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Foundation Drainage

Many problems can occur in a basement or foundation that was not designed to withstand the elements, especially standing water.

By damp proofing or waterproofing the foundation appropriately, installing appropriate foundation drains, and properly backfilling and grading the soil around a house, moisture problems can be averted.

Foundation Drains

Improper drainage around the foundation is a major cause of leaking foundations. When a drainage system is used in residential construction, it is usually a combination of a gravel drainage layer with a foundation drain, made of either a drain tile or perforated PVC pipe. However, as drainage occurs, small soil particles can plug up the drainage path, compromising the drainage system. Water pressure then builds up and eventually causes leakage through the foundation wall.

The typical foundation drainage system consists of a waterproofing membrane at the foundation with a preformed path (a grid system or a solid, porous board) and a filter to keep the drain path clear of small particle build-up. Filters have traditionally been a course or specially-graded aggregate, ranging from crushed stone or gravel to coarse angular sand.

Drainage Panel

Geotechnical fabrics, commonly called "filter fabrics," and other foundation drainage panels are now quite common. Compared to traditional granular fill, foundation drainage panels offer lighter weight; greater dimensional stability; dependable and increased flow; full flow continuity; and protection against freeze-thaw and backfilling damage.

Foundation drains should drain water by gravity away from the building to a daylight outfall, a sump pump, or drywell, depending on the site's conditions. When draining to an outfall, it is important to ensure that the outfall pipe is designed and located to minimize erosion.

For particularly wet sites, installing a radial drainage pipe system under the slab that directs water to a sump pump could be beneficial. Sump pumps are used to lower the water table to a point below the slab.

Appropriate Backfill and Grading

Poor surface and subsurface drainage can lead to water ponding around the house, leakage of ground water through the basement or crawlspace walls, and structural damage to the foundation from the build-up of hydrostatic water pressure. Successful drainage requires leading surface water away from buildings with appropriate grading and backfill.

The grading immediately adjacent to the building should be sloped a minimum of about 5% (or 3 inches every 5 feet) for at least 10 feet outward from a building foundation or as far as practical. In areas that receive a large amount of water, the ground around the foundation should slope away a minimum of 10 percent for at least 10 feet outward from the building foundation.

Backfilling the soil around the foundation of the house with appropriate materials is important. Avoid silt, heavy clay, or expansive clay backfill, particularly around basement walls. Use granular soils instead. Backfill should be tamped firmly to prevent excessive settlement and be covered with 2 inches of topsoil.

Keep the soil at least 8 inches below the point where the framing starts. Because foundation plantings-trees, shrubs, and flowers placed near the foundation of a home-may promote mold and fungus growth on siding that is protected from the sun, they should be planted at least one foot away from the foundation.

For more detailed information on backfill and grading measures, visit PATH's Durability by Design.

Content updated on 8/4/2006

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