Q: A well-respected and competent contractor who has done excellent work for us before is giving us a bid to install a French drain and surface drain in our sloped backyard.
For the drain that will run along the entire back wall of our house, he proposed a 4-foot-deep-by-3-foot-wide French drain filled with rocks — and that’s it. His bid does not call for perforated pipe or fabric. He said the water just needs to percolate into the soil, which will be well beneath the foundation. Do you agree with this assessment?
In addition, because we are on a slope, the foundation at the front of the house is lower than the back of the house. I have read that a French drain needs to be deep enough to protect the lowest part of the house, so would a 4-feet-deep French drain in the backyard be sufficient to keep the entire foundation dry?
A: With all due respect, we disagree with your contractor. A 4-foot-deep-by-3-foot-wide gravel-filled trench will only create an underground lake.
If your soil is sufficiently porous for water to percolate into the soil at the bottom of the trench, it would also hold true that the topsoil is porous enough to "perc." If this is the case, you shouldn’t be considering a French drain at all.
The purpose of a French drain should be to divert water away from the foundation, down the hill to either a dry sump, a decorative pond or, if code allows, to the street.
The basic concept behind a French drain is to construct a slightly sloped trench filled with round stone and perforated pipe encased in a filter cloth to divert surface and subsurface water away from your house.
Interestingly, the French drain is not named for the country in Europe. Henry Flagg French, a judge and farmer in Concord, Mass., conceived and promoted the idea of redirecting water via underground piping in an 1859 book about farm drainage. French made his drains with clay tiles, the same type of piping used in old sewer lines. Modern residential French drains are usually constructed with 4-inch-diameter perforated plastic pipes
How a French drain works
A French drain consists of buried perforated pipes slightly sloped to easily channel water flow, in this case, away from the house foundation.
Start by digging a 2-foot-deep-by-18-inch-wide trench 2 feet or so away from the foundation at the highest point where the rear yard meets the foundation. Continue the trench downhill around the house until it extends beyond the foundation at the front of the house and discharges away from the foundation where excess water will not cause a problem for yourself or a neighbor.
The bottom of the trench should be sloped about 1 inch for every 8 feet in the direction you want water to flow.
Fill the bottom of trench with about 2 inches of 1-inch washed rounded stones. Washed stone is necessary to prevent small pieces of stone called "fines" from plugging the holes in the drainpipes.
Lay landscape filter cloth on the layer of rocks at the bottom of the trench. Lay the perforated plastic pipe with the holes face down over the cloth, then wrap the sides of the cloth over the pipe. The cloth prevents any soil particles moving down through the rocks above from plugging the holes in the pipe. Fill the trench with stone.
If you want to plant grass or shrubs over the stone, fill the trench to about 6 to 8 inches from the top with stone and top it off with topsoil. Then indulge your green thumb.
When complete, surface and subsurface water moving downhill hits the stone and percolates through the spaces between the round gravel and into the perforated pipe at the bottom of the trench. Water travels freely through the pipe, which empties a safe distance from the house. The water can be diverted to a low-lying area of the property, a drainage ditch, a dry well, or the street.
Your contractor’s solution will create more problems, unless your soil passes the "perc" test, in which case you probably don’t need a French drain at all.
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