A French drain is not one job. It is three — and the wrong scope is how this fix turns into the same wet basement two or three rainy seasons later. The rest of this guide is how to tell which one you are actually building before you order stone.
First: what problem are you solving?
Soggy yard, downspout dumping at the foundation, surface puddles after rain
Shallow yard / surface French drain — this guide applies. Stone, fabric, pipe, outlet, and the common 8-12 in trench math below.
Basement water, wall pressure, water at the slab or footing
Footing-depth drain with waterproofing membrane and sump discharge — specialty waterproofing scope. This guide's 12-inch trench math does not apply.
No daylight outlet — flat lot, water would drain to a neighbor or back into a window well
Solve discharge first. Sump pit + pump, a dry well sized to the catchment, or a permitted discharge to curb. A French drain with nowhere to go is just a stone-filled trench.
Unknown source after a heavy rain
Observe the water path before digging — walk the lot during the next downpour. Most “mystery” leaks turn out to be one of the three above.
The shallow yard drain this guide covers
This guide is for the surface / yard case. You are catching soggy topsoil, surface runoff, or downspout discharge that is loading the foundation perimeter or pooling in a low spot in the yard. Typical trench is 8-12 inches deep × ~12 inches wide for a longer run with a buried pipe; shallower (8 in × 6-8 in) where the flow is small and the trench is short. The drain needs an outlet — daylighting to grade on the downhill side, or piped to a sump or dry well.
A surface drain is not a basement waterproofing fix. It moves water that is sitting on or near the surface before that water can saturate the soil against the foundation. If the water is already getting in at the slab or footing, the shallow trench is the wrong tool for the job.
Foundation water is a different job
A true exterior foundation perimeter drain has to run at or near the footing — typically 3-8 feet below grade depending on basement depth — to intercept hydrostatic pressure and groundwater before they reach the wall. That scope needs excavation shoring, waterproofing membrane against the wall, drain tile at the footing, compacted backfill in lifts, and a discharge plan (often a sump). Residential planning bands for full exterior waterproofing typically land in the $8K-$25K range, with a lot of variance for foundation depth, access, and finishes that have to be removed and reinstalled.
If that is the job, the math in this guide is not what you need. Get two or three waterproofing bids, and budget the line inside a larger rehab if there is one — see the gut-rehab cost calculator for that scoping.
The French drain stack
What is actually in the trench, top to bottom, on a shallow yard drain:
Stone choice — why #57 clean
#57 is a No. 57 graded angular crushed stone — ¾-inch nominal with the gradation cleaned of fines. The angular shape locks loosely (enough to stay put in a vertical column) while leaving roughly 35-40% void space between pieces for water to move. That void space is what a drain is doing — fines fill it, angularity preserves it.
- #57 angular clean— the default. Specify “#57 clean, no fines” on the ticket.
- #4 angular clean (1½ in) or #67 angular clean (½-¾ in) — acceptable regional substitutes when #57 is not stocked. Same intent.
- Crusher run / base aggregate— graded to include fines specifically to compact. The opposite of what a drain needs. Don't let a supplier or contractor talk you into it.
- Pea gravel — rounded, low void space, shifts under load. Only acceptable on low-flow shallow yard cases. Not for foundation or high-flow drainage.
- Crushed limestone clean — equivalent to #57 in void space and angularity in some regions, just a different naming convention.
Fabric and pipe — the details that prevent clogging
Non-woven drainage fabric reduces migration of soil fines into the stone column and helps preserve void space over time. It doesn't make a drain permanent — but skipping it shortens the working life materially. Specification per NRCS Construction Spec 4951:
- Non-woven, drainage-rated.Minimum 4 oz per square yard. Don't substitute plastic sheet — plastic traps water against the stone instead of letting it pass.
- Wrap the sides and bottom first. Fabric lines the trench before any stone goes in. Overlap fabric joins by at least 12 inches.
- Fold the fabric over the top. After the stone column is in and the pipe is buried, fold the fabric over the top with a ~6-inch overlap before backfilling. The top wrap is what installers most often skip — and it is where fines come in from.
- 4-inch corrugated perforated pipe. Black coiled pipe from the big-box is the residential default; PVC perforated is stiffer but more expensive. Larger pipe (6 in) for high-flow cases needs a wider trench (16-24 in).
- Holes down. Common residential practice for standard corrugated perforated pipe — water enters from the bottom of the holes as the trench fills with collected groundwater. Check your specific pipe spec; a few products are designed differently.
- Pipe in the stone column, not on dirt. 2-3 inches of #57 stone bedding under the pipe before it goes in. A perforated pipe sitting on trench dirt collects mud inside the pipe within months.
- Outlet first. Plan and dig the daylight outlet (or the sump connection) before the trench. A drain without somewhere to discharge is not a drain.
How pros actually do this
A gut rehab in the Chicago suburbs in 2018 is the one that taught me the surface-vs-below-grade split. Single-story house, mid-century build. The previous owner had cut an interior perimeter drain into the slab in the 1990s, dumping into a sump pit — that was handling below-grade infiltration fine. What the house didn't have was any surface-water management at the perimeter: gutter discharge was dumping directly against the foundation, and the topsoil over the first 12-18 inches stayed saturated for a day after every storm. The basement was still taking water.
