If you are experiencing problems with a waterlogged lawn or rising damp, it may be worth considering installing a French drain. These symptoms can be caused by excess water coming from higher ground, either on your own property or a neighboring property.

For centuries, French drains have been used to rectify these and similar problems. The earliest being simple ditches built on a gradient, today however, there are far more effective ways to build a French drain, one of which is described below.

1. Decide where to re route excess water

Probably the most important part of this project is deciding where the water should be re routed. Ideally this will be an area where the water will not have a negative impact on your own, or anyone else’s land or property. Having the drain empty into a soak away may be a solution, especially in areas with heavy clay soil.

2. Plan drainage route

Locate the most suitable route for your drain. To make the project easier for you, this should be free of obstructions (e.g. trees, shrubs) and if your foundations are shallow, situated at least 1 meter away from any walls. Take into consideration that creating a steeper gradient when building your drain will facilitate drainage. Though a French drain can and will work if the pipe is level, a drain which slopes at least 6" for every 50' you dig, will have a far greater impact. However, it will not work if the pipe has an upward incline at any point.

3. Digging the trench

Digging the trench is the most labor intensive part of the project. Your trench should be 5" - 6" wide, in extreme cases (where a large amount of water has to be re routed) a larger trench of 12" - 18" may be required. A French drain should be no deeper than the foundations of the nearest building. Your trench will run parallel to the wall of your building, horizontally across the slope. This will stop the water as it runs downhill towards your property and allow it to run off through your drain.

4. Line with gravel

On completion of the trench, line with 2"-3" of coarse gravel.

5. Lining the trench

Next, line the trench with landscape fabric (reinforced membrane) leaving some excess on either side, you will later fold these inward to create an enclosed drain pipe and gravel structure, this will help prevent dirt from clogging up the gravel.

6. Lay the drainage pipe

Lay the drain pipe on top of the lining and fill the trench with coarse gravel. Leave 4”-5” at the top of the trench. If you are using perforated pipe, ensure the holes are facing downwards.

7. Enclose the gravel and pipe

Fold the excess liner over the top of the gravel.

8. Fill and Finish!

Fill the rest of the trench by shoveling on some coarse sand, add a few inches of topsoil, seed or turf and the job is done!

The most difficult problem when building a French drain, will probably be where to run the water to. Once you have decided on this, it is a fairly straight forward project, though probably one for the weekend as digging the trench can be laborious and time consuming.

Building a simple French drain.

A simpler version can be built, without a liner or pipe. After following steps 1-3, fill 1/4 of the trench with small stones, 2/4 with coarse gravel and finish as in step 8.