What is a French drain and do you need one?

Just what is a French drain? While this might well be something you have heard of, when looking at various drainage systems, do you actually know what it is and how it could benefit you?

Here, we will be explaining what French drains are, why and where they are useful and how to construct one — it is a surprisingly simple DIY job yet can have a significant effect on your outdoor spaces. 

With so many different types of drainage system available, it is important to choose the one that is most appropriate for your home in order that excess and waste water can disperse effectively and to avoid issues that could damage your home, such as damp. 

What is a French drain?

French drains are used as a form of land drainage and are named after Henry French — a 19th-century farmer (and judge).

Unlike soakaways, which are a pit in the ground into which you run your rainwater drainage. French drains are used to prevent water logging and an excess of surface water. They consist of a trench dug, at a gradient, into the ground, lined with membrane, filled with aggregate and, usually these days, a perforated drainage pipe. 

They are a great way to transport surface water away from walls and for many people they are the ideal solution in gardens or driveways prone to flooding. They are also sometimes referred to as trench drains or land drains.

How do French drains work?

The idea behind French drains is that rather than sit around on the surface of the ground, water will seep down into these trenches to be taken away to a drainage area.

"Water runs into the gravel-filled trench and then usually into a perforated underground pipe within the trench," explain the experts at EasyMerchant. "The water will then travel along the pipe and empty either into a soakaway, drainage ditch, a well or any other suitable drainage area."

The trench is lined with a geotextile filter membrane, such as DRAINTEX Drainage Geotextile Membrane Landscape Fabric from Drainage Superstore. The purpose of the membrane is to prevent any sediment getting into the drain — the pipe is also sometimes wrapped in this. 

Where is the best place for a French drain?

In order to be most effective, your French drain needs to be installed at the lowest point of the flood-prone area you are dealing with — and in a position that means it can take the surface water to a drainage point elsewhere, or a soakaway. 

"Water has a tendency to reach the lowest point within an area, which is why it’s crucial for you to install your French drain at the lowest point of a landscape," explain EasyMerchant.

When planning on the route your French drain will take, make sure there are no pipes, cables or trees that could get in the way of the flow of water.  

What French drain depth and width should I aim for?

We will explain how to build a French drain in a moment, but it is useful to know how wide and to what depth you will need to dig your trench.

"Plan the width of the drain using 200-300mm as a rough guideline," advise the experts at EasyMerchant. "Though generally suitable, the width you use will be largely dependent on how much water needs to drain. Remember, it’s better to overestimate than build a trench that fails to function properly." In areas with large amounts of water, you could need a drain up to 450mm wide. 

In terms of depth, it is recommended to aim for 250-500mm, with a slope of at least 1:50 from the highest end of the trench to where it eventually discharges. So, for every 1m of trench, it should fall 20mm. Note that this is the minimum fall you should aim for. 

How to build a French drain

So, you know what a French drain is used for and think it could be just the thing you need — but how do you build one?

This is actually a task often undertaken on a DIY basis — but be sure to use the correct materials and think about where the drain will end and what you are going to do with the soil you dig out to form the trench.

Although traditionally French drains were constructed without a drainage pipe, these days this is the recommended way of doing things. 

"If you don’t have access to piping and landscape fabric, you can instead design a drain that functions according to traditional principles, but bear in mind that the drain will not be as effective and will not last as long," say EasyMerchant's experts.

Once you have dealt with the above issues you can get started:

1. Plot out your route using a marker spray or stakes and string.

2. Dig your trench — depending on the size of the trench, you may be able to do this by hand, otherwise you will need to hire a mini digger. When digging the sides of the trench, remember they will need to slope rather than be dug straight. "It’s important for your trench to slope at no less than 45 degrees, to ensure a smooth flow from the affected area and to maintain stable ground," say EasyMerchant.

3. Once the trench has been dug, it will need lining — use a permeable geotextile fabric to stop sediments from building up within the space and leave an overlap of around 300mm on either side. This will be folded over once the trench has been filled. 

4. The base of the trench can now be filled with a layer of aggregate — up to around a third of the way up. Use large, coarser stones here —  "the ideal size to use is around 10-20mm pea shingle," say EasyMerchant.

5. You can now place your perforated land drain pipe into the trench — according to EasyMerchant these will usually be 100mm wide, with perforated holes that should face the ground. "This is a crucial step because the position of your holes will be critical to the successful removal of water," they warn. "Otherwise the water will have to fill up the pipe before it drains out of the holes."

6. Once the pipe has been dropped in (make sure it is straight and has a 1:50 fall down to the drainage area), the rest of the trench can be filled up with more gravel to just beneath the top.

7. Fold over the membrane and add a more decorative layer to finish the whole thing off — fine gravel or even turf can be used.