Download French Drains Cost Background. An underground downspout drainage system costs $150 to $350 per downspout. The average cost to install a 100 foot, 2 feet deep exterior french drain in your yard is about $1,200, or $12 per foot for diy, with no trencher rental.
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Interior french drains are often installed as a last resort and are significantly more expensive than their exterior counterparts. A french drain or weeping tile is a trench filled with gravel or rock or containing a perforated pipe that redirects surface water and groundwater away from an area. How to install a french drain, everything from buying materials, costs, trenching, cutting walks, gravel, perforated pipe, connecting your pipe. During rains, water collects in my backyard and doesn't drain properly.