Drainage
What Is a French Drain and How Does It Work?
French drains explained: how gravity and gravel move groundwater, the anatomy of a working drain, where they help (and don't), costs, and design basics.
Despite the name (from Henry French, a 19th-century Massachusetts farmer who popularized them, not from France), the French drain is the workhorse of residential drainage - and also its most misunderstood tool. Understanding how it works tells you immediately where it will help and where it's the wrong fix.
The physics in one paragraph
Water in soil moves toward lower pressure and lower elevation, but slowly - through clay, sometimes inches per day. Open-graded gravel has huge connected voids, so water entering a gravel trench meets almost no resistance. The saturated soil drains into the trench, the water table around it drops in a curve (engineers call it drawdown), and the perforated pipe at the trench bottom - laid on a consistent downhill slope - carries the collected water to daylight, a storm connection, or a dry well. No pump, no power: just permeability contrast plus gravity.
Anatomy of a drain that works
Every component exists to defeat a specific failure mode:
| Component | Spec | Failure it prevents |
|---|---|---|
| Trench | 12 in wide, 18–24 in deep (typical yard drain) | Too shallow = misses the water |
| Non-woven fabric lining the trench | 4–6 oz geotextile, 6–12 in top overlap | Soil migration clogging the stone |
| Washed angular stone | 3/4–1.5 in, no fines | Fines cementing the trench solid |
| Perforated pipe, holes down | 4 in PVC preferred | Undersized/crushed corrugated pipe |
| Consistent slope | ≥1% (1/8 in per ft) | Flat spots collecting sediment |
| Protected outlet | Daylight, storm drain, or dry well | Water with nowhere to go |
| Cleanout access | Riser every 50–100 ft | No way to flush the pipe |
Full sizing math - stone tons, pipe sections, fabric area, elevation drop - is automated in the French drain calculator, and the cross-section diagram on that page shows how the layers assemble.
Where French drains help - and where they don't
Good applications: chronically soggy lawn areas fed by subsurface flow; intercepting hillside seepage above a house (curtain drain); relieving water pressure behind retaining walls; perimeter drains at basement footings; soggy ground below downspout discharge that can't be regraded.
Wrong tool for: roof water (pipe downspouts directly in solid pipe - don't dump roof volume into a French drain); surface runoff crossing a yard (a swale or channel drain intercepts it better and cheaper - see French Drain vs. Swale); negative grading against the foundation (fix the grade first - see Fix Standing Water in a Yard); and water rising from below (springs, high water table under a slab) which needs engineered solutions.
The design sequence (do this before buying anything)
- Find the outlet first. Locate a discharge point lower than the problem area - this determines whether a gravity drain is even possible. No outlet, no French drain (a dry well is a partial substitute in permeable soils).
- Trace the water. Where is it coming from - roof, slope, neighbor, water table? Interception works best across the flow path, uphill of the damage.
- Set the slope. Confirm you can maintain ≥1% from inlet to outlet over the full run: length × 1% = required drop. Check with the slope calculator and a string level.
- Size the materials. Trench dimensions → stone, pipe, fabric via the calculator.
- Call 811, then dig, line, bed, pipe, fill, cap.
Cost reality
Materials for a typical 50-ft yard drain run $500–1,200 (about 3.3 tons of stone, six pipe sections, 275 ft² of fabric, fittings). Professional installation adds excavation, disposal of spoil (~2.8 loose yards - see excavation volume), and warranty, landing at $1,250–3,500 for the same run. Interior basement systems are a different product entirely at $3,000–12,000.
When to call a professional
Water in a basement, foundation movement, slopes showing slumping or cracking, or any drain that must protect a structure justifies professional design - sometimes a geotechnical engineer rather than a landscaper, because chronic water against foundations changes soil behavior. Yard comfort drains are honest DIY; structural water management is not.
Frequently Asked Questions
How does a French drain work?
Water follows the path of least resistance. A gravel-filled trench is far more permeable than soil, so subsurface water flows into it, drops into a perforated pipe at the bottom, and gravity carries it along a 1%+ slope to a discharge point.
What's the difference between a French drain and a surface drain?
French drains collect subsurface water (soggy ground, seeping foundations). Surface drains - channels, catch basins, downspout pipes - collect water before it soaks in. Many wet-yard problems need surface fixes first, which are cheaper.
How much does a French drain cost?
DIY materials run $10-25 per linear foot (stone, pipe, fabric). Professionally installed exterior drains typically cost $25-70 per foot; interior basement perimeter systems $50-100+ per foot.
How long does a French drain last?
A properly built drain - washed stone, fabric-lined trench, real slope, maintained outlet - lasts 20-40 years. Skipping fabric or using stone with fines can clog one in 2-5 years.
Do French drains need maintenance?
Modest but real: keep the outlet clear, flush the pipe every few years via a cleanout, and watch for surface settling along the run. A drain with a buried, forgotten outlet fails silently.