Foundation Drainage Requirements

Foundations and Moisture

• Concrete is porous, letting liquid water and water vapor from the soil penetrate through foundation walls. Hydrostatic pressure from the buildup of water against the walls can cause wall failure or flooding if not relieved, and groundwater can seep through cracks and defects in the structure. The extent of drainage systems depends on the climate and soil conditions. Areas prone to flooding, with high water tables or expansive clay soils, need the most extensive water management.

Surface Grading and Drainage

• Managing water runoff at the surface is the first step in foundation drainage. Positive drainage -- away from the house -- is the fundamental principle of surface grading. Regulations call for the ground to slope at 5 percent, or 6 inches drop over 10 feet, for at least 5 feet from the foundation. The surface grade should be at least 8 inches below the top of the foundation wall and the house itself ideally is elevated 1 1/2 feet above street level.

Foundation Drainage

• Drainage tile, in the form of perforated or solid pipe known as a French drain, is placed at the bottom of the footings; the pipe is laid on a gravel bed channeled towards a safe discharge point or sump pit in the basement or crawlspace where a sump pump directs the water away from the foundation. In regions with well-drained soil, a gravel channel may be considered adequate for foundation drainage by your local building department, but that's not a common situation.

Damp Proofing and Waterproofing

• Damp proofing is a coating, usually with an asphalt base, applied to the outside of the foundation wall. The IRC requires damp proofing for any wall that retains earth and encloses interior living spaces. Damp proofing protects the foundation from water vapor created by soil moisture and is complementary to foundation drains. Waterproofing seals the walls with membranes or coatings that keep liquid water from entering the building. Waterproofing is required in regions with a high water table or severe soil-water conditions.