Providing proper site and building drainage is critical to moisture control for any foundation system. Proper drainage is needed to keep the foundation and underfloor areas dry. For a raised floor system, it is especially important that standing water be kept out of the crawlspace.

drainage
The ground beneath an open pier-and-beam foundation should be graded to provide drainage away from the structure.

Controlling moisture requires effective control of rainwater and ground water. Managing rainwater drainage from the building’s roof helps to keep the foundation and underfloor areas dry. Most important is the use of gutters, downspouts, and splash blocks or drainpipes to direct the water runoff away from the foundation. Also, floor areas of adjacent porches or patios should be sloped to drain rainfall away from the structure.

Whenever possible, the elevation of the crawlspace floor should be higher than the exterior grade. When that is not possible, perimeter drains should be included.

For open pier-and-beam foundations, positive drainage within the crawlspace is important. The ground should be graded to maintain a dry crawlspace and to drain water away from the structure. On a level site, this could involve slight crowning, centered beneath the raised floor. On a sloped site, the ground should be graded so the water exits through and away from the pier system.

For continuous wall foundations, drain tiles installed around the entire footing perimeter can greatly reduce moisture within the crawlspace. The drain tiles should lead to a storm drain, a sump with a pump, or other positive drain system that moves the water away from the structure.

The finish site grade should slope away from the structure to provide positive drainage away from foundation walls. This is important for keeping any type of foundation dry and trouble free. A minimum slope of 5% away from the structure is recommended. Typical site grading creates a fall of at least 6″ over the first 10′ away from a foundation. Drains or swales can also be provided to ensure drainage away from the structure.