What is a French Drain?
Although they sound quite European, French drains are in fact named for a New Englander, Henry French. French drains are a simple way of removing unwanted groundwater and consist mainly of a gravel-filled trench and some perforated drainage pipe. Water, either from the surface or in wet soils, is collected by the gravel and seeps into the pipe which carries it away.
Steps for Installing a French Drain
The steps include digging a trench whose bottom slopes, and that is long enough to lead to a discharge point.
In the area where water is to be collected, the sides and bottom of the trench are lined with filter fabric to keep dirt from washing in and clogging the drain. Perforated pipe is laid in the bottom of the trench atop this fabric. Beyond the water collection area, non-perforated pipe is used to transport the water away. Next, the collection area is filled to the top with gravel, and the section of the trench with the non-perforated pipe is backfilled with the soil that came out.
Draining Foundations
Every new house has some version of a French drain at the bottom of the foundation. Where the lot has enough slope and size for a drain to run downhill from the foundation to daylight, most builders prefer to install the drainage outside the footings. In many cases, older homes with foundation water issues lack a French drain at the footing level. They can be added and can make all the difference. This can be a very involved task that means digging all the way down to the footings—a job for a pro. However, if the problem is surface water reaching the foundation from uphill, then a much easier French drain whose trench is only a couple of feet deep can be used to intercept and divert the water. Four-inch-diameter perforated plastic pipe or coiled corrugated plastic pipe is laid on the ground at the footings with the facing up. This joins to solid pipe that slopes away to daylight.
Backyard Drainage Solutions
The pipe alongside the foundation is covered with a thick layer of clean, crushed stone, typically ¾ inch in size. On top of that goes the filter fabric, which is then covered to grade with native soil. With particularly wet soils, or in cases where the house will not have gutters, sometimes the foundation may be backfilled entirely with crushed stone. If the foundation drainage is also meant to handle the roof runoff, its capacity likely will need to be increased. This is a simple matter of adding additional collector pipes and increasing the diameter of the drain line. Here, it’s a good idea to have the system designed by an engineer.
Drywells
When the site won’t permit a daylight drain, other options are used. Sometimes the footing drain leads to a drywell. Drywells can be as simple as a hole in the ground filled with crushed stone, a precast concrete tank with drainage holes, or a similar plastic structure. Drywells work by being large enough to store the expected volume of runoff water for time, while eventually allowing it to infiltrate the surrounding soil. In other cases, the footing drain is run inside the foundation. Here, the collector pipes will drain to a plastic sump pit, where a pump ejects the water up and out to the surface of the ground or to a drywell. If the pump drains to the surface, it’s very important that the discharge be at least 10 feet from the house, otherwise the water can just drain back down to the footing and create a nearly endless cycle. One advantage of employing interior drains is that sealed sump pits are available which can also connect the sub-slab drainage pipes to a radon vent. One disadvantage of a pumped system is that it relies on electricity. If the power goes off, the basement can flood. Systems with battery backups are available.
French Drain in a Yard
Sometimes a section of lawn will remain too wet to mow until well into the spring, or a gravel driveway will develop muddy areas. Installing a French drain on the uphill edge of these areas is often all that’s needed. In this circumstance, the French drain is frequently called a curtain drain. Depending on the area to be drained, the trench can be dug by hand, with a trenching machine, or with a backhoe. The depth of the trench depends on the depth of the water table and the topography—the bottom of the trench needs to remain higher than its downslope outlet. In some cases, it will be necessary to have an engineer design the system. In other cases, digging down a couple of feet and sloping the bottom of the trench is all that’s called for. The downhill side of a curtain drain’s trench can be lined with heavy-duty plastic sheeting prior to backfill to create a dam and force more water into the drain. Typically, the trench will be filled to within six inches of grade with crushed stone, and the filter fabric folded over the top. Native soil covers the fabric, and grass is planted atop that. Along driveways, it’s common to fill the top of the trench with larger stone—oftentimes decorative river rock—that helps to drain surface water, too.
Benefits of a French Drain
French drains collect unwanted surface water, preventing flooding of lawns, patios, walkways, and drives. French drains divert ground water, drying lawns and foundations They are simplest to build during construction of a house but can be added later. Little to no maintenance is required.
