A solid wall of brick will resist the penetration of rain to its inside face by absorbing rainwater that subsequently, in dry periods, evaporates to outside air. The penetration of rainwater into the thickness of a solid wall depends on the exposure of the wall to driving rain and the permeability of the bricks and mortar to water.
The permeability of bricks to water varies widely and depends largely on the density of the brick. Dense engineering bricks absorb rainwater less readily than many of the less dense facing bricks. It would seem logical, therefore, to use dense bricks in the construction of walls to resist rain penetration.
In practice, a wall of facing bricks will generally resist the penetration of rainwater better than a wall of dense bricks. The reason for this is that a wall of dense bricks may absorb water through fine cracks between dense bricks and dense mortar, to a considerable depth of the thickness of a wall, and this water will not readily evaporate through the fine cracks to outside air in dry periods, whereas a wall of less dense bricks and mortar will absorb water to some depth of the thickness of the wall and this water will substantially evaporate to outside air. It is not unknown for a wall of dense bricks and mortar to show an outline of damp stains on its inside face through persistent wetting, corresponding to the mortar joints.
The general rule is that to resist the penetration of rain to its inside face a wall should be constructed of sound, well burned bricks of moderate density, laid in a mortar of similar density and of adequate thickness to prevent the penetration of rain to the inside face.
A solid I B thick wall may well be sufficiently thick to prevent the penetration of rainwater to its inside face in the sheltered positions common to urban settlements on low lying land. In positions of moderate exposure a solid wall l B thick will be effective in resisting the penetration of rainwater to its inside face.
In exposed positions such as high ground and near the coast a wall 2 B thick may be needed to resist penetration to inside faces. A wall 2 B thick is more than adequate to support the loads of all but heavily
loaded structures and for resistance to rain penetration a less thick wall protected with rendering or slate or tile hanging is a more sensible option.