French drains is another one of those confusing construction terms that really doesn't tell you what it does. If we used the phrase more often, like French doors or French toast, you might know what they are. We know sinks, bathtubs and washing machines have drains to move wastewater out of our homes, and we know there are pipes in the walls to carry the water away.
French drains are similar to these drains except French drains are found outside your house and they focus on moving water away from your home. French drains are used to move water away from foundations, retaining walls and other below ground walls that are subject to hydrostatic pressure. What's that?
We all understand gravity – when we drop a ball, it falls to the ground. Water as a fluid follows gravity or what I like to call the path of least resistance. Water that pools around your house foundation wants to move downward. It will look for the easiest path to move down and if the water can move sideways more easily, e.g. through cracks in your foundation,that's how it will travel.
Alternative to Using French Drains
Before jumping into an overview of how to build a French drain, it's important to understand that the ideal way to direct water away from your home is to grade the soil around the house so it slopes away from the foundation. When the land is flat and you're unable to build up the soil or when you have shallow places in your yard that are frequently waterlogged, French drains may be your best solution.
Swales are shallow ditches used to divert rainwater without the piping used with French drains. They typically follow natural property boundaries and rely primarily on existing topography for their required natural slope. They're not practical in areas that are very wet, often due to soil that doesn't drain well which might be washed away during peak rainfalls.
Materials Used to Build French Drains
French drains are basically trenches filled with gravel and most of them have a perforated pipe close to the bottom of the trench. Today's French drains use geotextile fabric to wrap the pipe and gravel to keep fine sediment from getting into the gravel where it slow the flow of water.
So let's start by looking at the materials that go into building French drains. You should consult with a local home professional as guidelines will vary based on your weather and soil type.
- Before digging, insure you're not going to hit any utility lines by calling DigSafe (Digging a Hole or Trench Safely).
- Now pick where you want the water to exit the trench, draw a line showing where the trench will go and dig, saving the soil to use when covering the pipe.
- Next you line the walls and bottom of the trench with geotextile fabric to keep as much dirt out of the drain.
- At the bottom of the trench, you want to place a minimum of 2 inches of crushed stone (three quarter inch minimum).
- Next comes the pipe. You want one made to be used in French drains, with two rows of round holes installed facing the bottom of the trench. Some experts recommend smooth wall piping while others use corrugated pipes.
- The pipe should be surrounded by stone, with 2 inches recommented and …
- The geotextile fabric should be folded over the pipe, completely covering it with enough overlap so you can wrap the fabric over a layer of gravel placed on top of the .
- Next is a layer of coarse sand, covered by at least 4 inches of topsoil.
- Finally you'll want some type of grate over the end of the pipe to keep the opening clear for maximum water flow.
Tips for Building French Drains
French drains are very effective at channeling water away from a house when part of the foundation is below ground, and if this space is finished. That's why it's common for French drains to be installed around the perimeter of a home’s foundation, when the house is being built.
- French drains should be less than 12 inches wide, and ideally 6 to 8 inches.
- French drains are typically 1 to 2 feet deep, although you need to go at least 2 inches below the level of your basement floor if that's the focus of your French drain. Longer trenches will also need deeper trenches to allow enough slope for proper drainage and it's best to consult a professional to analyze your requirements based on weather patterns and soil conditions.
- A grade or slope of 1 percent is often recommended for French drains, or roughly 1 inch of slope for every 8 feet.
French drains are common so if you're considering buying a house that has them, you should read the International Association of Certified Home Inspectors (NACHI) article on Inspecting French Drains.
French drain photo credit to BlalockLandscaping.com in Houston, TX.