How Does a French Drain Work

How Does a French Drain Work

If you have pools of standing water in your lawn or muddy areas that never seem to dry, then a french drain might be a solution to the problem. Installing a french drain can improve lawn drainage, reduce mosquito populations and even prevent mold, foundation damage and basement flooding. It may sound expensive and complicated; however, the cost of a french drain is minimal when compared to the cost of mold remediation, foundation repairs or installing a new lawn. Understanding how a french drain works is quite simple and we have listed the 5 main components of the drainage system below.

Sloped Trench

A trench will need to be dug with a slope providing a 1% to 2% grade. This sloped trench is designed to direct water using the inherent gravity of the slope. The water will be directed through the sloped trench using a perforated drainage pipe.

Perforated Drainage Pipe

Located within the sloped trench, a perforated drainage pipe should be installed to direct the water through the trench. This perforated drainage pipe can be either a flexible corrugated pipe or rigid PVC pipe and the holes of either should be installed facing downwards. This pipe is used to collect sub-surface water in problem areas of the lawn or landscape and relocate it to a desired location.


Gravel should also be installed around the perforated pipe with landscaping fabric to contain the gravel. This combination of gravel and landscaping fabric is used to create a void in the trench, filter debris and promote percolation of water. Most french drains are now being installed using an all-in-one perforated flexible corrugated pipe that already comes with a pseudo-granite material and landscaping fabric surrounding it.

Catch Basin

If you have specific areas of your yard that sit lower than others and collect water, consider placing a catch basin in these areas. Catch basins are simply grated boxes installed at ground level in low spots along the trench and are attached to the perforated french drain pipe below ground that collect and relocate surface water in problem areas. They also collect debris and are used to help keep the drain pipe clean.

Pop-up Emitter

Pop-up emitters are installed at the end of a french drain or at any other location along the drain where water is intended to be discharged. The valve-like device is opened or activated by the hydrostatic pressure in the drainage pipe and stays closed when not in use to prevent debris or rodents from clogging the drain line.

As you can see, how a french drain works is quite simple; however, digging the trench is a very labor intensive process and calculating the proper trench slope is extremely important, so consider using a professional landscaping contractor for a project like this.

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