Install a French Drain

If you have water that sits at the foundation of your home after a heavy rain or snow, you are asking for foundation problems!

Cracks in your foundation will also cause cracks in your drywall inside, and cause doors to stick.

To relieve the problem of standing water at your foundation, a French drain system is an ideal way to redirect the water away from your home!

So what is a French Drain? A French drain is basically a ditch filled with gravel, rock that redirects surface and ground water away from an area.

Optionally, you can improve French drain by actually using perforated piping, but the traditional gravel can be effective and is easier than installing piping.

French drains are common drainage systems, primarily used to prevent ground and surface water from penetrating or damaging home foundations. French drains are also commonly used behind retaining walls to relieve ground water pressure.

The earliest forms of French drains were simple ditches, pitched from a high area to a lower one filled with gravel. Lining the bottom of ditch with clay or plastic pipe increases the volume of water that can flow through the drain. Modern French drain systems can be made with perforated pipe surrounded by sand or gravel and landscaping textile.

French drains can lead to dry wells or environmentally friendly rain gardens where the extra water is held and absorbed by plants. If there is a creek on your property you can also redirect the water there.

French Drain Diagram

Variations on the French drain model include:

  • Filter drain is to drain ground water
  • Collector drain (or inteceptor drain) combines groundwater drainage with the interception of surface water or run-off
  • Dispersal drain distributes the waste water from a septic tank
  • Fin drain uses a perforated pipe with a thin vertical section (the fin) of drainage composite above. The advantage is that the fin drain is narrower (200mm or 7 inches) than a traditional French drain (450mm or 17 inches and up), and cheaper to build.

In the U.S., municipalities may require permits for building drainage systems as federal law requires water sent to storm drains to be free of certain contaminants and sediment.

Instructions for installing a French drain

What you need:

  • spade (or trencher)
  • landscape fabric
  • gravel
  • sand
  • sod
  • string level
  • string
  • 2 stakes
  • tape measure

Step One – Select Location to dig the trench
Locate an area where the excess water coming off the slope can be re-routed. This may be a matter of choosing “the lesser of two evils.” If water is pooling next to your house foundation, almost any other place is better. An ideal French drain leach field is any out-of-the-way area with sandy soil, where the water can percolate harmlessly.

Following the slope instructions in step two, begin by digging a trench in the area where you have drainage problems. The depth and width of the trench can vary, but 5 to 6 inches wide and 8 to 12 inches deep are common sizes and usually satisfy most needs.

Step Two – Ensure proper slope
Grading is a critical consideration; you must ensure that you have enough down slope for the water to actually flow in the right direction. You must ensure that a slope exists to carry water in the desired direction. Put two stakes into the ground to mark the start and end points for the trench. Tie a string to one stake and run it to the other stake. Tie it off loosely.

Attach a string level to the string and adjust the string to get it level. Once you have it leveled, tighten the string at the second stake so that the string is taut. As you dig, measure down from the string to achieve the desired grade. Hire a surveyor if you don’t think you can get the correct grading. Be sure that you have at least a 0.5 percent slope. A one or two percent grade is optimal.

Step Three – Add a drain pipe (optional)
While a French drain traditionally is just a trench filled with gravel, adding a perforated drain pipe will improve its effectiveness. Begin by laying an inch or two of gravel in the bottom of the trench and then set the drain pipe. The drain pipe should be wrapped in a filter fabric.

Step Four – Fill trench with gravel
Finish filling in the trench with gravel around the pipe. Be sure that at least 1 inch of gravel surrounds the pipe. Four- or 6-inch drain pipes are common and fit well in a 6- to 8-inch trench. Leave a few inches of space on top of the surface for step five. A good size gravel for French drains is 0.5 to 1 inch in size.

Step Five – Place sand on top
If you plan to plant lawn over your drain, cover the remaining space with about 3 or 4 inches of coarse sand. This will allow your turf to grow over the trench adequately. Be sure to use coarse sand so that water can drain well.

Step Six – Re-planting your lawn
You can plant lawn seed in the sand or just allow grass to grow back in from the surrounding lawn area in some cases. Alternatively lay sod over the sand. Washing the soil from the sod roots first will ensure you don’t contaminate the sand. If your drain is not in an area with lawn, another option is to place larger diameter (2- to 3- inch) decorative rocks over the top of the gravel.

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