All About French Drains: Benefits, Purpose, Properties, Installation, and Troubleshooting

Written By Adam Paul - Waterproofing Expert

A French drain is a trench that is used to remove water around the basement walls. French drain consists of a trench, gravel, perforated pipe, fabric, catch basin, and sump pump. It helps to stop water leaks into the basement, prevent basement flooding, reduce soil erosion, minimize health hazards, enhance the property value, protect the foundation and reduce the maintenance and repairs in the basement. The main function of the French drain system is accumulating water between the foundation walls and soil layer and pumping out away from the basement.

A french drain depth is around 18-24 inches (45-61cm). It has a width of 9-12 inches (23-31cm). You should use a perforated pipe with 4-6 inches (10-15cm) diameter. The hole’s diameter should be between 1/8 – 1/2 inches (3-13mm). You should use PVC pipe as the perforated pipe. You will need a non-woven geotextile fabric between the gravel and soil layers.

You should use a 1/2-1 inch (13-25mm) gravel size for the French drain. When you fill the gravel layer, it should be 12-18 inches high. You should place the catch basin at the lowest point of the French drain. You should maintain 1%-2% of the slope and you can use sandy soil, clay soil, and gravelly soil as the backfilling of the French drain. You should discharge the French drain away from the foundation wall to reduce the water impact.

When you install a French drain system, you should first dig the trench, then install the fabric and gravel, and then install the perforated pipe with a 1%-2% gradient. Then, test for the water flow. Then, you should install the fabric and gravel on the pipe. Then, place the catch basin and backfill the soil with a proper slope as water can move away from the basement wall.

When waterproofing basement walls, you can use both sheet membranes and liquid membranes. You should clean the French drains regularly in order to prevent clogs in the perforated pipe. If the French drain is clogged, you can use a hose or pressure washer to unclog the French drain. When you maintain a French drain properly, it will withstand 30-40 years. You can improve its lifespan by using proper materials proper techniques, inspecting regular signs of blockage and ensuring proper drainage and preventing sediment buildups in the gravels.

What is a French Drain?

A French drain is a trench that is used to remove water around the basement walls. The main components of the French drain are trench, gravel, perforated pipe, fabric, catch basin, and sump pump. French drain helps to prevent basement flooding, protect the foundation, reduce soil erosion, and reduce maintenance and repairs to the basement. There are two main types of French drains: interior French drains and exterior French drains.

The functions of the French drain are categorized by its components. The French drain trench ensures that is redirected. The trench runs parallel to the basement wall, which prevents water from entering the structure. Inside the trench, there is a perforated pipe that allows water to enter through its holes. Surrounding the pipe, the gravel acts a filter and provides the flow of water to the pipe. A catch basin included at the surface level collect water and directs it into the trench. The collected water is then sent through the discharging pipe, which carries it away from the property.

The main purpose of the French drain is to prevent water penetration, which could cause damage to areas like building foundations, basements, and low-lying areas in a yard. It prevents basement flooding, improves structural integrity, minimizes health hazards, prevents water damage to the basement, and drains underground water effectively, which are five benefits of installing French drains.

What Are the Benefits of French Drains?

These are the 6 benefits of the French drains

Prevent basement flooding
Improve structural integrity
Minimize health hazard
Prevent water damage to the basement
Drain underground water effectively.
Enhance the property value

Prevents basement flooding: The French drain system prevents basement flooding by accumulating seeped water near the soil close to the basement. It reduces the water seepage into the basement, which causes basement flooding. Prevention of basement flooding helps to stop mold, mildew growth, and the musty smell of paint peeling in the basement.
Improve structural integrity: The French drain system reduces the hydrostatic pressure by channelling the water away from the foundation walls and the basement, which improves the structural integrity of the house. Maintaining structural integrity extends the building lifespan, reduces repair costs over time, and reduces the risk of foundation damages like cracks and shifting
Minimize health hazards: The French drain system prevents water intrusion, reduces the risk of mildew growth, which can cause respiratory issues, and maintains a dry environment. It also mitigates the insect breeding grounds, which can cause health issues for the users. Minimizing health issues provides a safer and more comfortable living for the occupants and increases the property value, with positive feedback from the users.
Prevent water damage to the basement: Keeping the water out of the basement, which is done by the French drain system, not only prevents flooding but also protects floors, walls, and personal belongings from water damage. Preventing water from damaging the basement helps maintain a dry environment and usable space, prevents pest infestation, and improves the air quality.
Drain underground water effectively: French drain systems redirect and capture surface and groundwater without letting it collect around your property, especially in areas with high water or poor soil drainage. Draining underground water effectively helps prevent soil erosion and plant damage, improves soil quality, and provides foundation stability.
Enhance property value: Properties with French drain systems are less damage to the structure and more attractive to potential buyers. French drain system indicates that the property is well maintained and reduces the damage from water, increasing its property value. Ensuring property value helps to attract more buyers to a property and reduces financial savings.

