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FRENCH DRAINS

French drains are often required to remove excess water from low spots or excessively wet areas of the lawn.


This type of area is often located behind a house, at the base of a slope, where excess 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: 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 where to send the water. In most 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 THE CHANNEL WATER

>> 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 they can't be tied-in to existing storm sewers. Most storm sewer systems have been engineered for set flow rates, and it's usually difficult to get permission to tie-in to an existing storm sewer, even though this is the ideal solution.

The biggest problem with water dumped over the curb, is it ices-up in northern areas with a winter season. In freezing weather, the steady flow of underground water can create large "ice flows" along the curb, that create a sliding hazard to pedestrians and vehicles. In summer, water flow along a curb can create algae, as well as a favorable environment for mosquitoes.

Remember that connecting to a storm sewer requires advance permission from your local municipality. Therefore, if you must send water to the curb area, check applicable local laws and ordinances first.

French Drain at the top of a slope
French drain we saw in our travels...
It was installed at the top of a slope
to intercept water runoff from above.
Water flow was piped to the curb.

>> 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 cause wet basements. Adding to that foundation water flow can aggravate many 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 works well. Remember never to direct water onto a neighbor's property.

>> INTO A DRY WELL:  Old-timers often used a drainage solution 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 ONE CALL system. It usually takes 3 to 5 weekdays for all the utility lines to be marked, so plan ahead. 
  
>>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 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, the ditch should 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. You may 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 close the fabric over top of the gravel to prevent dirt from sifting down into the gravel and clogging it. If there isn't enough fabric to cover the top of the gravel, cut a new length of fabric for that purpose.
  
>>SOIL BACKFILL:  Once the fabric is in place, finish filling the trench with topsoil. 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.
  
>>NOTE:  For best all around results, the finished (surface) grade of the lawn should have a natural slope, to encourage surface water to runoff when the soil and french drain are frozen.


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