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 and travels 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 drainage pipe.
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 deep
Let’s look at each of these points individually…
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 is covered in drain sleeve fabric to further slow down any clogging by clay and 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” 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 length of French drain that is 12 inches wide has twice the drainage area as a 6 inch wide trench. There are also the 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” 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 12” 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 at 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 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 ride over the top of the obstruction.
Congratulations, you have finished the hard part (designing and digging). It was a lot of work and you don’t want to have to re-dig again, right? To make your French drain last a long, long time you need to use these following items:
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.