Read more: What you should Know about Basement Waterproofing?

How Does a French Drain Work?

A French drain consists of 6 main components. The function of the French drain is a combination of all the properties.

The water in the soil close to the basement accumulates into a perforated pipe through a water-permeable fabric which covers the pipe to prevent debris clogging the drainage system.

Surrounding this perforated pipe lies a layer of gravel, which facilitates the flow of water, providing structural support.

In some installations, a catch basin is used to collect water and direct it into the pipe. For areas with notable water accumulation, a sump pump can be installed to pump water away from the property.

The collected water is directed to a safer location away from the property from the discharge pipe.

Properly working French drain system helps to Reduce soil erosion, enhance the indoor air quality, prevent water damage to the property, improves yard drainage, and increase property value.

How Deep Are French Drains?

French drains should be deep, around 18-24 inches (45-61cm). The depth of the French drain should be at the bottom of the basement wall. The depth of the French drain helps to determine the drain’s ability to collect and redirect groundwater effectively, protect structures from water damage, and maintain soil stability. Deeper drains manage large amounts of water, making them suitable in areas with high water accumulation.

How Wide Are French Drains?

French drains should be around 9-12 inches (23-31cm) wide. The width of the French drain helps to determine the capacity of water that is collected and transported away from the property and ensures that the gravel surrounding the perforated pipe is sufficient to filter out soil and debris, preventing clogs and maintaining the long-term effectiveness of the drain.

What is the Diameter of the Perforated Pipe?

The diameter of the perforated pipe in a French drain is around 4-6 inches (10-15cm). A perforated pipe with this sufficient diameter helps to handle an adequate volume of water and ensures that the system can manage heavy rainfall and groundwater efficiently. An appropriate diameter facilitates easier maintenance and ensures the protection of the property.

What is the Hole Diameter in the Perforated Pipe?

The hole diameter in the perforated pipe of a French drain ranges between 1/8 inches, which is 3mm, and 1/2 inches, which is 13mm. The hole diameter on the perforated pipe should be less than the gravel size. It prevents the gravel from entering to the drainage path. Not maintaining the proper diameter results the gravel entering the perforated pipe and clogging the drain path which leads to water damage. Therefore, using the correct hole diameter is important for the long-term functionality of the French drain.

What Type of Pipe is Good for French Drain?

PVC (Polyvinyl chloride) pipe is an excellent choice for perforated pipes for French drains. In contrast to other pipes, such as corrugated plastic, which are more prone to clogging and trapping debris, and metal pipes, which can corrode over time when exposed to moisture, PVC pipes can withstand pressure and weight, and is durable, resistant to chemicals and moisture, and is relatively easy to install.

What Type of Fabric is needed for the French Drain?

Non-woven geotextile fabric which is made from materials like polypropylene or polyester, is the ideal choice for the French drain. The advantages of using non-woven geotextile fabric are enhancement of drainage efficiency, reduction of clogs and blockages, improvement of soil stability, ease of maintain, and increase in the long-lasting functionality of the French drain system.

What is the Gravel Size of the French Drain System?

The gravel used in the French drain system ranges from ½ inch (13mm) to 1 inch (25mm). The gravel should always be larger than the holes in the perforated pipe, which ranges between 3-13 mm, to prevent gravel entering and blocking the pipe. High quality gravel in French drain should contain fewer fine particles as fine particles can reduce the porosity of the mixture, and using clean coarse, consistent size gravel ensures optimal drainage of the French drain system.

What is the Gravel Height in French Drain?

The gravel should be filled up to a height of 12-18 inches from the perforated pipe of the French drain. Filling the drain with too much gravel does not allow enough soil to cover the gravel, which leads to erosion. In contrast, filling the drain with too little gravel results in not having sufficient drainage capacity, leading to water clogs.

How Much Gravel Need for French Drain?

