Masonry facades

The various mechanisms of water penetration are reviewed in Weathertightness. They apply equally to water penetration through masonry. For example, water will not penetrate a masonry wall unless there is some mechanism to make it do so. Capillarity coupled with gravity and air pressure differentials will permit water to be drawn into the fabric, while surface tension allows water to flow over it and kinetic energy will drive shattered raindrops into any openings, pores or fissures. Broadly there are 2 construction principles that apply to brickwork:

• the overcoat principle; and
• the raincoat principle.

The overcoat principle

An overcoat works by permitting a certain amount of water to soak into its fabric until such time as it becomes fully saturated. At saturation point, water is rejected. In the case of a masonry wall, water gradually soaks into the brickwork as a result of kinetic energy, capillarity and gravity. If the overcoat is thick enough, water will not reach the inside face; it will gradually dry out again by evaporation. Bricks that have a high moisture storage capacity function in this way - the more the brick has been burnt the less absorptive it is.

Soft, porous brickwork in lime mortar: it will gradually absorb water so is less likely to leak until the brickwork has become saturated

The raincoat principle

The raincoat provides a water resistant barrier on the external face. Glazed bricks or dense, well-burnt engineering bricks act in this way, at least in theory. Depending on the absorptivity of the surface and the rate of rainfall, a film of water can form and flow down the face of a wall. Surfaces of low moisture storage capacity readily become covered with a film of water that increases in thickness or volume flow toward the lower levels of multi-storey buildings. The flow of this film is influenced by surface texture, gravity and air movements along the wall face.

However, the downward migration of water tends to become concentrated at vertical irregularities in the wall surface. Surface depressions such as joints in brickwork are likely to attract flow rates many times greater than the average flow over the wall. Thus, while the fabric itself may be reasonably water resistant, the 'seams' in the fabric can allow water ingress, particularly if they have not been formed properly. Soldier courses of brickwork are particularly vulnerable because of the difficulty in battering the long faces of the brick with mortar and keeping it in place while the bricks are placed.

Similarly, laying a damp-proof course (dpc) dry will create a multitude of water paths that can take water into the fabric by capillary action and pressure differentials.

In the author's experience, walls constructed from well burnt or engineering bricks, particularly with a smooth face, tend to suffer more severe leakage problems than brick of a more porous nature.

Dense, well-burnt engineering bricks: leakage into the cavity is highly likely