Waterproofing of the horizontal slab can be as critical to the success of weatherproofing a building as waterproofing below-grade walls. This often-overlooked component has led to a tremendous amount of damage through moisture entry in both the liquid and vapor form. Moisture enters the concrete slab at all of the usual suspected areas, such as openings, joints, penetrations and cracks that form over time. Building damage created by moisture infiltration in this area ranges from floor system damage in the form of rust, stains or odors to an increase in heating costs. Moisture infiltration also contributes to mold and fungi growth, which can lead to sick building syndrome from the growth of mold and fungus. In today’s environment, these health issues create certain liability for the building owner.
Moisture infiltration at horizontal slabs can be eliminated through the application of a waterproofing membrane that is placed between the ground and the slab. A properly designed and installed waterproofing membrane will protect the slab from moisture infiltration in its four forms: hydrostatic pressure, capillary action, vapor migration, and condensation. Providing proper site drainage and a footing drain can minimize hydrostatic pressure. Applying a layer of permeable fill beneath the slab can reduce capillary water movement. Moisture in the form of vapor migration requires a waterproofing barrier, whereas condensation can be eliminated by the application of a vapor barrier. A best design practice in these conditions would be to apply a waterproofing membrane where there is a high water table, apply a vapor barrier when there is a low water table and a floor covering is being applied on the slab.