Q: While raking up leaves and pulling weeds, I came across some long, concrete, half-pipe-shaped tubing that extends the length of one side of our house — the uphill side, about a foot from our retaining wall. The contractor who built our backyard shed said it looked like a French drain.
What is a French drain and what are the advantages of using one vs. a gutter?
A: The half pipe you uncovered is probably an attempt to keep water away from your foundation, but it’s not a French drain. A French drain is a system for eliminating excess water from the soil. The classic French drain is simple — just a trench filled with gravel, with sand on top. It allows excess water to percolate through the sand and gravel, eliminating the puddles.
The modern version of the French drain is really an underground drainage system that directs groundwater away from building foundations to a downhill discharge area, a sump or to the street.
Gutters, on the other hand, direct rainwater from the roof to downspouts that discharge water either on the ground away from the foundation or into a pipe system that discharges away from the house.
Gutter systems and French drains can be used separately or in combination, depending on the conditions. In climates where rain levels and soil conditions do not combine to saturate soil, a gutter system is all that is needed to direct roof water away from the foundation.
But, in some areas with heavy clay soil and sloping lots, gutters aren’t enough. In these conditions, when water is retained in the clay it can migrate into a crawlspace or seep through the foundation into a below-grade living area. That’s when a French drain becomes a good solution.
A French drain is a good do-it-yourself project that will require very little money but a fair amount of hard work. You’ll need plastic drainpipe, landscape filter cloth to keep dirt out of the pipe, gravel and sand. The hard part is digging the trench and moving the sand and gravel.
Dig the trench 2 to 4 feet away from the foundation/exterior wall. The idea is to channel ground water through the gravel, into the drainpipe and into a sump or discharge area away from the house. Closer than 2 feet risks getting too close to the foundation and could allow water to wick from the gravel to the dirt adjacent to the wall. The wider the eaves, the farther away you can locate the trench.
Dig the trench about 8 inches wide and about 18 inches deep for a house with a standard 3-foot crawl space. Once the trench is dug, cover the bottom 2 inches with a 1/2-inch layer of gravel.
Grading is critical. Remember that water runs downhill and the purpose of the drainpipe is to move water downgrade to a discharge area away from the house. The slope should be about 1/4-inch fall to a foot of run. Check the trench with a level to ensure a proper slope.
When the slope is right, drop in the filter fabric. Fabric should cover the bottom of the trench with about 8 inches to spare. Next, drop in the pipe and wrap the fabric over the pipe.
Drainpipe has holes on one side that allow water to enter the pipe. Although it seems counterintuitive, place the pipe so the holes are down.
With fittings, build a riser at the uphill end of the pipe as a cleanout in case the drain gets plugged. A sweep 90-degree fitting, or two 45-degree fittings joined with a riser pipe topped by a screw-on cap, will do the trick.
If discharging roof water from gutters and downspouts into the drain, install a riser using a tee fitting and a riser pipe at each downspout. Connect each downspout to the riser.
Add gravel to the trench to within a few inches of the surface. Top off the gravel with 3 to 4 inches of coarse sand. This provides a medium in which turf can grow so that the trench will not be visible. If in a planting bed, cover the sand with mulch or rocks, but not dirt.
At the low point of the system connect a solid (no holes) 4-inch pipe and extend it to a point where it can discharge onto the surface. It may be necessary to discharge the water into a dry sump, a hole 6 inches or so deeper than the bottom of the discharge pipe and filled with gravel to allow the water to percolate into the ground.
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