Can French Drains Cure Subsurface Water in a Crawlspace?

French Drain Placement and Considerations

• Well-planned and properly implemented French drains should be able to at least partly alleviate crawlspace moisture. To function effectively, the drain must be placed next to the bottom of the footing or foundation on at least the exterior side of the footing. Ideally, one drain should be placed on either side of the footing. The trench must lead to an outlet or a sump pump to ensure that the pipe will drain and the trench must have a slope of about 2 percent

Implementing the Drain

• Dig a slightly sloping trench to the depth of the footing and place a 4-inch perforated PVC or corrugated plastic piping on the sloped trench bottom. The piping must extend to an outlet or sump pump. Cover the piping with at least 12 inches of clean, coarse gravel and cover the gravel with filter fabric or landscaping cloth that will allow water but not sediment to penetrate. In especially silty soils, the filter fabric may need to be entirely wrapped around the pipe and gravel. Once the fabric has been placed, the trench can be refilled with gravel or soil removed during the excavation.

Additional Possible Solutions

• French drains, though they will generally provide effective relief of the crawlspace moisture problem, are a relatively intensive solution that may be difficult to implement, particularly on the interior of the footings. As potential alternatives, there are additional, often simpler solutions. The landscape surrounding the structure should slope slightly away from the house. It may be necessary to re-grade the yard or install swales if this requires correction. Make sure all gutters remain free from debris and downspouts are directing water several feet away from the structure. Consider installing vapor barriers at the same time as French drain installation. If the grade is not manageable and other fixes prove ineffective, it may be necessary to install a sump system.

Controlling Crawlspace Dampness and Humidity

• Even without continuous or even occasional standing water, a crawlspace may have enough dampness and humidity to warrant corrective action. Vapor barriers, installed on the interior and, if possible, the exterior of the foundation walls, will block subsurface moisture penetration. Capillary attraction, a particularly important consideration in fine soils like clay, draws water to the ground surface in the crawlspace, often leaving a whitish residue. This can be addressed by installing a moisture barrier on the ground. Suitable barrier materials may include polyethylene and asphalt-saturated felt materials. Often, local building codes outline suitable vapor barrier materials and installation specifications.