How to Build a French Drain

Water management is an important aspect of designing a sustainable garden. Without water, nothing grows. As I’ve said before, it’s virtually the very essence of life itself!

In Permaculture, designing a garden to make optimum use of water is critical, especially in a country like Australia, the driest continent in the world, where water shortages are common.

French drains are one of the many tools used in Permaculture to best manage water.

Before delving into the details of French drain construction, it’s best to understand the very element we’re dealing with – water!

The Permaculture Perspective on Water

Water is one of the critical design elements in Permaculture. We work on capturing it, using it as efficiently as possible, reusing it where possible, and dealing with wastewater in a sustainable way.

Without understanding the nature of water and how it behaves, you cannot create an efficient Permaculture garden design. To understand how to use French drains in gardens, we’ll cover some water basics:

1. Water always seeks the lowest level – it naturally flows downwards due to gravity and runs from higher to lower levels until it settles at the lowest point. This may seem obvious, but we need to consider what it means in the context of nature, soil and plants. Consider rainwater falling on a mountainside. It will trickle down the slope quite quickly, picking up loose soil, debris, leaves and other organic matter, and carry them with it down the side of the mountain. The land eventually levels out into a valley, where all the nutrient-carrying rain ends up, resulting in a moist, fertile valley, which also probably has a lake at the bottom. The plants in the valley have all the water they can use, as it seeps deep into the soil, where it can be accessed by the roots of plants and trees. Conversely, very little water has soaked into the mountainside, as it runs along its surface, with very little soaking into the soil. Furthermore, soil and nutrient-rich material is potentially stripped away. If the water doesn’t flow into a valley, it forms a stream, which flows into a creek, which flows into a river, which finally flows into the sea – and with sea level being the lowest level, the water finds its way there, as it naturally does.
2. Water takes the shortest path to the lowest level – water flowing down the side of a hill or mountain will take the most direct route to the bottom, as will any running water, such as a stream or river. By taking the shortest route, it makes the minimum contact with the soil, and therefore soaks as little as possible into the soil on its downward journey to the lowest point.
3. Water is strongly absorbed by composted organic matter – humus, which is the result of the breakdown of plant matter, makes for a rich, dark soil with good water-holding properties. It is hydrophilic, that is, it draws and holds water. Soils that are dried out, leached, with little organic matter left in them don’t hold water; it runs off the surface. These are termed hydrophobic, that is, they repel water. If you’ve watered an area of depleted soil, only to dig into it afterwards to find it is bone dry underneath, then you’ve seen hydrophobic soil in action. If you want your soil to make the most of available water, then add lots of organic material to your soil; it’s that simple.
4. Water can be slowed and retained in the landscape – by utilising swales (contour trenches that run along the slope of the land at a constant elevation), we can slow the flow of water across the land and encourage it to soak into the soil. Swales work by diverting the flow of water from vertical (straight down the slope) to horizontal (along the slope) and by catching and pooling the water, allowing it to slowly seep into the ground rather than rushing downhill.
5. Water responds to design – by understanding water’s nature and applying design principles, we can manage how water flows, is retained, and is distributed across a landscape to meet our needs and those of the plants and animals living there.

Naturally, swales are more at home on rural hillsides than in urban backyards, though you can utilise the same concept on a smaller scale by creating a small trench on a slope to prevent run-off. For those of us with concrete in our backyards, which has 100% runoff and doesn’t retain any water, there is still hope! The next best thing to a swale is a French drain for capturing runoff and redirecting it back into the soil.

In the instructions below, we’ll go through a real-life example from my backyard. So, let’s have a look at French drains!

What is a French Drain?

A French drain is a long trench dug into the ground in the soil and filled with gravel to redirect water away from an area and carry it elsewhere. A French drain can have one or more perforated pipes (slotted Ag agricultural drainage pipes or “slotted Ag Pipes”) running along the bottom to improve drainage and carry away water more quickly.

Traditionally, French drains are used to carry water away from building foundations or to serve as outlets for septic tank sewage treatment systems, distributing the water over a large area so it can seep into the ground.

As you can see, it’s a fairly simple concept. Now, let’s have a look at a real-life example in design and construction.

How a French Drain Works and How to Build One

Pictured below is the rearmost part of my backyard, with the back fence in the background. The block of land is sloped, highest point of elevation at the back, the front of the house is the lowest point.

Now, to explain the problem…

The blue arrows show how the water flows when it rains. It flows over the concrete area pictured, down the concrete steps, along the walkway at the side of the house, then down the driveway, by which time it has travelled over thirty metres to the footpath, where it runs into the stormwater drain.

This small space above the step where the clothesline (the green post in the middle of the picture) is might seem trivial. However, there’s an additional point to consider. You can probably spot a line of pots along the back fence, all equipped with trickle irrigation. After watering, the runoff water, enriched with nutrients from the soil in the pots, flows down the slope to the stormwater system. Redirecting this water back into the garden would be more efficient, conserving water and recycling nutrients.

Considering the question of rainwater, the area in question is roughly 3.5m by 2.5m, totaling about 8.75 square meters.

If we remember our rainfall formula, 1 mm of water over one square metre of roof (or surface) yields one litre of water.

