Visual signs may include damp patches appearing on the internal walls resulting in discolouration of plastered or decorated surfaces. The moisture intrusion may be caused by leaks or by water permeating through the structural elements that make up the building.
The most obvious, and primary, causes of water intrusion are leaks in the building envelope. Missing roof tiles, defective joints around doors and windows, faulty flashings and leaky gutters are the major visual causes of rainwater penetrating into buildings and causing damp within them.
Perhaps less obvious is the potential for dampness from moisture which is absorbed through masonry walls. External walls can become saturated through leaking down-pipes or even by driving-rain, though the latter is mainly restricted to south and west facing walls which are most vulnerable to prolonged periods of wet weather.
Bricks and mortar are suprisingly porous; they able to absorb significant quantities of water when subjected to continuous wetting. The porosity of brick is attributed to its fine capillaries. By virtue of the capillary effect, the rate of moisture transport in the brick is ten times faster than in other building materials. Additionally, cracks in walls, even hairline cracks form a passage from water can be travel from the outside of a wall to the inside.
Buildings built with solid walls are generally more likely to be affected by damp contributable to moisture penetration than those built of cavity wall construction. Cavity construction incorporates an air-gap between the inner and out walls that keeps the water out.
However, cavity walls are not completely immune to the occurrence of penetrating damp. Wall cavities can become blocked in places providing a bridge for water to travel. Additionally, many cavity walls have been retrofitted with insulation that fills the gap between the walls. Where cavities are narrow and the walls are subjected to extended periods of driving rain, water can percolate through the outer wall and soak the insulation. The wet insulation then transmits the moisture to the inner wall, causing damp spot on the internal wall surface.
Treating penetrating damp is simply a matter of resolving the causes of water intrusion. Good building maintenance will eliminate the risk of leaks in the building envelope.
There are a number of steps that can be taken to safeguard masonry against the perils of moisture absorption in brick and stone walls. Water absorption is not only responsible for penetrating damp but also for spalling brickwork caused by absorbed water freezing within the masonry unit.
These actions can be taking without adversely affecting the visual appearance of brickwork or stonework.
It is advisable to use a masonry water-proofer that is 100% vapour permeable; that is one that lines the capillary pores of the masonry with silicones rather than one that blocks the pores. It is vital that moisture within the brickwork is allowed to evaporate in order to prevent it from freezing in cold weather and blowing the face off the bricks.