How does a French drain work?

A French drain is a buried path for water. It gives water that would sit in your lawn or press against a wall an easier route: into loose gravel, through the holes in a pipe, and downhill to somewhere it can do no harm. It does not make water disappear. Whether one works comes down to three things: water can get into it, the pipe keeps falling the whole way, and the outlet at the far end can take the flow.

The parts of a French drain

The Northern Virginia Soil and Water Conservation District, which advises Fairfax County homeowners, describes a French drain as "a perforated pipe surrounded by gravel and lined with sturdy landscape fabric" that carries runoff underground to a suitable outlet.^[1] Each part has a job.

• The trench sets the route and the fall. The district puts a standard French drain ditch at about 1.5 feet deep and 10 to 12 inches wide, varying with the pipe size, usually 4 to 6 inches, and the depth wanted.^[1]
• The gravel is the easy path. Water moves through the open spaces between stones far more freely than through packed soil or clay, so it drains toward the trench and down to the pipe.
• The perforated pipe collects what reaches the bottom of the trench and carries it away faster than gravel alone could.
• The landscape fabric keeps soil out. In the district's description the trench is lined on all sides with landscape fabric, and the fabric is pulled over the top of the gravel before topsoil and sod go on. Where the gravel is left exposed at the surface, the edges are protected so sediment cannot get into the drain.^[1]

Keeping soil out of the stone matters as much as the pipe, because a drain packed with silt stops taking water.

Where the water comes from

A French drain takes water from two directions. Rain and runoff soak down into the gravel from above, and the district notes that downspout pipes and sump pump pipes can be connected to a French drain, and a catch basin can be combined with it to help remove standing water.^[1] Groundwater seeps in from the sides and below: "Deeper French drains can also be used to drain groundwater."^[1]

Fairfax County's flood mitigation program explains the purpose around a house the same way: French drains "reduce pressure and remove excess moisture from the soil by collecting surface water and groundwater."^[4]

Where the water goes

Gravity does all the work. The pipe has to keep falling from its high end to the outlet, because a low spot along the run holds standing water and collects silt.

The outlet is the part people forget. The district's rule for any drainage that moves water is plain: "Whenever you are redirecting runoff, you must send it to a suitable outlet."^[1] It defines that as an area that can take redirected runoff without causing erosion or sogginess, such as a rain garden, a dry well, an existing swale or gently sloping vegetated land not immediately leading to neighboring properties.^[2] Sending water to the wrong place "will just move the problems downhill."^[1]

The common endings are:

• Daylight: the pipe simply ends on lower ground, often with stone at the end to slow the water.
• A pop-up emitter: a small cap set flush in the lawn. As pressure builds in the line, its spring-loaded cap pops up to release the water and closes again when the flow drops, and the maker places it at the lowest point in the system, away from buildings.^[5]
• A dry well: a gravel-filled pit underground. Water is stored in the spaces between the stones until it soaks into the ground, and the district notes that a dry well is only a good option if a permeable soil layer is found within a reasonable depth.^[1]

A yard drain is not a foundation drain

The trench across a soggy lawn and the drain around a basement footing look alike but do different jobs. The Department of Energy's Building America guidance covers the second kind. The standard foundation drain it shows is perforated pipe and clean stone wrapped in filter fabric, and it warns: "Do not connect the gutter drain pipe to the perforated foundation drain pipe; this practice will soak the foundation."^[6] Roof water belongs in solid pipe that carries it away, not in a perforated pipe beside the wall.

What a French drain will not fix

A French drain is one tool, and often not the first one to reach for.

• Puddles that clear in a day or two. If your yard dries out within a day or two after a rainstorm, the district considers that normal.^[3] For small, well-defined depressions it suggests replanting with water-tolerant plants, or filling and regrading the low spot.^[3]
• Ground that slopes toward the house. Grading comes first. The district's guideline: ideally, the ground keeps at least a 5% grade for ten feet around the house, which is a drop of six inches.^[2]
• A leak. Leaky water pipes, lawn sprinkler systems, outdoor faucets, ponds and pools can all keep a yard wet,^[3] and no drain fixes a leak.
• Heavy surface flow. When sheets of water cross the yard in a storm, an open channel may do the job more simply. The district describes a swale as "a wide, shallow ditch in which water can flow to a suitable outlet," protected with grass or lined with stone.^[1]

To work out which problem you have, read why your yard floods when it rains.

Before anyone digs

Every French drain is a trench, and every trench is excavation. Virginia law says no person shall make or begin any excavation without first submitting a locate request to the notification center.^[7] Virginia 811 asks for that request at least 3 working days before digging, the service is free, and when you hire someone, your contractor is responsible for contacting Virginia 811.^[8] Some lines are never marked: Virginia 811 says the water line from the meter to your house is private and will not be marked, and neither are lines to a detached garage or shed, invisible pet fences, or propane tanks and their lines.^[9] Tell the contractor where you know those run.

When to call a licensed pro

Designing and digging a French drain is a job for a contractor who does drainage work regularly. Getting the fall right over a long run, choosing an outlet that will not flood a neighbor, keeping silt out, and working around buried lines are where homemade drains fail.

Virginia's Board for Contractors defines landscape service contracting as altering or improving a land area by excavation, clearing and grading, and says the home improvement contractor specialty also covers that work.^[10] A Class C licence covers a single job over $1,000 but less than $30,000; bigger jobs need Class B or A.^[11] Check the licence on DPOR's License Lookup before you sign.^[12]

Ask each bidder where the water comes from, where the outlet will be, what fabric and stone they use, and how they will check the fall. For prices, read how much a French drain costs in Virginia; for upkeep, read whether French drains need maintenance; and if you are unsure whom to hire, see who to call for a wet yard.

Browse drainage and French drain businesses, find excavation and site work businesses in Blacksburg, or see more excavation answers.

Describe the wet spot and where the water could go, and licensed local contractors quote the drain.