A French drain is a simple drainage system designed to remove excess surface water from a specific area. Surface water often accumulates along retaining walls, house foundations under roof drip lines or downspouts, or where poorly drained soils such as clay are present. Standing water can damage foundations and create breeding areas for mosquitoes and other insects.
French drains were invented in the mid-nineteenth century by Henry French as a means of draining wet fields for crops. A traditional French drain is a horizontal trench or channel constructed without drainage pipes, allowing water to percolate into the soil, and replenish ground water.
How to Create a French Drain
French drains are a low cost solution to minor drainage problems. For large, complex drainage problems it is best to consult a professional. The materials needed to complete this project are available from most home and garden centers.
Tools and Materials
- Landscape fabric
- Coarse gravel 1/2 to 1 diameter
- Coarse sand
- Grass seed, Sod or other plant materials
- Wood Stakes or Yardstick
- Always call the municipality in which you reside and any utility companies before doing any type of construction.
- The first thing you will need to do is decide where to place the trench. Do not locate a French drain in an area where the water will drain onto someone else property. To determine the direction of the trench, locate an area of low elevation on your property. Avoid areas where there are trees, large stones or boulders, and existing wet areas or clay soils.
- Dig the trench a few feet away from the foundation and towards the lower elevation. Keep in mind that the trench will need to slope down towards the low point at a minimum grade of 1%, a drop of one foot per one hundred feet. For example, if your trench is 25 feet long, the drop in elevation from beginning to end will be 3 inches. This ensures that water will flow away from the source. The gradient can be determined by using the level and wood stakes or yardstick.
- The width of the trench can vary depending on the amount of surface water to be removed. A small trench might be 6 wide, whereas a larger trench might be 18โ€ wide. The trench should be at least 8 deep.
- Measure the length and width of the trench. The piece of fabric should be wider and longer than the bottom of the trench. The excess will be folded and wrapped under the gravel to hold it in place. Cut the landscape fabric as needed and place it over the excavated area. The purpose of landscape fabric is to keep soil particles from entering the gravel and clogging the drainage system.
- Place a minimum of 4โ€ of gravel over the fabric. If the trench is deeper than 8โ€, then the depth of the layer of gravel should be increased accordingly. In other words, if the trench is 12 deep, there would be an 8 layer of gravel instead of 4.
- Cover the gravel with a 4 layer of coarse sand or well-drained topsoil if you are planning on using plant material to cover the trench. Do not use fine grade sand, which will become compacted over time and obstruct the percolation of water into the ground.
- The final step is to plant grass seed or put sod down. Other options include leaving an open trench, covering it with flagstone, or planting it with flowers or other herbaceous material.