French Drains

French drains are used to remove excess water from low spots or excessively wet areas of a lawn


The type of area where a French drain is needed is often located behind a house (usually at the base of a slope) where excess water run-off is directed to a lawn area that's nearly level. Other times, the wet area might be along property lines where two adjoining lots were graded separately. Whatever the case may be, the goal is always the same: to move water away from the soggy area.

Once the decision is made to install a french drain, the next step is determining where to end the french drain, and most importantly, where to send the water. In many situations, the best place to channel the water is toward the front street curb (if you're on the high side of the street) but this presents other potential problems.


Where to channel water from a French drain?

TO THE CURB?  Piping storm water to the curb and dumping it into the street is often done with downspout connections in new building plans, especially when drain pipes can't be tied-in to existing storm sewers. Most storm sewer systems have been engineered for set flow rates, and it's often difficult to get permission to tie-in to an existing storm sewer, even though this is the ideal solution. Check with your municipality for local codes and regulations.

The biggest problem with water dumped over the curb is that it freezes in northern climates in the winter. In freezing weather, the steady flow of underground water can create large "ice flows" along the curb, creating a slipping hazard to pedestrians and vehicles. In summer, water flow along a curb can grow algae, as well as provide a favorable environment for mosquitoes. Protruding pipes can be crushed by vehicles and are difficult to mow around.

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French drain I photographed in my travels. It was installed at the top of a slope to intercept water runoff from above. Water flow was piped to the curb which was notched to reduce the height of the pipe.


INTO A DOWNSPOUT?  Tying your french drain into an existing downspout is another solution. This is often done when no other solution exists. The biggest downside to this type of arrangement is that you are sending water back toward the foundation area of the house. It's not uncommon for these downspout and foundation areas to already have drainage problems that can cause wet basements. Adding to that foundation water flow can aggravate any existing problems. Often times, downspout pipes and french drain pipes around the foundation of a house become disconnected as soil settles over the years. Therefore, before electing to use this solution for your french drain, make sure everything is connected properly underground so that you won't be adding to any existing problem with wet basement walls.

OVER THE HILL?  With a large enough building lot, it's often possible to direct french drain water onto the surface of the lawn. In effect, this only moves the wet area to a different part of your property, but in the right situations, this solution can work well. Remember never to direct water onto a neighbor's property.

INTO A DRY WELL?  Back in the day, a drainage solution they used was known as a dry well. A dry well is nothing more than a large hole in the ground, backfilled with rocks or large gravel. Water is piped into the dry well with the intent of it percolating out of the dry well into the surrounding soil. In ideal situations, with soil that drains rapidly, this solution works well. However, if the surrounding soil is heavy clay, the dry well may only act as a bath tub, filling with water, which then flows backwards up the french drain pipe, re-flooding the area you are attempting to drain.


French Drain Construction

LOCATION:  French drains should run along the center of the low area(s) needing drained, and end at the desired drainage point. There should be adequate "fall" in the french drain construction, for water to naturally run downhill, toward the drainage end.

BEFORE DIGGING: Before doing any digging, contact your local utilities to have them mark buried utility lines. This can usually be accomplished with one phone call to your state's 811 ONE CALL system. It usually takes at least 3 to 5 weekdays for all the utility lines to be marked, so plan ahead for this delay.

TRENCHING:   The next step is digging a narrow trench to accommodate the perforated pipe and gravel. It's best to do your digging during a dry spell, when soil is much easier to work. Depth of the trench is usually 18 to 24 inches deep, depending on the application, with the width of the trench being around 12 inches or more. The "floor" of the trench should run downhill toward the drainage end of the pipe, to encourage the natural flow of water. Note: You may want to first cut and remove lawn sod from the excavation area for later replanting, after the French drain is installed. Otherwise, you will have to reseed the disturbed lawn area.

INSTALLATION:  First, depending on the method chosen, the ditch can be lined with landscape fabric, making a "U" shape with the fabric laying in the trench. Next, lay the perforated pipe in the trench. Several types of perforated pipe are available. The flexible black pipe that comes in 100 foot rolls is easier to work with than rigid sections of perforated pipe, but it isn't as strong when it comes to bearing weight. Choose the right pipe for your specific project. You will also need to buy various pipe couplers to fit the pipe together.

GRAVEL:  Place 2b limestone (or equivalent) gravel around the the sides of the pipe and cover the top of the pipe with gravel, leaving enough room to finish off the top of the trench with at least 6 inches of topsoil (some installers like to run gravel all the way to the surface). Once the gravel is in place, cover the top of the gravel with landscape fabric to prevent dirt from sifting down into the gravel and clogging the French drain.

SOIL BACKFILL:  Once the fabric is in place, finish filling the trench with topsoil and lightly compact it to reduce future soil settling. If you are seeding the area, bring the grade even with the existing lawn. If you are using sod, leave enough room for the thickness of the sod to bring the grade even with the existing lawn.

WATERING:  Be sure to water the sod or seed as you would any newly planted lawn area, to ensure proper establishment.