In Melbourne, our dry climate averages about 700mm of rainfall annually, meaning each square metre of surface can collect 700 litres of water. When considering our small concrete patch, the calculation shows it captures 700mm multiplied by 8.75 square meters, totaling 6125 litres of rainwater, excluding runoff from irrigation and pots.

Now, the question is whether the 6000 litres of rainwater should be redirected to my garden or washed down the gutter into the stormwater drain. Unless you live in a swamp, the answer is clear! So, let’s start building!

Step 1. Mark the concrete for the cut

Cutting concrete can be dusty and messy, so it’s helpful to mark the cut line beforehand. In the picture below, I marked the cut with a dashed red line. In this example, the trench width is 15cm (6″).

To mark the cut, the traditional method involves using a builder’s chalk line tool. The image below shows a chalk line and a bottle of what’s called “chalk.” Although it’s called chalk, it’s actually a special marking material that is available in two types: temporary, which washes off with water, and permanent, which never comes off! It’s advisable to use the temporary type in case of mistakes. Chalk also offers various colors, so choose one that provides good visibility.

A chalk line is a string stored on a winder inside a container of powdered chalk. To use it, secure one end at your starting point—either by pegging it down or having someone hold it—then unwind the string to your desired ending point and pull it tight. To mark the concrete, lift the taut string like a bowstring and release, causing it to strike the ground with a sharp “snap” that leaves a chalk line. Repeat a few times if necessary for a clear mark. I own a chalk line and used it to make the mark—it only takes a few seconds.

If you don’t have a chalk line, you can use a straightedge—such as a long, straight object—and blackboard chalk instead.

Step 2. Cut the concrete

This is the tough and messy part. The professional way to cut concrete involves using a demolition saw. It resembles a large chainsaw motor with a sizable cutting disc, designed to create openings for doors and windows in concrete walls. These saws can perform wet cuts by directing water into the blade to minimize dust. Usually, these powerful machines are rented, which can be costly, so they’re not practical for small jobs. It’s advisable to check the daily rental rates. By the way, this is what a demolition saw looks like.

Since hiring a professional tool for a single 2.4m (8-foot) cut wasn’t cost-effective, I chose a cheaper but more challenging method: borrowing a friend’s 230mm (9”) angle grinder fitted with a masonry cutting disc designed for concrete. An image of this tool is shown below.

Here’s how the finished French drain looks and how the water now flows into the garden beds rather than the stormwater drain.

Since it’s an electrical cut, water cannot be used, which results in a huge amount of concrete dust. Believe me, you’ve never seen this much grit in one place! It will spread everywhere, so if you’re not fully covered in protective clothing, you’ll look like a statue covered in dust. Wearing protective eyewear, hearing protection, and a respirator is essential because the concrete dust is hazardous to your eyes and lungs. The noise is deafening, and cutting through thick concrete while keeping the saw steady and heavy is a slow, exhausting process.

Use these two monster tools for a clean, straight cut. If you prefer a less precise approach, you can cut the concrete in any way you like. Methods such as using a club hammer and cold chisel, then prying out the pieces with a wrecking bar, are also effective but require more effort and tend to be less messy.

I guarantee that after finishing the concrete cutting, you’ll question why we ever chose to cover all our spaces with this god-forsaken substance!!!

Step 3. Fill the Trench with Gravel (and Optional Drainage Pipe)

After all the drama and commotion, hopefully, you’ll have a neat, straight trench. Remove any broken concrete pieces and dig the soil to the necessary depth. For mine, I made it 15cm (6”) deep. Next, lay the slotted Ag pipe (agricultural drainage pipe) and fill the trench with gravel, or omit the pipe and simply fill with gravel.

The slotted Ag pipe (shown below) significantly improves drainage. Use it if needed, as it is inexpensive and helps reduce the amount of gravel required to fill the trench. Ensure you choose the slotted type, which features holes or perforations along its ribs, allowing water to enter and flow through easily.

After adding the gravel, level it using a flat board, spade, or the back of a rake. Your French drain is now finished.

Results and Takeaways

This completed French drain is modest in appearance but highly effective.

If we retrace the water flow, as we did before, using the blue arrows, we see that the water is redirected sideways along the trench.

Let’s take a closer look at the French drain.

Clearly, the drain ends must lead somewhere! In this case, the right side flows beneath a mulched path into the garden beds.

Water flow into the garden beds is indicated by the blue arrows.

The other side of the French drain also directs water into a garden bed, near the lemongrass, which benefits from the additional moisture.

Overall, cutting the concrete with an angle grinder was extremely difficult, but it’s worthwhile considering the garden receives an additional 6000 liters of water annually from this small effort. This significantly improves the garden’s resilience and drought resistance, while also conserving water over time. Additionally, although traditional swales may not be feasible in urban permaculture, we can adapt similar principles through creative approaches.

Permaculture is centered on creative solutions, so embrace your creativity and don’t hesitate to get your hands dirty. Your efforts can make the world a better place!

Want a more decorative French drain? You can plant it with the right type of plants to maintain water drainage and prevent weeds from growing.

This is what the French drain looks like with Creeping Thyme planted on it.