life expectancy and many components are replaced because they are obsolete for other reasons. Designers and those choosing life expectancies for life cycle cost or value engineering exercises should consider all forms of obsolescence.

There are six main forms of obsolescence. They are: physical, economic, functional, technological, social and legal. Some apply equally to buildings and components and some to complete buildings only.

These factors will ultimately determine the life expectancy of a component in specific circumstances.

The results of the BCIS surveys of common building components (see below) give typical life expectancy, minimum life expectancy and maximum life expectancy as well as providing some general guidance to the causes of premature failure and the factors to be considered when assessing life expectancy.

The latest survey (incorporating the results of previous surveys) will be published by the BCIS in February 2006.