What Are French Drains and How Do They Work?

8 June 2021

Whilst no two properties are ever going to be the same, there are certain similarities shared by certain structures that make them easy to maintain and repair throughout their life. Systems like wiring, ventilation, insulation, and of course, plumbing, are all staples of any property. It’s these systems that you need to understand if you want to keep your property in tip-top condition.

So, if you are the owner of a property that makes use of a French drain, then you might be a little concerned about how they work, and what you need to do to maintain a French drain. Alternatively, you might have recently come into ownership of a property that you might believe would benefit from the use of French drain itself.

Either way, it’s going to be useful to understand not only what a French drain is – but how it works too.

What Is a French Drain?

A French drain in principle is very easy to understand: a pipe is laid within the ground that allows water that pools around your property to drain away from the building itself. This reduces the risk of water damage occurring as a result of hydrostatic pressure in below-ground structures or from dampness if any pooling water makes its way into the building itself.

It’s not only pooling water that is directed into a French drain either. Subsoil and other loose sediment can be easily washed away along with water if that is also providing an issue to your property.

What makes French drains so special is that from a practical standpoint, they aren’t special at all. Typically, they are installed at the edge of a building so that the sitting water can be drained away right at the source of the problem. They are usually compromised of a drain, which leads to a pipe located within a trench in the ground – which is in turn filled with aggregate.

This is all very simple and easy to understand – and to some, it might seem like a perfect solution for troubles with pooling water or flooding within a property. However: Before you start digging up your driveway or garden, it’s important to seek out expert advice on water control in your property for a number of reasons, which we will explore a little further down.

How Does a French Drain Work?

As mentioned earlier, the main purpose of a French drain is to divert water that is pooling against or leaking into a property away from the building itself so that it can be safely drained away.

Digging Out The Trench

A French drain will always be installed within a trench. The trench should be dug out at least a meter away from the property itself, and great consideration should be taken as to whether your French drain trench could interfere with any building foundations, piping, electrical inputs, or other below-ground amenities.

As a rule, your trench should be dug no deeper than the foundations of your building, so bear that in mind when planning.

You will also have to plan for the actual excavation of the drain itself. The soil you have removed will need to be recycled safely. The actual siting of the trench will have to ensure the water will drain safely. On that subject, the trench itself must be dug out towards the planned draining area on at least a 1% decline. Otherwise, the water flowing into the French drain isn’t going to travel anywhere.

Installing the Right Type Of Pipe

Once your trench is dug out, you will be able to install the pipe that will be responsible for directing and controlling the flow of pooling water. You can choose here between two types of pipe: perforated or non-perforated.

Usually, its recommended that a perforated pipe is used, so that any water travelling down through your trench will naturally find its way into the pipe itself, and you will avoid any debris entering and blocking the flow of water within the pipe as a result.

Filling in the Trench

You are also going to have to fill the trench in. Firstly, lining the sides of the trench itself, and lying under the pipe should be a water-permeable fabric. This is a measure taken in most modern French trenches to make sure that no unwanted sediment or other solids make their way into the pipe and block it.

On top of that fabric, you should begin to lay the aggregate. A layer that reaches approximately a third of the way up the trench should be filled to start with, with the pipe inserted into the bottom of the trench afterwards.

One thing to bear in mind when it comes to laying perforated pipe is that the holes need to lie face down. That’s because otherwise, the water draining in the pipe will be unable to leave or move too far unless the pipe itself is completely full.

You can then fill the rest of the trench with gravel until it sits just below the top level of the trench itself. It’s then up to you, but it is possible to cover the top of the French drain with some turf or whatever type of gravel or aggregate might cover your property in that specific area.

Do You Need a French Drain?

An example of what rising damp can do to masonry

If you have problematic water pooling either next to or underneath your property, then the chances are that you could well benefit from a French drain being installed on your property. French drain’s themselves are cheap, and if the option to have one dug out presents itself, it can be a very cost-effective way of limiting water damage on your property.

One of the biggest questions you are going to have to ask yourself when it comes to the matter of French drains is whether or not your property would benefit from having one installed.

Having Your Water Problem Checked By A Specialist

The very first step in figuring this out would be by having your water issue looked at by a professional surveyor. It could well be that the problematic pooling water or ineffective drain can be easily fixed with property maintenance or drain cleaning. However, if this isn’t an option for your property, then you will have to decide on whether a French drain is the correct course of action to rectify the problem.

You will have to establish a few things before a French drain is the course of action you settle on. First off, you will need to be sure that the French drain will be installed at a location that would be considered a low point on your property. As water will naturally always find its way to the lowest point a French drain will need to be located at a low point to ensure that the problematic water can be drained safely.

The Depth Of The Trench

You also need to think about the depth of the trench containing the French drain versus the depth of your properties foundations. If your foundations are going to be located higher than the overall depth of the French drain itself, then you will need to reconsider the installation as a whole, as it will not work.

The Distance From The Property

Also, take into account the distance from the property itself – in most cases (but especially with older properties) that your French drain should be located at least a meter away from the building itself to avoid compromising your buildings structural integrity.

Where The French Drain Will End Up?

You will also have to think about where your French drain is going to end. Realistically, you will want all of that problematic, excess water to end up draining into a safe and contained area. A specifically designed soakaway, existing gutters leading to established drains, loose stone, or even a well would all make for safe drainage.

Deciding On a Drainage Solution

If you are looking to dispel and remove problematic water from pooling around your property, contact our team today to learn what our expert surveyors might have to say about your situation.

With Timberwise having over 50 years of property care experience, you can depend on our highly qualified surveyors to fairly and accurately assess your property and decide what actions should be taken to safeguard your property against water damage.