The volume of the gravel needed fill the French drain depends on the size of the drain and the pipe. The volume of the gravel needed can be equal to the volume of the excavated trench as well. To calculate the volume, you can use the formula; width*height*length. For example, if the trench is 12 inches(1 foot) wide, 25 inches(2 feet) long, and 18 inches (1.5 feet) deep, you can calculate it by; 1 foot* 2 feet*1.5 feet. The answer will be 3 cubic feet of gravel.

Where Should You Place French Drain Catch Basin?

The catch basin for a French drain should be placed in low-lying areas for groundwater or on top of a French drain during rainfall. The purpose of the catch basin is to collect, filter out debris, and redirect them to the drainage pipe. French drain helps to Improve water management, reduce the risk of clogs, and manage excess water, protecting the property from water damage.

What Type of Backfill Soil is Good for French Drains?

The best 3 types of backfill soil for French drains are sandy soil, clay soil, and gravelly soil. Sandy soil has large particles that allow water to easily pass through. Clay soil, which is a balanced mixture of sand and clay, offers good drainage and retains some moisture. Gravelly soil consists of small stones, and coarse particles prevent soil from washing away. These soil types are suitable for backfilling because they contain high porosity, prevent waterlogging and support efficient functioning.

What is the Slope of the French Drain Backfilling?

The slope of the French drain backfilling has a gradient of about 1%-2% ( 1-2 inches fall for every 8 feet length). The backfilling slope should slope away from basement walls and other structures to ensure efficient water flow and prevent water from pooling near the foundation. This gentle slope guides water to the drainage system, away from the property.

Where to Discharge French Drain?

The discharging point for the French drain should be a location away from the property, an area where water can be safely absorbed like a dry well, storm sewer, or a drainage ditch. The purpose of selecting an appropriate place is to ensure that the drainage of water won’t create soil erosion or flooding. Choosing a right discharge location protects the landscaped property and maintains the effectiveness of the French drain.

How to Install a French Drain?

These are the 6 steps of installing a French drain

Dig a trench: After deciding the best spot to direct water away from your house, dig a trench typically 18 inches deep and 12 inches wide, ensuring a slope to facilitate water flow.
Install the perforated pipe: Install the perforated pipe with a 1%-2% gradient.
Install the fabric and the gravel: Line the trench with a water-permeable fabric, leaving enough fabric to wrap around the pipe. Then fill the trench with about 2 inches of gravel.
Install a catch basin with the pipe: Install a catch basin that works for the diameter of the pipe. Then, install you perforated pipe.
Cover the pipe with gravel and the fabric: Add another 2 inches of gravel. Then, cover the gravel and the pipe with the excess filter fabric.
Backfill with soil: Cover the fabric-wrapped gravel with soil, filling the trench to ground level. Make sure to spread it evenly.
Discharge the water: Direct the discharge pipe to a location like a drywall, drainage ditch where water can disperse naturally. Make sure the drain is flowing successfully.

What Waterproofing Method is Good for Exterior French Drains?

For external French drains, sheet membrane waterproofing and liquid membrane waterproofing are two effective waterproofing methods. Sheet membrane waterproofing is durable and resistant to chemicals. It can be waterproofed immediately after installation, has consistent coverage, and is easy to inspect. Liquid membrane waterproofing is flexible and repairable, offers rapid curing, is seamless, and can be easily applied.

How to Clean French Drain?

You should follow below 5 steps to clean a French Drain

Locate the access points: Identify the access points of the French drain system. Remove any grates protecting the access points.
Insert a garden hose: Inspect the drain for any visible blockages. Place a garden hose as far as possible into the drain and turn on the water.
Power wash the drain: For more stubborn blockages, use a pressure washer. Stand to the side when you do this, as the water can hit you back after it hits the clog.
Flush with clean water: After removing the clog, flush thoroughly with clean water. Ensure all debris is cleared and water flows freely.
Replace Grates: Once the drain is clean, replace the grates over the access points to protect the system from future debris.

How to Unclog French Drain?

Following are the 7 steps to unclog a French drain.

Identify the clog
Remove the grate or cover
Insert a garden hose to the drain
Use a drain snake
Power wash the pipe
Flush with clean water
Replace the grate or cover

How Long Can a French Drain Last?

A French drain can last between 30-40 years. It depends on various factors like quality of the materials, type of soil, amount of water to manage, and proper maintenance.

How to Improve French Drain Lifespan?

Using high quality materials for pipes and fabrics
Using proper installation techniques
Inspect regularly for signs of blockages
Ensure proper drainage
Prevent sediment build-up that clogs the gravel