The components used to construct buildings are often fabricated, assembled or formed on site, often by hand, in conditions that may be less than ideal, and using materials with inherent ‘imperfections’. Whilst it is easy to draw a straight line on a drawing, to give an precise dimension, an exact mix or position it is impossible to construct, for example, concrete that is perfectly straight, simply by virtue of the inherent properties of the material itself.
It can be difficult therefore to determine whether a variation from perfection is simply a function of the nature of a particular type of construction or material, or whether it constitutes a defect. For this reason it is important to specify allowable variations, or ‘tolerances’, that are not considered to be defects.
The concept of tolerances is also be important when assembling a number of components, as some items may have very little flexibility to accommodate variations in neighbouring items. For example, it may be relatively straight forward to adjust the setting out of brickwork to accommodate a slight variation in the size of a timber beam, or simply to cut the beam on site, but if a double glazing unit is even a millimetre larger than the opening for which it is intended, it simply will not fit. Even when individual items appear to be reasonably close to what was specified, variations can accumulate when a number of components are assembled, and this can create a clash with an item that may have a low tolerance.
This is becoming more important as the number of items prefabricated off site has increased, and so there is less scope for changes on site to make things fit.
There are a thousands of different standards available setting out accepted classes and ranges of tolerance for different materials, components, systems, construction techniques, fabrication methods, installation techniques and building types. These may range from relatively large tolerances for site layouts or landscaping to very precise tolerances for manufactured components.
 Related articles on Designing Buildings Wiki
Featured articles and news
An Arc de Triomphe for the late-20th century, the La Grande Arche of Paris.
Richard Hayward of Legrand asks whether technology could help developers meet the needs of an increasingly diverse population.
Thomas Heatherwick's ambitious steel structure begins construction.
The principles, practice and formwork of one of the most important components of modern architecture.
New report claims that inappropriate standards and regulations are holding back the use of composites.
The global smart homes and smart light commercial market will grow fastest in the UK.
Have a look at our article explaining the different types of construction contractor.
Futurist Thomas Frey explores the concept of Disposable Housing - could it be a reality sooner than we imagine?
ICE to host new exhibition offering a window onto the civil engineering achievements beneath our feet.
Do you know all the various types of defects in brickwork?
US museum reveals plans for an installation made entirely of paper tubes.