Installing an exterior french drain by David Beaulieu

What are French drains? When are they useful?

If your neighbor’s land stands at a higher elevation than yours, you may be experiencing problems with excessive moisture on your property. Water from your neighbor’s property may be running down the slope (just one of the challenges of landscaping on a hill) and spilling onto your property. You need better yard drainage. One option in such cases is installing French drains.

When some people speak of a “French drain,” they refer to a trench in which a drain pipe is laid, but the traditional French drain is basically a trench filled with gravel.

Difficulty: Average

Time Required: Depends on extent of water flow and ground to traverse.

Here's How:

1. Determine a spot on your property where the excess water coming off the slope could be re-routed. Determining such a location may end up being a matter of choosing “the lesser of two evils.” If water is currently spilling out at your house foundation and excessive moisture threatens to damage it, obviously almost any other spot would be preferable. The ideal French drain leach field would be an out-of-the-way area with sandy soil, through which the water could percolate harmlessly....
2. But be sure your attempt at yard drainage will not adversely impact anyone else’s land. Otherwise, installing a French drain could land you a lawsuit! Check your city codes before digging. Another preliminary step that could save you headaches later is checking with your local utilities concerning the whereabouts of underground cables and the like, so that you'll know where not to excavate for a French drain. There's a quick way to check: just dial the Dig Rite phone number. 
3. Locate the best area for a French drain. Find an area along the slope on your side of the boundary where excavation would be easiest for your French drain (i.e., free of obstructions). Trench lines should be plotted out before you begin digging French drains. You need to create your own mini-slope to carry the water down to its destination. A grade of 1% (i.e., a drop of 1 foot for every 100 feet in length) is often recommended for French drains; others advise a drop of 6" for every 100'. Getting the water to go where you wish is essential for improving yard drainage; the grade will facilitate your efforts. 
4. So how do you measure the grading for a French drain? Pound 2 stakes into the ground to mark the beginning and end of the trench. Tie a string tightly to one of the stakes, then run it over to the other stake and tie it off there, too, but loosely (for now). Attach a string level to the string, adjusting the string to get it level. Once it’s level, tighten the string at the second stake. Make sure the string is taut. Now begin digging the trench. As you dig, you’ll be able to measure down from the string to make sure you are achieving the desired grade for your French drain. 5.
5. Check yourself as you go. For instance, if the trench for the French drain is to be 100’ long and the grade 1%, then by the time your trench is 50’ long, it should be 6" deeper than where you began excavating. 6.
6. You'll be digging a horizontal trench across the length of the slope. The digging is the most labor-intensive part of installing French drains. The trench will slope down toward the area where you've determined the water will be re-routed (if it doesn’t quite reach that spot, you’ll have to dig a connecting ditch down to it). Trench width will depend on the magnitude of your moisture problem. Bigger moisture problems call for wider trenches. Small trenches are often dug to a width of 5”-6”. 7.
7. Before applying gravel, line the trench with landscape fabric. The landscape fabric will keep dirt out of the gravel. You want to preserve the porosity of the gravel, which promotes percolation of water through it -- one of the underlying principles that make covered French drains work. Shovel a coarse gravel onto the landscape fabric. Wrap the ends of the landscape fabric over the top of the gravel layer. 
8. You now essentially have a tube of landscape fabric filled with gravel. To fill in the rest of the trench, shovel in a layer of coarse sand, cover it with more landscape fabric, add 4" of topsoil and lay sod on top. Your French drain is complete!

Tips:

1. Hire a surveyor. If you don't think you can get the grading right for a French drain on your own (Step 3), hire a surveyor. Or simply hire a pro to do the whole job.
2. Hire a backhoe operator. If you're not inclined to dig a French drain trench by hand, you could hire a backhoe operator. But that will jack up the cost for the French drain -- not only for the digging, but for the extra gravel you'll need (since a backhoe can't dig as small a trench as can a person wielding a spade). Another alternative is suggested by reader, Matt Fisher, noting that "many rental shops rent trenchers now. These machines cut very thin trenches...."
3. Bigger not necessarily better. If you can get away with a small trench (Step 6) for your French drain, you'll save money, as there'll be less gravel needed to fill the trench.
4. Terminology. "Gravel" is a term that can be used differently in different regions. Here, "gravel" refers to small pieces of rock.
5. Where to channel the runoff. The most difficult problem in installing French drains can be figuring out where to channel the water. For possible solutions, consult this article on what to do with runoff.