Since slab foundations do not enclose below-grade space, traditional waterproofing is often not required. However a continuous layer of capillary break / vapor retarder materials is required between the ground and the interior / above grade portions of the building. Depending on foundation design, this can include subslab vapor retarders, sill sealers, gaskets, waterproofing membranes, or other appropriate materials.
Rain water can be properly managed by using a well designed gutter and downspout system and by grading the ground around the foundation (6 inch drop in 10 feet) to channel water away from the foundation (Lstiburek 2006). The slab should also be elevated at least eight inches above grade to prevent water accumulating at the foundation (PATH 2006).
Since slab foundations place all the living space above grade, subgrade drainage is not always necessary. In some cases where seasonal surface water pooling may occur, or on sites with impermeable soils, it is recommended that a foundation drain be installed directly beside the bottom of the footing as recommended for basements and crawl spaces. The foundation drain assembly includes a filter fabric, gravel, and a perforated plastic drain pipe typically 4 inches in diameter. The drain runs to daylight or a sealed sump.
For more information visit Minimum Thermal Bridging within the Building America Solution Center.
