Foundation Drainage Basics

Foundations and Drainage

• Water collecting around foundations causes flooded basements, weakens concrete footings and can even create problems for adjacent properties. Building codes are stringent about measures to prevent moisture from entering the foundation; they call for surface and subsurface drainage and extend from the structure to the siteworks. Even downspouts are considered as part of the potential threat to safe conditions around the foundation. The measures include ventilation to keep the foundation dry and impenetrable to moisture.

Lot Grading

• The Council of American Building Officials (CABO) code, which forms the basis of many local codes, specifies the ground around the foundation should be sloped to fall 6 inches in 10 feet -- 5 percent -- minimum; or 6 inches minimum if the horizontal distance isn't available. All paved surfaces and attachments to the house, such as porches and entry ways, also need to be sloped away from the building. Roof water has to be discharged at least 5 feet from the foundation.

Damp Proofing and Waterproofing

• Damp proofing defends against the migration of moisture vapor through the wall; waterproofing prevents liquid water from entering the wall. A drainage system needs to be installed at or below the foundation to capture excess moisture and drain it away. Drainage tile, perforated pipe, gravel or crushed stone, or any other approved system is permitted. Damp proofing requires the application of an impenetrable membrane to the foundation wall -- typically Portland cement covered with a coat of an approved bituminous material applied to the exterior foundation wall.

Wood and Crawlspaces

• No untreated structural wood can be placed closer than 8 inches to soil, and soil should be no closer than 6 inches below untreated wood or composite siding. In other words, the foundation wall extends 6 inches above the finished grade. In wet regions, the crawlspace floor should be as high as the outside finished grade or an approved drainage system provided. The floor can be crowned to shed water. Cross ventilation should be available in crawlspaces.