The rain aim of cavity construction is to provide a break between the damp outer leaf of a wall and the dry inner leaf to prevent the movement of moisture from one to the other. Rain penetration occurs in this type of construction when the cavity is bridged and moisture can move across it. This can occur in numerous ways, the most common being where mortar droppings have fallen into the cavity during construction and have collected on wall ties, flashings or above windows and doors. It may also occur where the cavity is incomplete because of the type of brick bond used. This typical symptom of this type of problem is a damp patch which continually appears on a wall after rain.
Rain penetration through solid brick walls.
Water penetrates solid brick walls, notably under conditions of strong wind and rain, via the mortar joints and fine capillary cracks that normally occur in walls. Instances have been noted where the water has been seen running freely down the inside face of a solid wall.