Last Modified: March 3, 2024

What is a French Drain

12 Things You Need to Know

A gravel blog page template

By Ben Parrot - Landscape Gardener & Amateur Geologist

Overview

The French drain is a simple yet brilliant solution for dealing with excessive build-up. Contrary to what the name suggests, it is not a Parisian architectural wonder. This system, misleadingly named after its American populariser Henry Flagg French, ingeniously redirects unwanted water away from vulnerable areas via a gravel-filled trench and a strategically placed perforated pipe. It's a testament to the power of smart design in managing nature's challenges.

This guide aims to 'mansplain' the humble French drain, offering insights and tips from years of professional landscaping experience. Whether you're a homeowner about to seek expert advice for damp arising from excess moisture or a DIY landscaper taking measures to ensure that your new gravel pathway or drive will stand the test of time, understanding the ins-and-outs of French drains is vital to proper installation and drainage. Don't 'run-off' now, 'cause we're just getting started!

Polar ice chippings alongside a drivewway. Jpg

Key Takeaways

  • Gravel Size Matters: The size of the gravel used impacts the effectiveness of the drainage system.
  • Filter Fabric: Laying a geotextile membrane can prevent the system from clogging.
  • Gradient: For a French drain to work effectively, it must be installed with a proper gradient.
  • French? Despite the name, French drains did not originate in France.
Metal grate over a gravel drain

What is a French Drain?

A French drain is a popular drainage system used to redirect water away from a specific area. It consists of a trench filled with gravel or rock and a drainage pipe placed at the bottom. The trench is sloped to cause the flow of water to be directed away from the intended area. French drains are used around the foundation of buildings, in gardens and areas with ineffective drainage.

The Way a French Drain Works

A French drain works by utilising gravity to redirect water away from a specific area and allow it to drain away safely. The trench, filled with gravel or aggregate, creates a path of least resistance for the water to flow. The perforated pipe is laid at the bottom of the trench allows water to enter and be carried away, preventing it from pooling or causing saturation near the sub-base or foundations. The slope of the trench ensures that water naturally moves away from the intended area.

Next we jump straight into the twelve facts and common mistakes that you need to know before installing a French drain.

Trench dug in preparation for gravel drainage

12 Facts and Common Mistakes to Avoid About French Drains

French drains are one of the most common solutions for managing excess water, but there are some common issues and mistakes to avoid when you install drainage systems. Here are 12 important facts and tips about the modern French drain:

1. Call Before You Dig

Not calling the appropriate authorities to mark any underground utility lines before you get your trench dug. It’s important to take this simple step so that you can dig safely and avoid any serious accidents or disruptions to essential services. Remember, safety should always be a top priority, especially when undertaking any DIY project.

2. Choose the Right Type of Drainage Gravel

Using the wrong type of drainage gravel or shingle can significantly impact the effectiveness of your land drainage system. This tip is to select a gravel type that allows for the ideal water flow because using gravel that is too small can lead to clogging, and those that are too large can create gaps and prevent proper drainage. The ideal size is typically 18-25mm (¾ inch to 1 inch) in diameter. Aim to use clean, washed drainage gravel that is free from sediment or dirt.

Washed pea shingle 20mm close up. Jpg

3. Line the Trench with Drainage Fabric

Not lining the base and sides of the trench with a geotextile filter membrane is a common mistake. This water-permeable fabric acts to prevent soil and sediment from clogging the drain pipe, which reduces its effectiveness. It helps to stabilise the soil and prevents the perforated drainage pipe from becoming clogged over time. Therefore, it is highly recommended to include a layer of drainage fabric in your installation.

4. Select a Non-Woven Landscape Fabric

Using the wrong type of landscaping fabric can be a common mistake. It is important to use a permeable, non-woven landscape fabric. Non-woven fabric is specifically designed to allow groundwater to flow through while preventing debris from passing. It is often called French drain fabric.

French drain fabric, a type of filter membrane, is typically made of a non-woven geotextile fabric, which allows water to pass through but prevents soil and debris from clogging the drain. For maximum durability, it is important to use a high-quality type specifically designed for drainage applications, such as the crate of a soakaway.

Close up of non woven geotextile landscaping fabric

5. Using Soil to Backfill the Trench

One common mistake to avoid is backfilling the trench with your excess soil, as it will severely hinder the proper functioning of your drain by creating a barrier for water to flow through the drain pipe within. You need to use suitable drainage gravel for filling the rest of the trench. This allows for proper filtration of the water within and increases the effectiveness and longevity of your drainage.

6. Grade a Positive Slope for Your Trench

One factor to consider during excavation is making sure there is a proper slope for the drain. Without a positive slope, water will not be able to flow efficiently, especially after heavy rain. A minimum slope of 1% is recommended to allow for effective water drainage. This means that for every 30 metres (100 feet) of drain pipe, there should be at least a 30cm (1-foot) drop in elevation.

7. Avoid Gravel Only Drains Without a Drain Pipe

When installing a French drain, it is essential to include a drain pipe along the trench filled with crushed aggregate. Using gravel alone without a drain pipe markedly lessens the effectiveness of the drain. The drain pipe acts as a conduit, allowing for water to enter the drainage and flow through, preventing clogs and water pooling. This could result in damage to your property. A french drain isn’t as effective without a drain pipe and therefore is an inexpensive but significant addition to any installation.

