French drains are a time tested approach to eliminating water. The
method is actually quite straight forward. In its simplest form, it involves
digging a trench and filling it with gravel and sand. More advanced designs
include burying a drain pipe at the bottom of the trench and covering it
with gravel and sand. Once in place, the gravel covered French drain
can be covered by turf. When complete, one hardly knows the drain exists at
all. So, here's a basic approach:
1. Dig the trench for the French drain. Although there's no
prescribed size for French drains, a 6 inch wide by 12 inch deep drain is a
common size. Properly sloping the French drain is very important. The
slope allows the water to flow in the correct direction. Try to achieve a 2
percent grade if possible. A .5 percent grade is the minimum grade one
should use for a French drain.
Digging and sloping the trench is not an easy task, especially when running
it over long distances. For help with the math, a 1 percent slope equates to
a one foot drop over a distance of 100 feet.
2. Add the gravel and drain pipe for the French drain. Use .5 to
1 inch gravel. Lay a couple of inches of gravel and then lay a
perforated drain pipe (4 to 6 inches) on top of the gravel. The drain
pipe should be wrapped with a filter fabric. Make certain when filling
the balance of the trench with gravel that at least an inch of gravel
surrounds the drain pipe. So you can clean the pipe later, make
certain that you run the top end of the pipe to the surface. This will allow
you to run a snake down the pipe.
3. Cover the gravel with a coarse sand (so the water will drain
through it). The sand will help your grass grow. Three to four
inches of sand should do.
4. Seed or sod the French drain. You can plant seed, allow the
grass from either side of the French drain to grow in, or plant sod. If you
sod the drain, wash all the dirt off of the roots of the grass. You
don't want soil infiltrating your drain.
These projects can get complicated, particularly when you are running long
French drains. There are engineers and contractors that build these drains
every day. Proper grading is critical and if you don't get the grading
right, the system will not work properly.
If you are not a professional, we suggest that you find a licensed
contractor to help you with your project.
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