What is a French Drain and How Does it Work?

A French drain is a special type of drainage system created by laying a perforated pipe inside a gravel-lined trench. It works by allowing water to enter the pipe through the perforations (holes) in the pipe. From there, the excess water is channeled along the length of the pipe to a safe drainage area. This ingenious system prevents drainage trenches from flooding and keeps water flowing downhill, even in flood conditions.

Man installing french drain

What is a French Drain?

A French drain is created by combining 3 simple things: a drainage trench, gravel, and a perforated drain pipe. The creation of this very effective drainage system is simple. Below are the components of a French drain.

  • A drainage trench at least 9 inches wide and 18 inches deep, slanting downhill at a slope of at least 1 inch every 10 feet.
  • The bottom of the drainage trench is filled with a 3-inch deep layer of gravel.
  • A perforated pipe is laid in the trench, with the holes angled downward.
  • Gravel is used to fill the rest of the trench, around and above the drain pipe.

This is a french drain in its simplest form. It seems almost too simple to work, but it does. But at first glance, it seems like something you’d find on a farm hundreds of years ago. Funny story…

Why is it Called a French Drain?

The French drain is named after Henry French, who invented the system and included it in his 1859 book, “Farm Drainage.” Henry French was a native of Massachusetts who spent years studying agriculture and drainage practices in the United Kingdom and the Netherlands to improve his own farm.

  • Named after Henry French, who invented the French drain in 1859.
  • French drains have been used to control surface water stormwater in the US capitol, including at the Lincoln Memorial.

The French drain system is so efficient and easy to maintain that it has been used for generations. This time-tested system has been used for ground drainage around public monuments, farms, and backyards.

How Does a French Drain Work?

The French drain works by channeling stormwater into a gravel-lined drainage ditch. From there, the water rises until it enters the drain pipe through the holes along the lower sides of the pipe. Once inside the pipe, the water flows downhill and empties into a pond, bed of gravel, or dry well built in the ground.

  • Rainwater flows into the drainage trench and filters down through the gravel.
  • The water rises until it flows into the holes in the pipe.
  • The pipe conducts the flow of water downhill to the drain’s exit.

The gravel in the French drain is essential for allowing groundwater to flow in among the rocks and enter the drain pipe. A perforated pipe laid directly on the soil will become clogged with soil and will not work. Also, it’s important that with proper french drain installation, the perforations in the pipe are facing downward, not up. The water cannot drain until it reaches the level of the perforations and enters the pipe.

French Drain Holes Up or Down?

The holes in the drainpipe should be facing downward in a French drain. This allows the water that fills the trench to enter the pipe and flow downhill. The closer the holes are to the top of the trench, the higher the water has to rise before it can enter the pipe and flow away. This leads to a flooded and ineffective drain.

  • When installing a perforated pipe in a French drain, angle the holes downward.
  • Most perforated drain pipes have 2 lines of holes. Set the pipe so that the floor of the pipe is solid, with the lines of holes low on either side.
  • By angling the holes downward, you allow water to easily enter the pipe for proper drainage.
  • Do not face holes upwards when installing a French drainpipe. The water will be forced to rise until the trench is flooded before it can enter the pipe and flow away.

A properly installed French drain with downward-facing holes drains water away well before the trench floods. A good French drain stops flooding altogether, even in cases of heavy rain.

Where Does a French Drain Drain To?

You can direct your French drain to an aboveground outlet such as a pond, water garden, gravel bed, or municipal storm drain. Alternatively, you can have your French drain terminate below ground into a simple dry well, or a 3-foot hole filled with gravel.

  • Pond or water garden.
  • City storm drain
  • Aboveground area where water will disperse and evaporate.
  • Belowground dry well, which you can construct yourself with a shovel and gravel.

Whether you choose an aboveground water feature or want to disperse drained water below ground, out of sight, you can terminate a French drain easily in a way that prevents flooding.

What is the Difference Between a French Drain and Other Drains?

The main difference between French drains and other drainage systems is the inclusion of the perforated pipe in the French drain. This system allows for water to enter the drainage ditch at any point and be directed downhill.

  • A perforated pipe is the main feature of a French drain.
  • Solid pipe drainage systems must have water enter at one end (such as from a gutter spout).
  • Drainage ditches without a pipe (gravel-filled trenches) do not direct water downhill as well as French drains.

Instead of focusing on draining water away from a single area, such as it is done with a solid pipe system, French drains allow for entire flooded areas to be drained, and for the water to be safely directed away. A pipeless trench drain will not conduct water downhill as quickly or efficiently.

When Should a French Drain be Installed?

A French drain should be installed wherever your yard or garden has excess surface water or is frequently flooded by runoff. As long as you have determined the slope of your yard to be adequate for drainage, you should consider a French drain. Some common applications for French drains are:

  • Along the edge of a garden.
  • Around a house to prevent basement flooding.
  • Behind retaining walls, to prevent water flowing downhill from undermining your wall.
  • Along frequently flooded or boggy parts of your property, to encourage drainage.

If your yard or garden is prone to flooding, or if water is infiltrating your basement, consider adding a French drain. It’s also good practice to add a French drain on the uphill side of any rock or retaining wall, to prevent water from undermining and toppling the wall. The French drain is made from simple materials and solves water issues efficiently.

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