Water from above, water from below

Much is written about water in masonry. It is important in estimating how the mass of masonry will perform. I would like to distinguish the contributions to performance from water deposited on a masonry wall from above—such as from rain, poor capping, roof overflow, etc.—compared to water rising by capillary action from below.

Water from above

• Pores carry water to the interior, under gravity. Any pore structure that provides continuity provides flow.
• Capillary size does not matter. In fact, the larger the pore openings, the less resistance there is to flow.
• Capillary uniformity does not matter. Under the acceleration of gravity, water can flow from a small pore to a large pore, back to a small pore, and indeed fill any cavity made available to it.
• Medium cavities can be filled. It may be difficult to fill large void cavities—that much water is rarely available. But the range of sizes that may be filled by gravity flow is great.
• Water can spill out. The course of water under gravity can continue to the exterior and run down a face, either exterior or interior.
• Efflorescence can occur. If the water picks up salts from within the masonry, then spills out onto a face, it can carry that salt to the surface and deposit it. The pattern made by the salt often indicates the site where the water exits the masonry mass.
• Heat will increase evaporation. The immediate effect of heat applied to a wet surface is to increase the rate of evaporation. Evaporated water from the surface may be immediately replenished if the water source is not exhausted.
• Salts are flushed and carried to surface. Rainwater has been much cleaner chemically than the earlier days of acid rain. This means that rainwater is rarely a source of contaminants, particularly salts, within masonry assemblies. Rainwater can pick up salts from with the assemblies and carry them to the surface.
• Salts can clog pores. If rainwater carries internal salts to the surface, the rate of water evaporation may easily exceed the rate of salt diffusion back into the mass. Thus this process may lead to unwelcome accumulation of salts on the surface or below the surface, and this accumulation may lead to spalling from subfluorescence.
• Ice can form and damage masonry material, at least in sky-facing surfaces. There are many instances of freeze-thaw damage where masonry faces the sky.

Water from below.

Water in the soil supporting the masonry wall structure may be wet and saturated with water. With saturation, water may rise in the porous masonry structure.

• Capillarity carries water upward, and outward.
• Capillary size matters. The attraction of masonry surfaces for water is balanced against the weight of water being lifted above the source level. Capillaries a millimeter in width are way too large to lift water. A simple glass capillary tube 100 microns in diameter can lift water only about 1 cm.
• Cap uniformity matters. If a vertical capillary encounters a bulge of greater diameter, it likely cannot continue upward (unless the bulge is filled by water from above). Only a very uniform and fine porous structure can be expected to fill uniformly.
• Only small cavities can be filled. A non-uniform matrix will have water filling only the pores of the smallest diameter.
• Water cannot spill out. If water could spill out, then a tiny turbine placed to catch the water could become a perpetual motion machine. No, water from below cannot, ever, spill out of the wall.
• Efflorescence cannot occur. Efflorescence requires surface water, an impossibility with a capillary source.
• Heat will drive back capillary margins. Capillary rise is greater at lower temperatures, less at higher temperatures.
• Salts are distributed uniformly. Capillary water in masonry is rather stable, affected only by changing temperature at the margins, and changes in source strength. A stable matrix of water tends toward a stable value of salt concentration, sharing that concentration with the source.
• Salts cannot clog pores. Recall that heat drives back capillary margins. Can this lead to salt crystallization? For that to occur, the rate of evaporation would have to substantially overpower the diffusion of salt away from the perimeter, and that’s not likely.
• Freeze-thaw damage cannot occur in capillary-only pore filling. Explanation for this is forthcoming. Meanwhile, there’s this.

Of course, a wall can get water from both above and below. In that case a mix of effects, described here, are possible, with effects from above overpowering capillary effects. The above discussion is written for above-grade walls. For below-grade walls it’s important to note that almost all the water comes from above.

The lesson here is simple. Fix the roof. Provide overhangs. Do everything possible to keep precipitation away from masonry. And recognize the relative safety of capillary water in masonry.