French Drain Guide: Gravel, Pipe, Fabric & Slope

💧 How a French Drain Works

A French drain is deceptively simple: a trench, filled with clean stone, wrapped in filter fabric, with a perforated pipe along the bottom. Water takes the path of least resistance — through the open voids between the stones — collects in the pipe, and flows downhill to an outlet. There are no moving parts; the whole system runs on gravity.

Four things are happening at once:

• Gravity carries water down a sloped pipe to a discharge point lower than the line.
• The perforated pipe both collects water (through its holes) and conveys it (down the bore).
• Filter fabric keeps soil fines out of the stone and pipe so the voids stay open.
• The drain intercepts the water table, relieving the hydrostatic pressure that pushes water against a foundation or up through a soggy lawn.

The technique is named for Henry Flagg French, a Massachusetts lawyer who popularized it in his 1859 book Farm Drainage — though the Romans were building gravel-filled drains long before him. The same design protects foundations, dries out yards, relieves pressure behind retaining walls, and carries downspout water away from the house.

🏡 When You Actually Need One (and When You Don't)

A French drain manages subsurface and intercepted water. It shines when:

• A chronically soggy spot in the yard won't dry out between rains.
• Water collects against the foundation or seeps into a basement.
• A hillside sends groundwater toward the house (a curtain drain intercepts it).
• Hydrostatic pressure is building behind a retaining wall and needs relief.

But a lot of “wet yard” problems are really surface-grading problems, and trenching a French drain where a simple fix would do is wasted effort. Before you dig, check whether one of these solves it:

• Regrade the soil to fall away from the house — re-sloping the first 10 ft (about a 6″ drop) fixes many “wet basement” complaints with no drain at all.
• Cut a vegetated swale — a shallow planted channel moves more surface sheet-flow than a buried drain and needs no gravel or fabric.
• Extend the downspouts on solid pipe to daylight — keep this clean roof water off the perforated collection run.

🛠️ Choosing the Pipe: Size & Material

4-inch perforated pipe is the residential standard. Use 3″ only for minor, short runs and 6″ for high flow, long runs, or commercial drainage areas. Capacity climbs steeply with diameter — a 6″ pipe carries far more than a 4″ at the same slope — so when in doubt, size up rather than chase a tight grade.

Pipe material is a genuine trade-off, not a single right answer:

Corrugated HDPE is cheap, flexible, and shrugs off freeze-thaw movement — the common pick up North. PVC SDR-35 is rigid with a smooth bore that holds a precise slope over distance and cleans out with a snake. Schedule 40 PVC has the thickest wall for traffic loads under a driveway. Use perforated pipe on the collection run and solid pipe for the non-perforated outfall.

🪨 Choosing the Gravel: Why #57 Stone

The right aggregate is ¾″ clean washed #57 stone — open-graded, angular crushed stone with high void space and no fines, classified under ASTM C33 / AASHTO M43. The open voids are what carry water to the pipe, and the angular shape interlocks without compacting. #67 stone (¼–¾″) is an acceptable finer alternative.

For ordering, #57 stone runs about 105 lb per cubic foot, or roughly 1.4 tons per cubic yard. As a planning rule, a 12″-wide × 18″-deep trench needs about 1.4 cubic feet of stone per linear foot before you subtract the pipe. Quarries sell stone by the ton, so the calculator converts your cubic-yard volume to tons using the correct density, then adds a waste factor (10% is typical) before you order.

🌯 Choosing the Filter Fabric & the Burrito Wrap

Filter fabric is what keeps soil fines from migrating into the stone and clogging it — and on a foundation drain, IRC R405.1 requires a filter membrane. For residential work, use a non-woven, needle-punched geotextile at 4–6 oz/yd² (heavier, 8–12 oz, for clay-heavy or high-sediment soils), selected per AASHTO M288. Non-woven is preferred for filtration and flow; woven fabric offers higher strength but lower flow, so use it only where strength governs.

To estimate fabric for a burrito wrap, the wrapped cross-section is the trench bottom plus both side walls: (2 × depth + width) × length, plus about 20% for side laps and the top fold. A 100-ft trench at 12″ × 18″ works out to roughly 480 ft² — which the calculator converts into linear feet of a chosen roll width.

📐 Slope: The Number That Makes or Breaks It

A French drain works only by gravity, so slope is non-negotiable. The minimum is 1% — which is ⅛ inch of drop per foot, or about 1 foot of fall per 100 feet. The practical target is 1–2%; foundation outfalls are often run at ¼″ per foot (about 2%).

• Too flat (under 1%): water stagnates in the pipe — the single most common design failure.
• Too steep (over ~5%): fast flow scours the gravel and strands sediment; add erosion protection at the outlet or ease the grade.

Check your fall before you dig with a string line and a line level, or a laser. Set a stake at each end, run a level string between them, and measure down to the trench bottom at the outlet end — the difference is your fall. The calculator gives you a pass/fail against the 1% minimum and the exact total drop in inches.

🔧 Installation, Step by Step

1. Call 811. Utility locates are free and required — never dig a drain without them.
2. Mark the route and verify slope with a string line or laser before you break ground. Plan the outlet first: it has to be lower than the line.
3. Excavate the trench to width and depth, holding a steady fall of at least 1% toward the outlet. Keep spoil and equipment back from the edge.
4. Lay the fabric into the trench for a burrito wrap, with enough overhang on each side to fold over the top later.
5. Bed 2″ of #57 stone in the bottom, then set the perforated pipe on it with the holes facing down.
6. Envelope the pipe in stone to within 6″ of the surface (IRC R405.1: at least 6″ of cover on a foundation drain).
7. Fold the fabric over the top of the stone, overlapping the seam by 6–18″.
8. Backfill the remaining depth, and add a cleanout riser at the ends and at any turns so the line can be flushed later.

💸 What Drives the Cost (No Price Quotes Here)

We don't publish dollar figures — prices vary too much by region, soil, and access to be honest. But it helps to know what moves the number, because on a French drain the materials are usually the smaller share and labor plus excavation dominate. The main cost drivers:

• Length, depth, and width — every extra foot is stone, pipe, fabric, and digging.
• Soil type — clay and rock are far harder (and slower) to excavate than loam.
• Accessibility — can a machine reach the trench, or is it all hand-dug around landscaping?
• Interior vs. exterior — an interior basement drain means breaking and repouring slab.
• Discharge method — a simple daylight outlet is cheap; a dry well or a sump-pump lift is not.
• Permits and inspections — often required for a storm-sewer connection.

Use the calculator's material quantities to get accurate supplier quotes for the stone, pipe, and fabric, then get labor bids locally. Pair it with the gravel calculator if you're also stoning a wider area.

⚠️ The 8 Most Common French Drain Mistakes

🧰 Maintenance & Service Life

A properly built drain — clean #57 stone, a full fabric wrap, holes-down pipe, and real slope — can last 30 to 50 years. A poorly built one (pea gravel, no fabric, flat grade) can clog in 5 to 10. The difference is almost entirely in the build, not the maintenance, but a little upkeep keeps it honest:

• Flush the line through the cleanouts every few years, or any time drainage slows.
• Inspect the outlet — keep the daylight emitter or dry well clear of leaves, silt, and rodent nests, and make sure the rodent screen is intact.
• Watch for warning signs — water resurfacing above the trench, a soggy line that won't drain, or backflow at the outlet all point to a clog.