Stone Masonry for Drainage

Cavities in Masonry Walls

• Moisture can condensate in, or intrude into, the cavity space in regular twin-skin masonry walls. The Brick Industry Association advocates a two-inch cavity and weep holes must be built into the base of the exterior wall so that accreted moisture -- primarily the condensation when the house is either warmer or colder than the outside atmosphere -- can leach out. In this situation, the masonry itself becomes the duct to the places where the moisture is expressed; the stone walls are effectively a drain.

Masonry Landscaping as Draining Device

• Some masons recommend the use of a dry 7-to-1 mix of masonry sand and Portland cement as a 2-inch screed below pavers, and the same mix for use brushed between the pavers. The use of a cement mix below and between masonry pavers effectively blocks any moisture from percolating between the stones. This system is only workable if the entire landscaping project is canted in such a way that the masonry serves as a culvert, directing precipitation toward a drain capable of accepting and routing the maximum volume of water that could fall on the landscaping. In areas of low rainfall, this is feasible. In locations where massive volumes of water fall in short periods, such as in hurricane or tropical storm conditions, ponding and the problems inherent in ponding are likely.

Masonry Landscaping That Facilitates Drainage

• If large amounts of rain are expected to fall on a masonry landscaped area, both the screed below the pavers and the fill between them should be of play sand. This fine material allows water to percolate through the masonry into the substrate. As long as the substrate is permeable -- and typically being soil, it is -- the masonry facilitates drainage by allowing water to pass through. This technique markedly reduces the volume of surface water that reaches the areas where ponding occurs.

The Pumped French Drain

• A pumped French drain, as commonly used in domestic and small commercial premises, is a shallow trench dug at the low point of a masonry landscaping feature that does not direct water run-off to a regular drain. The trench should be lined with impermeable material or a perforated pipe, crowned with a linear grating of the type used around swimming pools, and angled downward so as to vent into a large drum which has also been dug into the ground.

  The drum -- 55-gallon rain barrels are frequently used -- is equipped with a submersible pump actuated by a float valve similar to those used in a toilet cistern; the pump’s vent is connected to a pipe that runs to a regular drain. When the French drain directs water into the drum, the float valve lifts and actuates the pump, which expresses water under pressure from the drum to the regular drain. This system again uses the masonry above the French drain as part of the drainage system.