Unless they are resistant to or protected from wetting, the components in a roof assembly may be subject to corrosion, rot, expansion, loss of strength and freeze-thaw deterioration. The freezing of ice to a membrane and subsequent forces produced by contraction and cracking can be sufficient to split the membrane. Rain and snow also impose structural loads on the roof. Snow varies in density. In many parts of the country, snow drifts two metres high are not unusual.
Ice can also accumulate to a considerable depth.
These loads must be considered in calculating deflections and differential movements. In shading the building's interior from the sun, the roof itself is exposed to solar radiation. Roof materials must be selected and assembled to control conduction, convection and radiation and thereby limit energy consumption to an acceptable level. Daily and seasonal temperature variations in the roof components themselves will affect their performance over time. Materials have different coefficients of thermal expansion and contraction.