Last edited 28 Jan 2020

Thermal expansion

Variations in the temperature of a structure can result in thermal movements of its constituent parts. Most building materials will expand with rises in temperature caused (in most cases) by solar heat gains. In many solid materials (i.e not liquids), the expansion will usually be greater along the long dimension of the material as opposed to the short. Liquids tend to expand in all directions when heated.

Different materials are likely to have different rates of thermal expansion. Indeed, it is possible for variations to exist even among samples of the same material. The degree to which solid materials expand as a result of temperature change is expressed by their coefficient of linear thermal expansion (CLTE). The expansion of a material can be calculated from the percentage change in its length per degree of temperature change, its length, and the change in temperature.

Knowing the coefficients of expansion allows designers to calculate how much thermal movement to accommodate (particularly on a hot summer’s day when there are solar heat gains will be higher) by allowing for expansion joints.

Thermal movements tend to be reversible, i.e for every millimetre of expansion, there will usually be the same amount of contraction as the temperature drops.

In the UK, the typical seasonal variations in temperature – i.e between a hot summer’s day and a cold winter’s night – can be as much as 30°C. This can induce strain and damage a building. When materials are restrained excessively and cannot expand, the release of built-up internal stresses can result in cracking, bowing, buckling and other forms of deformation. For example, in brickwork, thermal expansion can cause cracking of both mortar and bricks, allowing rainwater to penetrate through the external leaf.

Expansion joints usually prevent this happening.

In brickwork, 10mm-wide vertical expansion joints filled with a suitable filler material can allow for thermal expansion and are typically installed every 10m-12m for main walling runs. But in freestanding walls and parapets, this is usually reduced to 6m-8m because these constructions are typically more exposed to the elements and have less weight above them (and therefore less restraint).

Movement in the vertical plane should also be considered and accommodated with a horizontal movement joint (sometimes referred to as a ‘soft joint’) at the appropriate spacing.

Lead, zinc and copper can be formed (in roof applications) with traditional ‘drips’ which are steps that break up the length and accommodate thermal expansion. Alternatively, a rubber (neoprene) expansion joint can be used.

Expansion joints are typically filled with a material capable of being compressed by up to around 50% of its original thickness and which can recover after the thermal movement is reversed.

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