Rain Penetration (also known as "penetrating damp") is a common form of dampness in buildings. It can occur through walls, roofs, or through openings (e.g. window reveals).
Water will often penetrate the outer envelope of a building and appear inside. Common defects include:
Roof defects such as faulty flashing, cracked or missing slates or tiles.
Faults in the brickwork or masonry such as missing or cracked pointing. Porous bricks or stones.
Missing or defective mastic around windows and doors.
Blocked weep holes.
Missing or defective trays in cavity walls.
Rain penetration is most often associated with single-skin walls, but can also occur through cavity walls - e.g. by tracking across wall ties.
The primary cause of rain penetration through walls is defects in the masonry - e.g.:
Porous masonry (i.e. under-fired bricks, porous stone, or porous mortar)
Cracks
Defective pointing
Unfilled joints and perpends,
Defective seals around doors and windows
Holes in walls - e.g. where cables or pipes protrude
Defective render
Where a wall suffers from one or more of the primary causes of rain penetration listed above, the problem can be made worse by one of the following exacerbators of rain penetration:
Defective rainwater goods
Moss growth on roof tiles (causing blockage of rainwater goods)
Defective or missing window sills (causing high concentrations of rainwater to bear on section of wall below window)
Non-breathable coatings such as acrylic masonry paints - especially where applied to a poorly prepared masonry substrate
Location/aspect of wall - e.g. walls facing prevailing wind are more prone to rain penetration problems (see BS8104)
Periods of extreme rainfall (e.g. winter of 2013/2014) - walls that are normally thick enough to prevent rainfall from reaching the inner face can be overwhelmed during periods of heavy, persistent rainfall
