French Drains: Dimensions, Rock, Pipe, Fabric and Installation

Written By Adam Paul - Waterproofing Expert

A French drain is a trench that is used to remove water around the basement walls. A French drain consists of a trench, gravel, a perforated pipe, fabric, a catch basin, and a 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 to accumulate water between the foundation walls and the soil layer and pump 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). The French drain should be 1%-2% of the slope towards the catch basin. You should use a 1/2-1 inch (13-25mm) crushed stone for the French drain. The French drain rock layer height should be 12-18 inches, and its width is similar to the French drain width.

You should use a perforated pipe with a 4-6 inch (10-15cm) diameter. The hole’s diameter should be between 1/8 – 1/2 inches (3-13mm). You should use a PVC pipe as the perforated pipe. You will need a non-woven geotextile fabric between the gravel and soil layers. You should place the catch basin at the lowest point of the French drain. You should discharge the French drain away from the foundation wall to reduce the water impact.

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 subsurface drain system designed to collect and redirect water from the basement, foundation, and crawlspace areas. The main components of the French drain are a trench, gravel, a perforated pipe, fabric, a catch basin, and a sump pump. A French drain helps to prevent basement flooding, protect the foundation, reduce soil erosion, and reduce maintenance and repairs to the basement. There are 10 types of French drains available for the home with different designs and purposes.

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 as a filter and provides a flow of water to the pipe. A catch basin included at the surface level collects water and directs it into the trench. The collected water is then sent through the discharging pipe, which carries it away from the property.

Use Aqviz French Drain Calculator to calculate all the materials and their costs.

How Does a French Drain Work?

A French drain works according to the following 6 steps

Surface water leaks into the ground and increases the ground soil saturation and creating an excess water pressure
Then excess water seeps into the French drain rock layer through the geotextile fabric from the saturated soil
French drain rock blocks the silt and dirt in the water and creates a path to the perforated pipe
Seepage water enters the perforated pipe through small perforated holes
The water in the perforated pipe flows towards the catchment basin with the help of gravity
A catch basin has a sump pump . It discharges the accumulated water from the French drain pipe.

A properly working French drain system helps to reduce soil erosion, enhance the indoor air quality, prevent water damage to the property, improve yard drainage, and increase property value.

What is the Depth of the French Drain?

The depth of the French drains should be between 18-24 inches (45-61cm). 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.

What is the Width of the French Drain?

The width of the French drains should be between 9-12 inches (23-31cm). 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 Slope of the French Drain?

The slope of the French drain should be 1%. It means each and every 8-foot French drain should be sloped 1 inch. Slope should be towards the catch basin. If the slope is low, it causes to slower the drain more slowly, and if the slope is higher, you have to use more materials during the installation process.

What Type of Rock is Good for French Drains?

Limestone, granite, or basalt crushed stone is good for the French drains. These crushed stone with an angular shape resists compaction, offer high permeability, and provide excellent structural support around the perforated pipe.

Read More About: French Drain Rocks: Everything You Should Know

What is the Size of the Rock for a French Drain?

The most commonly used rock size for the French drains ranges from 1/2 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 from entering and blocking the pipe.

What is the Rock Layer Height in French Drain?

The rock should be filled up to a height of 12-18 inches, including 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 Rock Is Needed for a French Drain?

To calculate the amount of rock for the French drain, you should find the rock volume first and then reduce the pipe volume. To find the rock volume, multiply the drain width, rock height, and drain length. Then find the pipe volume. Then subtract the pipe volume from the rock volume.

French drain rock volume = French drain width × Rock height × Drain length

Pipe volume for 4 inch pipe =Length × π × (0.165 ft)² ≈ Length × 0.0856

Net Gravel Volume = Total Trench Volume – Pipe Volume

Is this difficult to calculate? So check our French Drain Rock Calculator

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 inch, which is 3mm, and 1/2 inch, 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 in 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 Pipe Material is Good for a 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 maintenance, and increase in the long-lasting functionality of the French drain system.

Where Should You Place a 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 it to the drainage pipe. A 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 the 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 7 steps of installing a French drain

Inspect the basement and plan the French drain layout
Gather relevant tools and materials for the French drain
Excavate the French trench
Set up a catch basin
Lay the geotextile fabric at the bottom of the French drain
Add rock base and install a perforated pipe
Cover the perforated pipe and backfill

Read More: How to Install a French Drain?

How to Clean a French Drain?

You should follow below 5 steps to clean a French Drain

Gather relevant cleaning tools and materials
Locate the cleanout or drain outlet
Flush the line with the garden hose or jetter
Use a plumbing snake or auger for tougher clogs
Clean the outlet and catch basin
Backflush the French drain if accessible
Maintain landscape and surface conditions

Read More About: How to Clean a French Drain?

What Are the Benefits of French Drains?

The benefits of a French drain is preventing basement flooding, improving structural integrity, minimizing health hazards, preventing water damage to the basement, draining underground water effectively, and enhancing the property value of the house.

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. A 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 costs.

Read more: What you should Know about Basement Waterproofing ?

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 Unclog a French Drain?

The 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