Drainpipe going into the ground with plum slate chippings ground cover

8. Don't Tie a Down Pipe Directly Into the Drain

Tying a down pipe directly into the French drain is a common mistake to avoid, even though it may seem like an efficient way to divert water away from your property. In practice, it can actually lead to clogs and drainage issues. The hydrostatic pressure of the water coming from a down pipe can overwhelm the drain and cause water to back up. It is important to plumb them into a specific drain system for downpipes that flow into stormwater drains or even waterbutts.

9. Install the Correct Type Of Pipe

The most commonly used pipe for French drain installations is perforated PVC pipe. This type of pipe allows water to enter through the perforations and be carried away from your property. It is also beneficial to choose a pipe that is the appropriate size for your needs. A larger diameter pipe will have a higher capacity to handle larger volumes of water. Additionally, when purchasing, make sure it is durable and designed to withstand underground conditions.

Perforated flexible drain pipe close up. Jpg

10. Maintenance Is Crucial

Particularly where they perform a critical role, regular inspection and maintenance should be conducted annually to keep your drain free from debris and obstructions. This is the only way to help ensure that it continues to function effectively.

To assess the conditions, locate the ground-level access point, which is typically an above-ground grate or the endpoint of a down pipe or above-ground pipe. Run a hose down the pipe so that you can observe how efficiently the drain is operating. If any issues are found, clear any leaves, dirt or other debris that has accumulated.

11. Keep Your Distance

The distance from your property is an important factor to consider when installing a French drain. The drain should be positioned at a minimum distance from the foundation of the property to effectively divert water away. It is recommended to place the drain at least 3-4 metres (10-12 feet) away from the building in order to prevent any water damage or seepage into the foundation. Bear in mind that if the drain is too close it might not be able to efficiently redirect standing water which could lead to moisture or more serious problems.

Fines and gravel around a home drainage system. Jpg

12. What's in a Name?

The origin of the name "French drain" is often attributed to Henry French, a judge and farmer from Massachusetts, USA, who popularised the technique in his 1859 book called "Farm Drainage." However, the technique of using perforated pipes in trenches to redirect water had been used long before his book. It is believed that the term was named after French for bringing widespread attention to the method.

Is a French Drain Right For Your Project

When considering whether a French drain is right for your landscaping project, it is important to assess your specific drainage needs. In landscaping, they are especially beneficial for features being built at the garden's lowest point, or where there is poor soil drainage or regular, heavy rainfall. However, if you have minimal water accumulation issues, there may also be simpler solutions and a French drain may not be necessary.

If you have serious damp issues in your house and you believe it is caused by water build-up near your foundations, it is best to consult with a professional.

Trench for french drain through a garden

Need to work out how much gravel your drainage project needs? Try our free gravel calculator.

Summary

In conclusion, we've demystified the concept of French drains, breaking it down into understandable chunks, or 'mansplaining' it if you will. We've explored the importance of proper installation, the necessity of regular maintenance, and many of the potential pitfalls. So, from here, you're probably ready to 'go with the flow'.

Either way, we wish all the success as you journey towards a beautiful garden feature that is also likely to stand the test of time! Happy gardening!

Drainage system in the forests of sao vicente madeira

FAQs

Why would you use a French drain?

The purpose of a French drain is to direct surface water away from properties or landscaping features in order to prevent flooding, where water accumulation issues exist. It is particularly useful in areas with heavy rainfall or poor soil drainage. It provides a long-term solution to maintain the structural integrity of the property and helps to prevent moisture-related issues such as mould and mildew growth.

When should you not use a French drain?

While French drains are an effective solution for managing water drainage, there are certain situations where they may not be the best option. If the issue is caused by a high water table or a natural spring, it won't be effective in diverting the water away.

If you are working in clay-heavy soil that doesn't drain well, it also may not provide the desired results, as the subsoil will lack the permeability required. In these cases, alternative drainage solutions like surface drains or grading the landscape may be more suitable.

Can a French drain just end underground?

A French drain can end underground, but it is not recommended. When a French drain ends underground, it can lead to several issues. The water will pool at the termination point, causing potential flooding and water damage to the surrounding area. The water may also just seep back into the soil, which will eventually cause the drain to become clogged with sediment and debris.

For proper functioning, it is best to connect the drain pipe to a discharge point such as a storm sewer, dry well or a designated place away from the area.

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About the author 

Ben Parrot is a landscape gardener and amateur geologist. He is the founder of Outdoor Aggregates, with the mission to create the first encyclopaedia for decorative aggregates. From natural stone through to decorative bark, these aggregates are critical to garden and landscape design, universally.

The problem: There isn't any place where all the different types of decorative landscaping materials and construction aggregates can be found, nor are the specifications of each type readily available. So, enriched by his extensive experience, and inspired by the timeless elegance and raw beauty of stone and rock, etched by nature's artistry over millennia, Outdoor Aggregates was born.