Plumbing – How French Drains Work

French drains which, despite their name, originated in the United States, essentially work by providing invasive groundwater with a path of least resistance by means of which it can be redirected away from a structure or low-lying section of lawn. They are named for a new Hampshire man, Henry Flagg French, who, in 1860, published a book with the intriguing title: Farm Drainage – The Principles, Processes, and Effects of Draining Land with Stones, Wood, Plows, and Open Ditches, and Especially with Tiles.

Nowadays, French drains are generally used to combat flooding problems caused by surface and/or groundwater that a home owner may be having, especially affecting their lawn, foundation or basement. They are also sometimes used to drain off liquid effluent from septic tanks.

The basic design, a gravel-filled trench, is simple but for it to continue working over the long haul, it’s important that it be well executed.

Flooding problems are usually associated with sloping ground, non-porous clayey soil, or a combination of the two. For example, if your property is built on a slope with your neighbors’ house occupying a lot higher up the slope, heavy rainfall can precipitate an accumulation of groundwater rushing down from their property and onto your own. If your soil is not able to absorb all that water, you could very well experience damage to your house’s foundation, or leakage into a crawlspace or basement below the ground floor of the house.

A linear French drain is a simple, cost-effective solution to such a problem. In this scenario, it acts as a moat that protects your house by intercepting the groundwater rushing down the slope and directing it around and away from your house’s foundation.

A linear French drain is a doable D.I.Y. project, if you don’t mind doing some backbreaking work (this does involve digging a trench, which after all is a thing closely akin to a ditch) and you have the proper tools and materials (1″ round washed gravel, 4″ PVC pipe with drainage holes, a trenching spade or power trencher and a builder’s level)

So, let’s get down to the nitty-gritty both of how to build a French drain, and how it works. First of all, you’ll need to dig an L-shaped or U-shaped trench system, 6″ wide and 24″ deep, four to six feet from the house. It’s important not to build the drain too near the house because, if you do, you’ll be bringing water up against the foundation, which is exactly what you don’t want.

The main leg of the trench system should be dug up the slope from the house. For a U-shaped French drain, it should be level and connected to two pipes on either side of the house with 90 degree PVC elbow joints. For an L-shaped drain, the main leg should slope down, at a pitch of at least 1/8 inch per foot of fall, to the second leg which will run alongside the house, also connected by means of a 90 degree PVC elbow joint.

When you are designing your drain system, you want to make gravity work for you. Just like a river, groundwater flows downhill, so you’ll have to work with the natural slope of your property and, if possible, have the exit pipe come out above ground to give the groundwater an easy exit point.

Once you’ve decided on the layout of the system and done the heavy work of digging the trenches, it’s time to install the working parts of the drainage system: the gravel and pipes. First of all, tamp down any loose soil in the bottom of the trench and line it with 1 to 2 inches of gravel, lay the PVC pipes on top of this first layer of gravel, with the holes pointing down, and then fill in the trench with more gravel, to one inch below ground level. Then all you have to do is cover the trench with sod or another decorative touch of your own choosing. And you’re done. The next time there’s a heavy rain, excess ground water will enter your newly installed French drain and be diverted around your house and discharged at the end of the exit pipe or pipes.

It’s commonly recommend that a French drain be lined with geotech fabric and the piping be wrapped in a geotech sock to prevent it from becoming clogged with silt. I don’t recommend doing either. If you were going to use geotech fabric anywhere, the place to put it would be on top of the trench to prevent silt and sediment from filtering down from above and filling in the air spaces between the gravel. Most of the water that enters a French drain is groundwater flowing sideways underground, not downwards from the surface. Groundwater is not silty, it has already had the silt and sediment filtered out of it as it trickled down through the topsoil. If you doubt this, just ask yourself whether underground spring water and well water are clear or muddy. Both of them are of course usually crystal clear because soil is a natural water purifier.