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Tim Yates and Kathryn Bourke
BRE
Stone selection nowadays is made almost exclusively on an aesthetic basis, but the medium-to-long term durability of the stone is also important. Often a stone is chosen because the architect has used it before and is comfortable with it. Indeed, in the past, stone was selected on the basis of local availability and a knowledge of what performed well in the immediate environment. Current approaches to the design of façades (including elements such as cornices, sills, overhangs, plinths etc.) and detailing (the presence of lips, drips, bed orientation etc.) tend to be based on the architect’s aesthetic decision and the strictures of British Standards such as BS5390, BS8298 and BS6093. Too little consideration is given to the effect design and detailing have upon the stonework’s future durability and façade maintenance and hence upon the Whole Life Costing and Performance. In order to demonstrate stone’s value for money, therefore, it is usually necessary to undertake a Whole Life Cost and Performance Assessment. This looks at the material’s (initial and long term) physical, functional, environmental and economic performance and assesses whether competing alternatives provide satisfactory performance at similar cost. The performance criteria set by a client’s brief may include:
- Service life requirements (e.g. design life of not less than 60 years)
- Aesthetic requirements (e.g. no significant discoloration during the first 10 years of use)
- Maintenance frequency requirements (e.g. scaffolding to be erected at intervals not exceeding 10 years)
- Maintenance expenditure requirements (e.g. no excessive maintenance requirements within 25 years)
A detailed comparison is required to appraise these requirements and to consider alternative specifications (and associated costs). Where this is done in association with a Whole Life Costing and Life Cycle Assessment it should also include the aspects shown in figure 1.
It is important to remember that cost and environmental impacts are measured over the service life of the product, and that maintenance or replacements accrue new impacts that make a long low-maintenance service life extremely desirable. Where a client has set high targets for service life, appearance, low maintenance, repair costs and low risk of unexpected failures, the performance of stone is likely to be highly competitive. Equally, where the assessment covers the full service life period, stone is likely to show competitive environmental impacts.
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