How to Install a French Drain

Installing a French Drain

If you consistently notice water or moisture in your basement, or pools of water around the outside of your home after heavy rain, then it may be time to look into a more effective waterproofing techniques. A French drain is one of the best defences against basement flooding. The installation process is a little more involved than other waterproofing techniques, but when combined with a sump pump a French drain is an extremely effective and almost foolproof way to keep water out of your basement.

What is a French Drain?

A French drain consists of a continuous system of piping running beneath the floor and around the entire perimeter of the basement. The pipe is surrounded by coarse gravel and is perforated to let water inside. Water is collected and then directed away from your home. The French drain is primarily used to prevent ground and surface water from damaging concrete or seeping through building foundations.

Materials and Tools

  • Landscaping paint
  • Hammer
  • Stakes
  • String
  • Line level
  • Pencil
  • Tape measure
  • Shovel
  • 3/4-inch aggregate gravel
  • 4-inch perforated PVC/corrugated plastic drain pipe (or flexible drain pipe)
  • Landscape fabric
  • Hand tamper

Step 1: Locate Any Underground Pipes and Utility Lines

Before you start digging your trench you’ll want to know where there are underground pipes and utility lines so you can avoid them during the installation process. Check the building codes in your area or ask a city official to come to your property and mark the locations or areas you should avoid.

Step 2: Locate the Problem Areas

Locate the area (or areas) where water tends to pool outside your home. These are the areas where water will need to be directed away from. If you are unsure where the problem areas are take a walk around your home after a heavy rain and look for obvious signs of trouble.

Step 3: Plan the Route of Your Trench

Find a location to direct the water to. Storm drains or ditches are ideal, and make sure the water doesn’t end up on your neighbour’s property. It’s also best to pick a point that’s lower than your home, if possible, so you can use gravity to your advantage. Use landscaping paint to plot the route of your trench from the problem area to your end point.

Step 4: Create a Slope

To effectively drain water away from your home you’ll need a downward slope.  If the path you’ve plotted for your trench doesn’t have a natural slope you’ll need to create one. To do this, hammer stakes into the ground in the location where you want your trench to begin and also where you want it to end.

Tie a string between the two stakes, making sure it’s good and tight. Hang a line level on the string and make adjustments until the string is level. Use a pencil to mark the location of the string on the stake at the end point.

For the best results you’ll need a slope that’s 1 inch for every 100 feet of length. Measure the distance between the stakes and slide the string down on the second stake to the appropriate distance. For example, if the distance between the stakes is 50 feet, move the string down ½ inch on the second stake. If it’s 25 feet move the string down ¼ inch, and so on.

Step 5: Create a Trench

Dig a trench between the two stakes, making it 6 inches wide and 24 inches deep from the string to the bottom of the trench. Measure the depth as you go to ensure you have the correct slope. Line the trench with two inches of gravel.

Step 6: Lay the Pipe

Choose one of two types of pipe: PVC with predrilled holes or flexible drain pipe cut with slits. PVC will last longer and can be easily cleaned with pressure or a plumber’s snake if you encounter a clog. Flexible pipe, however, is less expensive and generally easier to work with.

Lay your piping of choice on top of the gravel and place the landscaping fabric over top. This will prevent dirt and debris from entering the pipe and clogging it up. Fill the trench with more gravel until there is only 2 inches between the gravel and the top of the trench. Use the dirt and turf that was dug out of the ground to cover the trench and flatten the top with hand tamper.

For more information on how to install a French drain, watch the following video:


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