What Is a French Drain?
A French drain is a drainage system installed at the base of a house or building. It consists of a perforated pipe surrounded by gravel or filtering material, buried all around the foundation.
Its purpose is to collect water from the soil and direct it toward a discharge point.
This system reduces moisture problems, prevents water infiltration, and protects the structure from water damage.
In short, installing a French drain helps:
- Reduce the risks of moisture, efflorescence, mold, and concrete deterioration
- Maintain the long-term strength of the structure
- Avoid costly repairs caused by untreated water infiltration
- Improve the quality of life inside the home and increase its value
Types of French Drains (Interior and Exterior)
There are two main types of drainage systems:
- Exterior French Drain
This system is installed around the foundations, on the outside of the building. It captures groundwater to prevent it from reaching the walls. It is the most common option in new construction or major renovation projects. - Interior French Drain
This solution is used when it is not possible to install a system outside. It collects the water that has already entered and channels it away, helping keep the basement dry.
Steps for Installing an Exterior French Drain
1. Site Preparation
- Locate underground utilities (water, gas, electricity, etc.) around the foundation to avoid damage during excavation. The trench path may be adjusted depending on the presence and location of these lines.
- Mark the work area on the ground.
2. Excavation
- Dig a trench around the foundation, typically about 4 feet deep and 1 foot wide.
- Use a mechanical or manual excavator depending on site accessibility.
3. Installing the Drainage System
- Place perforated drainage pipes (PVC or polyethylene) at the bottom of the trench on a slight slope to direct water toward the discharge point.
- Add ¾" clean crushed gravel all around the pipe (typical thickness: 6 to 12 inches). This gravel allows water flow and protects the pipe.
- Wrap the gravel and pipe with a special geotextile membrane. This filtering fabric prevents fine soil particles from entering, avoiding clogging and extending the drain’s lifespan.
4. Connection
- Connect the drainage system to a discharge point, such as a municipal storm sewer or a sump pit. This directs water away from the house and prevents accumulation near the foundation.
- Check the slope to ensure proper flow toward the outlet.
5. Backfilling and Finishing
- Backfill the trench around the drain: first with clean gravel, then with topsoil or sand depending on the project and site conditions.
- In some cases, add an extra membrane or insulation near the foundation walls to reinforce waterproofing.
- Compact the surface soil to stabilize the ground.
- Restore the site: reinstall grass, landscaping, or patios as needed.
- Perform a final inspection of the system’s operation (pipe position, condition of connections).
Steps for Installing an Interior French Drain
1. Basement Preparation
- Move objects and protect surfaces to allow access.
- Identify where the drainage system will be installed, usually along the walls.
- Cut the concrete around the basement perimeter to create a trench deep enough for the system.
2. Installing the Drain and Membrane
- Place a perforated drainage pipe in the trench.
- Add gravel around the pipe to help water circulate.
- Install a geotextile membrane around the system to prevent clogging from fine soil particles.
3. Connection to the Discharge System
- Connect the drain to an interior sump pump or the storm sewer system, depending on the building’s layout.
- Check the slope to ensure water flows effectively toward the discharge point.
4. Restoring the Basement
- Backfill the trench with gravel and pour new concrete over the restored surface.
- Clean the area and put back the items that were moved or protected.
When Should a French Drain Be Installed?
Signs That Installation or Replacement Is Needed
- Water infiltration or moisture marks in the basement or on walls
- Cracks appearing in the structure or foundation
- Presence of mold, musty odors, or high humidity levels
- Water pooling near the foundation after rainfall
- White deposits (efflorescence) forming on concrete
- Damp or warped basement floors
- Difficulty with doors and windows closing properly
- Standing water in the basement after snowmelt or heavy rain
- Unpleasant odors coming from the basement or drains
Homes at Risk
Some houses are particularly exposed:
- Older buildings (over 40 years), often lacking a reliable drain or equipped with an outdated system
- Homes built on clay or low-permeability soils
- Foundations that are unprotected, cracked, or already prone to infiltration
- Houses with basements or semi-basements, especially in humid or flood-prone areas
- Properties where the slope of the land directs water toward the foundation
Cost of Installing a French Drain
See our full article on how much a French drain costs
Hire a Qualified Professional
If you have a French drain installation project, you can rely on our services and excavation expertise to carry out the work efficiently and to the highest standards.