How Does a French Drain Work?

Adding a french drain to a yard with drainage problems can prevent soil expansion, water accumulation and structural damage. To avoid any potential hazards during the digging process, contact your local utility company to have buried lines marked before beginning the job.

Essentially, a french drain is a trench filled with gravel and a perforated pipe. The gravel encourages groundwater to flow down into the pipe, where it’s carried away.

The Trench

A French drain is essentially a slightly sloped trench filled with gravel and a perforated pipe that diverts water away from your house. Gravity does its job as water moves down the incline, through the unobstructed gravel, and into the drainage pipe that is wrapped in a water-permeable fabric to prevent gravel, dirt and other debris from entering the drain.

The trench should have at least a 1 percent grade for optimum effectiveness. If your yard has a lower slope than this, a French drain is not likely to work and should be considered a different type of drainage system.

A French drain is ideal for areas that tend to pool or collect water, rerouting it into an area that can either drain or be useful, such as a rain garden or a vegetative planting bed. It also helps protect your home from structural damage caused by hydrostatic pressure in below-ground structures. It is a great preventative measure that can save you from costly repairs down the road.

The Pipe

The main function of a French drain is to collect water that pools in low spots, like a lawn or garden, and redirect it. The drain carries the water through underground tubing to where it needs to go, such as a waste area or a municipal drainage system.

The pipe is typically a perforated drainage pipe, surrounded by gravel or rock and wrapped in landscaping fabric to prevent weeds or dirt from clogging the holes. The fabric also helps the pipe look a little more natural, while still allowing it to take on the shape of the surrounding soil.

The trench is often dug on a slope, which allows gravity to work its magic and speed up the flow of water. It’s also common to add a dry well or Flo-well to act as a repository for the water once it’s been channeled elsewhere on the property. This will help reduce the amount of time water remains in the drain, preventing issues with basement flooding or sand erosion around the home.

The Landscaping Fabric

If your yard looks like a lake after it rains, you might need some help redirecting that water. French drains are great for areas where standing water jeopardizes the integrity of a home’s foundation, retaining wall, or other structure. And unlike a ditch, this drainage path is totally underground.

They work by taking advantage of liquid’s natural tendency to seek out the lowest point. In this case, that’s empty pockets in loose soil. A pro will dig a trench that contains a perforated pipe wrapped in landscaping fabric and then filled with gravel.

The landscape fabric (also known as geotextile fabric) keeps sand, soil, and plant tendrils from making their way into the pipe. It also stops the French drain from becoming clogged with debris that would otherwise slow or impede flow. A grate is then installed at the end of the pipe to direct excess water away from your property. This helps reduce the risk of damp basements and flooded crawl spaces.

The Outlet

In the end, a French drain should connect to a sewer drain or some other spot on your property where water can safely discharge. If you are unsure where to connect yours, consult a drainage expert who will be able to suggest an appropriate outlet.

The drainage system uses gravity to force water down into the gravel-filled trench and then into the perforated pipe, which is situated at a downward-sloping angle. This helps to minimize clogging.

A French drain is a great solution to wet basements, flooded crawl spaces and saturated yards. The key is the trench’s slope, which needs to be at least 1%. If it is not, the water may simply flow into the drain instead of where you want it to go, potentially leading to further problems down the line. The drainage trench should also be well away from any buried utility lines, if possible. This will prevent the trench from leaking into your sewer and cause more costly damage.