Moisture Management of Foundations: Perimeter Sub-Surface Drainage

The finished soil grade will be sloped away from the building so that surface water will naturally drain away from the structure. As an additional best practice for managing site and foundation drainage, we have installed a sub-surface “French drain” system at the base of the foundation walls to insure that sub-surface water can never accumulate and seep into the crawlspace. Storm of the century or flood?? …bring it on. This house will stay high and dry.

The building pad is graded so that sub-surface water will naturally flow to the sides and then the rear of the lot. Gravity put to good use.

A French drain or sub-surface perimeter drain is a trench filled with gravel encapsulating a perforated pipe at the bottom. The pipe and gravel are wrapped in a protective “geotextile” fabric to prevent fine soil from entering and clogging the pipe holes. Moisture accumulating in the trench percolates down and enters the pipe which transports the moisture to a point of discharge away from the building.

Here is where it all comes together at the rear (lower grade) of the foundation. The drain pipe coming from under the crawlspace floor from the 4″ gravel drainage plane joins the exterior perimeter drain pipes and runs down and away 20 feet to a gravel pit. There is a backflow preventer as well so that neither air nor water can enter the building through the drain pipe.

The perforated pipe used for French drains is typically manufactured with two parallel rows of perforations (round holes) which are positioned on the under side of the pipe. This guy must be praying to the gravel gods or something?

The perforated drain pipe has been fully encapsulated with at least 6 inches of 3/4″ drain rock on all sides.

The landscape fabric is carefully folded over the top of the gravel to form a barrier against soil intrusion into the drain trench. This foundation drain system is complete and ready for backfill.

The French drain system properly designed and installed below grade at the base of the foundation walls relieves hydrostatic pressure and is a very valuable component of our site and foundation moisture management. This is a best practice required by the EPA Indoor Air Plus certification we are following on this project.