A significant moisture content is normal in all porous structural materials due to hygroscopicity, absorption by capillarity from contacting sources of moisture, or accumulation of condensation, but structural materials are only considered to be ‘damp’ if their moisture content is excessive, in the sense that damage can occur to materials or decorations. The level at which moisture content becomes abnormal, representing unacceptable dampness, is difficult to define, although it is usual to consider the extent of saturation of the material, as discussed in section 4.2. Fluctuating moisture contents can cause dimensional changes, as discussed in section 2.3, as well as changes in the thermal properties of materials as discussed in section 3.2. However, perhaps the most important property of moisture in building materials is the way in which the water content is an essential component in the processes of physical deterioration due to movement, salt crystallisation and freezing, chemical deterioration due to aggressive atmospheres and soils, and biological deterioration due to plants, mosses, lichens, algae, fungi, bacteria and various arthropods, particularly insects. These various deterioration processes are discussed in detail in other sections of this chapter, as well as in sections 6.4, 6.5, 7.6 and 8.6.