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.
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
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.