How To: Install a French drain
The French drain is a time-honored system for eliminating excess water from low points and other areas prone to saturated soil. French drains are quite simple: in essence they are just trenches filled with gravel, with sand on top of that. Often, you'll see French drains defined to include a drain pipe as well, though the traditional design is simply the gravel-filled trench. The advantages of French drains are low cost and easy installation. In addition, they can be covered over with turf after installation, making them less conspicuous.
- Dig the trench
A French drain starts with digging a trench. The depth and width of the trench can vary, but 5 to 6 inches wide and 8 to 12 inches deep are common sizes and usually satisfy most needs.
- Ensure proper slope
Grading is a critical consideration — you must ensure that enough slope exists for the water to actually flow, and flow in the right direction. It might be adequate to check very short stretches of drain with a level to ensure that a slope exists to carry water in the desired direction. However, you should take whatever measures are necessary, including a survey and grading, if needed, to ensure that you have at least a 0.5 percent slope. A 1 or 2 percent grade is better.
- Fill with gravel
Add gravel to the trench to within a few inches of the surface. Gravel for this use is typically 0.5 to 1 inch in size.
- Top off with sand
On top of the gravel, lay at least 3 or 4 inches of coarse sand. This provides a medium in which turf can grow so that the trench will not be visible. But remember that the sand must be coarse or it won't allow water to properly drain through.
- Turf the trench
Turf may be seeded into the sand or simply allowed to grow in from the adjacent stand, if the turf is a spreading type. Or, you can lay sod over the sand. However, if you do this, be sure to wash the soil from the sod roots before laying it so that you don't contaminate the sand with finer soil.
Adding a drain pipe
If you wish to use drain pipe, the process is similar to that just described, except you will first lay an inch or two of gravel in the bottom of the trench, on which a perforated drain pipe will rest. The drain pipe should be wrapped in a filter fabric. Then, finish filling in with gravel around the pipe, ensuring at least 1 inch of gravel surrounds the pipe on all sides. Four- or 6-inch drain pipes are commonly used, and are easily accommodated by 6- to 8-inch trenches.
Be sure to run the upper end of the drain pipe to the surface and cap it so that you can, if necessary, access the pipe to unclog it.
Don't get in over your head
Consider hiring a site engineer or similar professional for complex systems that cover large areas and require multiple lines. Proper grading, pipe-sizing and other design factors can make or break a complex drain system.
A 1-percent slope equals a drop of 1 foot over a distance of 100 feet. For shorter distances, the drop can be quite small. For example, a 20-foot run needs a drop of just 0.2 foot (a little more than 2 inches) for a 1-percent grade. Shorter drains may be easy to grade with the help of a good level. However, when in doubt, hire a surveyor.
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