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Last edited 18 Dec 2020
Rule of thumb
Rule of thumb is an English phrase that has been in common use since the 17th century. It describes a method of doing things which, although not scientifically verified, is broadly correct although it may not be strictly accurate or reliable in all applications, even though it may have proved successful on past occasions.
 Use in construction
Builders of the past would have used rules of thumb in construction. Gothic cathedrals eventually would soar in height as master masons learned through experience to understand certain relationships e.g, the height of a tower in relation to its base and wall thickness; the depth of foundations according to soil conditions, or the diameter and spacing of piers to support the masonry above.
Advancement would typically be through making mistakes - frequently catastrophic - and learning from them. A classic example is the collapse (in 1237) of the main tower at Lincoln Cathedral, and the collapse (1210) of the south-west tower of Chichester cathedral followed in 1635 by the collapse of its north-west tower.
Examples of rules of thumb are:
- The Evans rule, which arose out of a Royal Academy of Engineering paper by Evans et al: the 1:5:200 ratio. This states that if initial building construction costs are ‘one’, over the years ‘five’ will be the operating and maintenance costs, while ‘200’ will be business operating costs.
- Health and Safety Executive (HSE): in assessing noise risks for large, dynamic sites …’as a general rule of thumb, the noise level is probably 80db or more if the noise is intrusive but normal conversation is possible between people 2m apart – comparable to a busy street, a typical vacuum cleaner or a crowded restaurant'.
- BSRIA has a publication called 'Rules of thumb - guidelines for building services (5th Edition) (BG 9/2011)', providing a source of approximate engineering design, environmental performance and project cost-data for building services projects.
- The '45 degree rule' and the '50:50 rule' for determining rights to light.
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