How to dig and install a French Drain
To collect and move rain water away from your home.
Please first read Steps 1 through 4 of our Drainage Guide
This page provides information on:
- How to install a French drain
- Tips for a long-lasting system
- Drain pipe or tile advice
On this page we will dig into how to install a French drain system to reduce drainage problems in your lawn, yard or garden. Officially a French drain is a gravel drain with no pipe. The water just collects in and travels through in a gravel or stone filled channel that starts from the surface or just below it. But in modern times, and for practical purposes, we will consider a French drain to include a drainage pipe in the gravel.
Another difference you will notice here is that while other drainage guides recommend fairly narrow trenches for their French Drains - often only 6 inches wide - my recommendation is that your trenches be between 8 and 12 inches wide for the following reasons:
- A wider French drain will last longer
- The capacity to collect and disperse water is better when wider
- It is easier to dig and grade a wide French drain, especially when it gets deep
We will look at each of these points in detail...
A wider French drain will last longer because the primary cause of failure in a French drain is silting up of the spaces between the gravel by clay and soil - and a wider system simply has more gravel in it and takes longer to clog up. It is also best if the French drain trench is wrapped in permeable landscape cloth (the type water flows through) and the pipes are covered in drain sleeve fabric to further slow down any clogging by roots or soil particles. Digging a drainage system is a big job - be sure to do all you can to make your hard work last as long as possible.
The capacity to collect and disperse water is better in a wide French drain. This is a common sense type thing. Think about how much more water can flow through a 10 inch channel compared to a 6 inch channel. Also remember that we want to allow as much of the collected water as possible to soak into the subsoil in dryer parts of the yard, and a French drain that is 12 inches wide has twice the drainage area as a 6 inch wide trench. There is also the potential for small drywells that you can add to the bottom of the French drain trench - it is much easier to dig these in a wider trench.
It is easier to dig and grade a wide French drain, especially when deep, because you can get your digging tools, hands, and even feet into a deep 12 inch trench - but not a 6 inch one. Grading the bottom of trench to obtain the proper slope is also much easier in a wider French drain, especially when digging by hand. There is also the issue of using a posthole digger in the bottom of the French drain trench to add drywells, as mentioned in the paragraph above. For those using a trencher, many of skidsteer type trencher attachments, and a few walk-behind trenchers, will handle a 10" or 12" wide trenching chain - ask at the rental yard.
So plan on doing the job well, and therefore only doing it once in your lifetime. Take a little extra time to dig the wider width and spend a little extra for the additional gravel that will be needed. It will be well worth it!
French Drain and Drain Tile Installation
A few days before you begin to dig contact your local utility company to have all the underground utility (gas, water, sewage, electric, phone, etc) lines on your property located and marked.
In your yard, stake out the drainage system you designed in Step 3. Read the instructions on our Determining Drainage System Slope page to figure out how deep to dig and what slope the different parts of your system should have.
Before you begin to dig think about how you will handle all the leftover dirt. Installing a French drain is much different than normal trenching in that much of the dirt will not go back into the trench (because of the gravel). Most often people load the extra subsoil into a wheelbarrow and move it to the road for pickup and removal, or move it to another part of their property for fill. Be sure to plan to pile the subsoil so that you do not have to cross the open trench with a wheelbarrow. The topsoil is usually piled on the side opposite the subsoil since it will be spread back over the top of the trench.
Often it is best to start digging at the farthest end of the drainage system, usually a ditch, the street, a drywell, or other low or dry area you are draining to. This way you lock in the fixed end and if troubles occur further upstream in the system (like a boulder or major roots) you can sometimes just increase the slope of the French drain piping to go over the top of the obstruction.
So gather your friends and some good trench digging tools, and get to it.
Installing the Drainage Pipe and Gravel
Congratulations, you have finished the hard part (designing and digging). It was a lot of work and you do not want to have to do it again, right? To make your French drain last a long, long time you need to use these following items:
- Clean or washed gravel (not crushed limestone - it becomes cement like)
- Permeable landscape fabric (they kind water passes through)
- Rigid plastic perforated drain pipe (not flexible corrugated pipe - read why not)
- Correct drain pipe fittings (to allow Roto-Rooter cleaning - read how to choose)
- Fabric drain pipe sleeve or sock - (keeps roots and soil out of the pipes)
- Downspout leaf seperators: either plain or fancy (to keep leaves out of the pipes)
French drains only fail when the gravel becomes full of clay or soil particles or when the drainage pipe becomes full of soil or roots. By using the above items you will maximize the life of your French drain system and make it possible to easily clean it out with a Roto-Rooter or sewer snake if it does become plugged.
Here is what your French drain trenches should look like...
Note that there is always at least 2" of gravel all around the 4" drain pipe. Also note the fabric drain sleeve and that the landscape fabric is overlapped and completely surrounds the gravel. Remember that the rigid drain pipe is always installed with the holes on the bottom.
For information on how to select and install the right pipe and fittings for a French drain system that will work well and be
easy to maintain, go to the next page:
How to install French drain pipe and fittings for easy Roto-Rooter cleaning in the future