Other symptoms are cracks near the corners, displacement of shelf angles, and the deformation of the wall near the corners. During a rain storm, forces acting on an exterior wall typically include wind, internal building pressure, gravity, rain drop momentum and capillary suction. Fine capillaries draw and hold small volumes of water with high suction so they seldom contribute to rain penetration. But greater amounts of water are held in large capillaries such as cracks and unbonded interfaces. Under the influence of wind, raindrops approach the wall with enough velocity to carry them through large openings.
Gravity pulls the water downward.
Water running down the sides of vertical cracks or joints can be diverted inward by surface irregularities. A pressure drop through a wall is produced by wind pressure on the face of the building. Then, if there is a large amount of water at the surface, openings up to three-eighths inch or more are bridged with water, which is readily forced through passages by even small differences in air pressure. Creating air space in the wall system can control water entry from air pressure differences, but the air pressure in the space must be equal to that of the wall face. This can be accomplished by providing sufficient free area of opening to the exterior to allow the wind pressure to maintain equalization.