Perimeterdrainage

Perimeter drainage is an exterior subsurface drainage system installed around the foundation of a building to collect and convey water away from the structure. Its primary purpose is to reduce hydrostatic pressure on foundation walls and minimize water intrusion into basements or crawl spaces. It is commonly used in areas with high groundwater, clay soils, or poor surface drainage.

A typical perimeter drainage system consists of a trench that runs around the building’s footprint, a perforated drainage pipe placed on a gravel bed, a geotextile filter, and backfill that directs water to a discharge outlet. Outlets may lead to daylighted drains, a storm sewer, or a sump pump. The trench is usually sloped to promote gravity flow. In some constructions, exterior drainage is used in combination with interior drainage or waterproofing membranes for additional protection.

Design considerations include soil type and moisture conditions, the level of the water table, foundation design, surface drainage around the site, and local building codes. Materials commonly used are perforated pipes, clean gravel or crushed stone, geotextile fabric, and appropriate backfill. Installation requires careful trenching below the footing line, proper seeding of the pipe with a continuous slope, and an unobstructed outlet.

Maintenance involves keeping outlets clear of debris, inspecting for sediment buildup, and periodically checking the integrity of the filter fabric and backfill. Perimeter drainage can reduce dampness and moisture-related foundation issues but requires adequate design, proper installation, and ongoing maintenance. Alternatives include interior perimeter drainage systems or surface drainage improvements that divert water away from the foundation.