The fix wasn't a below-grade footing drain — that would have meant excavation to footing depth (4-6 ft on this house), waterproofing membrane against the wall, and waterproofing-contractor scope. It was a 90-foot shallow yard / surface French drain at the perimeter, intercepting gutter discharge and topsoil saturation before either could load the wall. 12 in × 12 in trench, #57 stone, fabric wrapping the column with a top overlap, 4-inch perforated pipe in the stone column, outlet daylighting 15 ft from the house on the downhill side. That was the right split for this house — the interior drain handling below-grade water, the new exterior surface drain handling surface saturation. The lesson isn't that this two-part setup solves basement water everywhere; it's that surface drainage and foundation drainage are different jobs, and conflating them is the most common French drain mistake.
Run the gravel math
The calculator below opens on the drainage preset and the worked example dimensions — #57 clean stone, 12-inch loose depth, 30 ft × 1 ft trench. Adjust length, width, and depth for your own trench; it returns cubic yards and tons.
This calculator sizes stone material only. It does not design slope, discharge, footing-depth drains, or waterproofing. Use it after the drainage scope above has been chosen.
Many supply yards have 3-5 cubic yard or ton-equivalent delivery minimums — call the dispatcher before assuming bulk delivery. Below the minimum, a small landscape yard or bagged from a big-box is often the practical move.
Where French drains fail
- Wrong problem.Using a shallow surface drain to try to fix basement / foundation water. The shallow trench is not footing-depth, the membrane isn't there, the slab keeps taking water. Diagnose first.
- No outlet. A French drain with no daylight outlet and no sump pump is a stone-filled trench. Water still collects; it just collects in your drain instead of in the soil. Plan the discharge before the inlet.
- Crusher run or fines in the column.Fines pack into the void space and the drain stops draining inside a couple of years. Some installers will fill the bottom with crusher run for “stability” — the fines migrate up and the column silts from below. Single type of clean angular stone, full column, every time.
- Fabric skipped — or top not wrapped. Bottom and sides without the top fold is the most common partial install. Backfill settles and fines come in from above. Wrap the top.
- Pipe sitting on trench dirt. Mud enters the pipe from the bottom within months. 2-3 inches of stone bedding under the pipe, every time.
- Trench too shallow for the water source.6 inches of trench against a saturated topsoil layer 12-18 inches deep doesn't intercept enough water to matter. Match the trench to the water you are seeing.
- Dumping water somewhere worse.Outlet that daylights at a neighbor's lot line, in a window well, or against another foundation wall isn't a fix — it's a relocation. Solve discharge to grade on the downhill side, to a sump, or to a permitted curb discharge.
Frequently asked
What size gravel is best for a French drain?
#57 angular clean stone — ¾-inch crushed rock with no fines. The angular shape holds void space for water to move through; the clean grading keeps that space open over time. #4 or #67 angular clean can substitute regionally. Avoid pea gravel on anything but the lowest-flow shallow yard run, and never use crusher run or base aggregate — the fines pack into the flow path and the drain stops draining.
How deep should a French drain be?
Depends on what water you are moving. A shallow yard / surface drain catching downspout discharge or surface runoff is commonly 8-12 inches deep × 12 inches wide. A footing-depth foundation drain runs at or near the footing — often 3-8 feet below grade on a typical basement — to intercept water before it reaches the wall. The footing-depth job is specialty waterproofing scope, not the shallow yard trench this guide covers.
Do I need fabric for a French drain?
Yes. Use non-woven drainage fabric, line the trench sides and bottom before the stone goes in, and fold the fabric over the top of the stone column before backfill. The top wrap is the part installers most often skip — without it, fines migrate in from above and the void space silts up. Don't substitute plastic sheet; plastic traps water against the stone instead of letting fines wash through.
Which way do the holes on a French drain pipe face?
Common residential practice for corrugated perforated pipe is holes pointed down. Water reaches the pipe by moving through the stone column above; the bottom perforations let it enter as the trench fills. Check your specific pipe spec sheet — a few products are designed differently — but for the standard 4-inch corrugated black pipe at the big-box, holes down is the default install.
How much #57 stone do I need for a 30-foot French drain?
At a 12 in × 12 in shallow yard trench: 30 cubic feet of stone = 1.11 cu yd exact, ~1.25 cu yd practical with a small cushion, and about 1.69 tons at #57's loose density (~100 lb/ft³). Many supply yards have 3-5 cu yd or ton-equivalent delivery minimums — call the dispatcher before assuming bulk delivery. Below the minimum, a small landscape yard or bagged from a big-box is often the practical move. Footing-depth drains scale linearly with depth and land in bulk-delivery range fast.
What I'd do next
- Size the stone order in the gravel calculator
Pick drainage preset, enter trench length × width × depth, get cubic yards + tons.
- Use general gravel yardage math
If the scope is bigger than a single trench (yard drain, dry well, base prep, retaining wall back-fill).
- If basement / foundation water is involved, put it in the gut-rehab budget
Footing-depth waterproofing is a line on a bigger rehab budget, not a Saturday project.
- 1. NRCS Construction Specification 495 — Geotextile — non-woven separation fabric weight (4 oz/yd²) and overlap (12-inch) specifications for drainage applications. ↩
By James Wu. Stone choice (#57 angular clean) and per-type density (~100 lb/ft³ loose) trace to supplier spec sheets cross-checked against InchCalculator (1.4-1.7 tons/yd³ industry range). Geotextile specifications from NRCS Construction Specification 495 — Geotextile. Not geotechnical or basement-waterproofing advice — for high water table sites, structural foundation issues, or any drainage scope beyond a shallow yard / surface trench, consult a licensed waterproofing contractor or a geotechnical engineer. Full methodology.