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Walls, being vertical, are not subject to water pressures induced by gravity unless a crack or joint in the wall slopes toward the interior of the building. Gravity, moreover, works to the advantage of walls, stripping water off them before it has much time to penetrate. But wind forces on walls are usually much greater than those on roofs, driving raindrops hard against wall surfaces, moving water sideways and sometimes upward across the surfaces, and pumping water through even the most minute openings where the air pressure is lower on the inside than on the outside. We often build walls of more porous materials than roofs, and we invariably riddle our walls with cracks or joints between pieces of material and with holes to accommodate windows and doors, so that water penetration problems are frequently encountered.
Masonry materials all absorb and transmit water to some extent. Mortar joints that have deteriorated over the years afford particularly easy paths for water to pass. Water adheres to the interior surfaces of the pores of the materials, and if, as in most cases, the adhesive force between the water molecules and the wall material is greater than the cohesive force between the molecules themselves, the water will be drawn in by capillary action. A strong wind can increase the rapidity of absorption. A possible solution to this problem would be to apply an impervious coating, such as a heavy paint or a synthetic rubber, to the outside face of the masonry. This would repel rain water quite effectively, but it would also stop the outward migration of water vapor, thus leading to rupture of the coating and subsequent peeling and leaking. The coating would also be susceptible to tearing because of thermal movement in the underlying wall.
A more reliable approach to making a masonry wall watertight is to assume that some water will penetrate the outer layer of masonry but to provide a continuous gap just behind the outer layer to break any capillary path that might conduct water to the interior of the building. This is the logic of the masonry cavity wall, in which the outer wythe (vertical layer) of stone or brick held a couple of inches (50 or 60 mm) from the inner wythes by means of stiff metal ties (ill. 9-12). The effectiveness of the cavity depends on keeping it free of mortar droppings during construction and providing weep holes at frequent horizontal intervals to drain water from the continuous flashing at the bottom of the cavity. Sheet metal or plastic flashings also must be installed around window and door frames to avoid capillary paths across the cavity. An incidental benefit of the cavity wall is that it transmits heat much more slowly than does a solid wall, and foam plastic thermal insulation is usually installed in the cavity to enhance this effect.
If water penetrates into brick, stone, or concrete and freezes there, it can cause spalling, the chipping off of flakes from the surface by the expansion of water as it freezes. Spalling was a major force in the destruction of concrete walls and pavements until it was discovered that concrete deliberately produced to contain microscopic air bubbles (air-entrained concrete) provides expansion space for the water as it freezes and so virtually eliminates spalling. Obviously, spalling can also be eliminated by keeping water off the wall with roof overhangs or other protective devices.