There are at least six generally recognized forces that cause movement in buildings along with potential problems as identified below:

Thermal expansion and contraction. It is well documented and understood that heat causes solid materials to expand and cold causes them to contract. In situations where materials are unrestricted on at least one end, then there is really no problem-they will grow and shrink without impact on things around them. However, when the materials are rigidly connected to other materials, the expansion and contraction pushes or pulls on the adjacent connecting materials causing structural forces of stress and strain. How much movement and how much force will depend on at least two variables: the material itself and the variation in the amount of heat or cold.

Every material has, among its other physical properties, a thermal sensitivity factor that is expressed as a coefficient of linear expansion. Because this physical property is so well known, virtually all common building engineering materials have been tested and coefficients of expansion (contraction) have been determined. These coefficients are expressed in terms of very small fractions of an inch for each degree Fahrenheit of temperature change (see Table 1). At the low end of the scale, wood and glass tend to expand and contract the least when subjected to temperature changes while metals such as bronze, brass, and aluminum tend to move the most.

The amount of heat or cold that affects a material depends on its source. Materials exposed to the weather throughout the year will experience the cyclical daily, weekly, monthly, and annual changes relevant to the local climate. That means that the materials could be exposed to temperature swings of up 100 degrees or more in some locations over the course of a year with instances of 30 to 40 degrees of temperature swings possible within 24 hours. Materials that are kept inside a heated and cooled space may experience very little temperature change, perhaps 20 degrees or so. In addition to the air temperatures, there may also be isolated sources of heating and cooling that occur both inside and outside such as direct sun exposure, mechanical equipment proximity, industrial processes, etc. Since the amount of movement of a material is a direct result of the difference between the starting temperature and the ending temperature that is experienced, planning for the correct range is significant.

Water impact. Water in the form of vapor, rain, or other precipitation can cause materials to expand and contract if they are porous or if their water protection is compromised. In normal temperatures, water can soak into certain materials, including many masonry products, causing them to swell. As the water evaporates, the material will shrink back. Either form of movement may cause forces on the material affected or on adjacent materials. In freezing climates, the presence of water takes on an additional concern. Water is one of the few materials that performs inversely to most materials, meaning that it expands as it gets colder (freezes) and contracts as it gets warmer (unfreezes). Hence, water that seeps into a crack or opening in a building might harmlessly drain away or evaporate. However, if it gets trapped inside a material and freezes, it will expand and cause significant force on that material. This force is one of the chief causes of spalling in masonry and concrete work.