drainage - Knowledgebase Question

irving, Te
Question by josblackcats
September 10, 2009
What is the best way to install a french drain along the front and side of our home to prevent future flooding?


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Answer from NGA
September 10, 2009

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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. 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. 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. Add gravel to the bottom of the trench, set your perforated pipe if you are using one and then add more 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. 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 may be seeded into the sand or simply allowed to grow in from the adjacent stand, if your turf is a spreading type. Or, you can lay sod over the sand. Best wishes with your project.

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