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Calculate gravel, perforated pipe, and fabric for French drains
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A French drain is a gravel-filled trench containing a perforated pipe that redirects surface water and groundwater away from an area. Named after Henry French, who popularized the concept in 1859, French drains are used to prevent water damage to foundations, eliminate soggy yards, and reduce erosion.
Water seeps into the gravel trench and flows down through the stones into the perforated pipe. The pipe then carries water to a discharge point (daylight outlet, dry well, or storm drain). Landscape fabric prevents soil from clogging the gravel.
Common applications include:
Example: 50' × 1' × 1.5' = 75 cu ft ÷ 27 = 2.8 cu yd = 3.9 tons
Extra width covers trench sides and provides overlap at top
Use clean, washed crushed stone for best drainage. Avoid limestone which can cement together. Gravel should be angular (not rounded river rock) for stability.
Corrugated: Flexible, easy to install, most common for DIY. Use 4" diameter for most applications.
Rigid PVC: Smoother interior, better flow, lasts longer. Requires fittings for direction changes.
Sock-Wrapped: Pre-wrapped in filter fabric, prevents clogging, costs more.
Use non-woven geotextile fabric (not woven weed barrier). It allows water to pass while filtering sediment. Wrap the entire trench, overlapping fabric at top.
Spring-loaded cap opens under water pressure to discharge. Closes when not flowing to prevent debris and animals from entering. Install at daylight outlet location.
Mark the trench location from problem area to outlet. Maintain minimum 1% slope (1 inch per 8 feet). Call 811 before digging!
12" wide × 18" deep minimum. Keep sides vertical. Smooth the bottom for consistent slope.
Drape landscape fabric into trench, leaving excess on both sides to wrap over gravel later.
Place 2-3 inches of gravel at bottom. This creates drainage space under the pipe.
Place perforated pipe with holes facing DOWN. Connect sections. Use solid pipe at outlet.
Cover pipe with gravel to within 4-6 inches of surface. Fold fabric over top.
Top with soil and sod, more gravel, or decorative stone. Install pop-up emitter at outlet.
Pipe holes must face down. Water rises from below; holes-up causes pipe to fill with sediment.
Without fabric, soil migrates into gravel and clogs the system within 5-8 years.
Using pea gravel or round stone doesn't drain as well. Crusite or fines clog quickly.
Flat or reverse slope areas create standing water in pipe. Minimum 1% grade required.
Water must go somewhere! Plan daylight outlet, dry well, or storm drain connection first.
For foundation drains, bottom of trench should be at or below footing level.
For a typical 50-foot French drain (12" wide × 18" deep), you'll need approximately 3-4 tons (2.5-3 cubic yards) of 3/4" washed gravel.
With proper installation including landscape fabric, French drains last 15-20+ years. Without fabric, expect 5-8 years before sediment clogs the system.
Yes, French drains are a common DIY project. Main challenges are digging (consider renting a trencher) and maintaining proper slope. Call 811 before digging.
For foundation drainage, dig to the level of the footing or slightly below (typically 24-36 inches). The drain should intercept water before it reaches the foundation.
4" perforated corrugated pipe works for most residential applications. For longer runs or higher flow, use 6" pipe. Rigid PVC lasts longer but costs more.
Ideally to "daylight" at a low point on your property with a pop-up emitter. Alternatives include dry wells, storm drains (if permitted), or rain gardens.