In some areas, heavy soils may retain surface drainage and cause water pressure against the foundation walls or slab. Surface drainage problems may also occur because the crawl space is below the finish grade outside the house. The minimum depth of the crawl space should be 24 inches under the floor joists or 18 inches under the girder. An 18-inch clearance beneath any ductwork is also desirable. Moisture problems in crawl spaces may be due to the construction of a crawl space in an area of high water table, improper grading of the lot for drainage or the omission of moisture control devices such as vapor barriers, ground cover and ventilation openings.
Capillary action causes moisture to travel upward into the slab.
Gravel beneath the slab can slow the movement of moisture, but 6-mil plastic below the slab will provide a more effective vapor retarder. Capillary rise occurs in nearly all areas where the soil is clay or silt. Capillary action is the effect of surface tension that causes water to rise up a narrow tube, against the effect of gravity. In building construction, capillary action can occur between two surfaces placed together, or within porous materials. This relates to the installation of wood flooring in that moisture can be drawn through both the subfloor and the concrete below it.