French drains can solve a flood of problems

Oregon weather is almost synonymous with rain. Many of us live on plots of land that do not drain during the rainy season. We put up with muddy yards, standing water, and even flooding into our homes and storage areas. There is often a straight forward, energy-efficient, low maintenance solution: a French drain.

French drains require no electricity to operate, but rather rely on gravity for power. Properly installed, maintenance is low. The one essential ingredient necessary to build a French drain is an area of land, ditch or stream that lays at a lower elevation than the flooded area. If one of them is available nearby, planning and installing a French drain is relatively simple for a knowledgeable and skilled builder. The key to a successful French drain installation is finding the path of least resistance for the problem water to follow.

The first planning step is to find the lowest elevation level of the standing water (“the problem area”) and to locate the elevation of the drainage area or drainage stream. The second step is to map out a path for a ditch that will travel from the problem area to the drainage area. The final planning step is to calculate the average slope of the ditch (determined by dividing the length of the ditch by the total difference between the two elevations).

Once planning for the French drain is complete, installation can begin. The first step is to dig the ditch in accordance with the plan. As the ditch is being dug, the accuracy of the slope must be checked frequently. Any inaccuracy in slope must be addressed by evening out the bottom of the trench. Any areas that are too high must be lowered and any areas that are too low should be filled with crushed rock. This process is called grading. Three-quarter inch minus ( 3/4″- ) crushed rock is a common choice for grading the bottom of the drainage trench. The bottom of the drainage trench should also be tamped.

Once the trench is sloped correctly and tamped sufficiently, a 4”-diameter perforated drainage pipe should be placed carefully into the graded drainage trench. The perforations should be placed facing up. The source end of the pipe should curve up above ground level and it should be installed with a removable cap. This will allow the pipe to be cleaned out if should ever become plugged.

Next, the drainage pipe should be surrounded with 1 ½” round river rock for the entire length of the drainage trench. It is best to cover the round rock and drainage pipe with rolled drain trench barrrier paper. Commonly available at building supply stores, this paper will help keep top soil from filtering down into the drainage trench and into the drain pipe.

The final step is to back fill top soil into the last 2” at the top of the drainage trench to restore the original vegetation that was displaced by the new French drain.

One Response to “French drains can solve a flood of problems”

  1. elena amodeo says:

    hey Dan,
    this was really helpful. thanks.

    Elena (WBB)

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