Last edited 23 Jul 2016

Construction tolerances

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.

In addition to dimensions, tolerances may be used to specify allowable variations in strength, stability, the mix of a material, the performance of a system, temperature ranges and so on.

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.

It is important that tolerances are clearly specified in contract documents and that they are properly understood by designers, fabricators, contractors and site supervisors.

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