How to Make a French Drain: Slope, Depth, Stone and Pipe Specs

Knowing how to make a French drain is mostly knowing where its numbers come from. The familiar 18 to 24 inch depth and 12 inch width are not folklore: they fall out of two public specs, the 2 inches of stone under the pipe and 6 inches over it in International Residential Code Section R405.1, and the 1 foot of cover and 8 inches of extra width in Wisconsin DOT Standard Specification 612. This page stacks those figures into a trench you can dig, shows how much deeper a flat-yard trench gets at its outlet, and turns your length into tons of stone.

The build specs, and which document says so

Every row below comes from an extension service, a building code, a state transportation spec or a manufacturer, read in November 2026. Where they disagree, the table says so. Highway and foundation specs are written for bigger drains than a backyard needs, so treat them as the upper end, not the minimum, for a lawn.

Item	Published figure	Source
Fall (slope)	At least 1 percent	Clemson HGIC, 2020; NDS installation guide
Fall (slope)	0.5 to 1 percent	WSU Extension, Clark County
Fall (slope)	At least 1 to 3 percent	Colorado State GardenNotes #219
Depth	About 8 inches to 2 feet for yard work	NDS
Width	5 to 6 inches for small problems	NDS
Stone under pipe	At least 2 inches of washed gravel or crushed rock	IRC R405.1 (foundation drains)
Stone over pipe	At least 6 inches	IRC R405.1
Stone over pipe	To 1 foot above the top of the pipe	WisDOT 612.3.5(1)
Stone width	At least 8 inches wider than the pipe’s outside diameter	WisDOT 612.3.5(1)
Stone size	No. 57 (100 percent passes 1.5 in, 0 to 10 percent passes No. 4); listed for shoulder drains	NCDOT Table 1005-1
Stone size	At least one sieve size larger than the pipe’s holes	IRC R405.1
Pipe	4-inch smooth or 5-inch corrugated (highway underdrain)	FDOT Index 440-001, note 1
Holes	Perforations on the underside	WisDOT 612.3.3(3)
Fabric	Filter membrane around the pipe or over the stone	IRC R405.1
Fabric laps	18 inches at joints (WisDOT); 1 foot (FDOT)	WisDOT 612.3.2; FDOT note 8
Outlet pipe	Solid (nonperforated); bends made with 45 degree elbows	FDOT note 9
Ends	Cap the upper end; screen the discharge end	WisDOT 612.3.3(4) and (5)
Dig order	Start at the outlet and work uphill	WisDOT 612.3.1(2)

Why 18 to 24 inches deep and 12 inches wide?

Stack the code minimums and the depth appears. Allow about 4.5 inches for the outside of a 4-inch pipe, put 2 inches of stone under it and 6 inches over it per IRC R405.1, and add a 4-inch soil cap so grass can grow. That is 16.5 inches, which rounds to 18. Swap in WisDOT’s 1 foot of cover and the same stack reaches 22.5 inches, which rounds to 24.

Layer, top to bottom	Code-minimum stack	Highway-cover stack	Basis
Soil or sod cap	4 in	4 in	Our assumption; WSU says the fabric is covered with soil
Stone over pipe	6 in	12 in	IRC R405.1 / WisDOT 612.3.5
Pipe (4-in, outside)	4.5 in	4.5 in	Allowance for corrugated tubing
Stone under pipe	2 in	2 in	IRC R405.1
Total depth	16.5 in, dig 18	22.5 in, dig 24
Width: pipe plus envelope	12.5 in	12.5 in	WisDOT: 8 in wider than the pipe

The deeper option buys more storage and more filtering stone between the surface and the pipe. The shallow option is 25 percent less digging and about a third less stone per foot (1.06 vs 1.56 cubic feet). For a lawn that is soggy only after storms, 18 inches is the usual call; for groundwater seeping from a slope, the 24-inch stack intercepts more of it.

How much fall does a French drain need?

Clemson Extension says to keep a 1 percent slope along the trench, WSU Extension gives 0.5 to 1 percent, and Colorado State says at least 1 to 3 percent. One percent is 1/8 inch per foot, 1.2 inches per 10 feet, or 1 foot per 100 feet. The popular “1 inch per 8 feet” works out to 1.04 percent; “1 inch per 10 feet” is 0.83 percent, inside WSU’s range but under Clemson’s.

Run length	0.5 percent	1 percent	2 percent	Outlet depth on flat ground, 18-in start at 1 percent
10 ft	0.6 in	1.2 in	2.4 in	19 in
25 ft	1.5 in	3.0 in	6.0 in	21 in
50 ft	3.0 in	6.0 in	12.0 in	24 in
100 ft	6.0 in	12.0 in	24.0 in	30 in
150 ft	9.0 in	18.0 in	36.0 in	36 in

• 0.5 percent (WSU low end)
• 1 percent (Clemson)
• 2 percent

Source: Slopes from Clemson, WSU and Colorado State Extension; fall = length x slope, our arithmetic Colorado State's minimum runs from 1 to 3 percent; 2 percent shown as a mid value.

Show the numbers

	0.5 percent (WSU low end)	1 percent (Clemson)	2 percent
10 ft	0.60	1.20	2.40
25 ft	1.50	3	6
50 ft	3	6	12
100 ft	6	12	24
150 ft	9	18	36

The last column is the problem most guides skip. On a level lawn, a 100-foot drain that starts 18 inches deep must end 30 inches deep, and a 150-foot run ends at 36 inches. Your outlet has to sit at least that low. If the lowest ground you control is not 12 inches below the trench bottom per 100 feet of run, you need a pop-up emitter in a lower spot, a dry well, or a shorter run, and our French drain cost guide prices each outlet.

To set grade, drive a stake at each end, run a mason’s line, level it with a line level, then lower the line at the outlet stake by the fall in the table: 6 inches for 50 feet at 1 percent. Measure from the line to the trench floor every 10 feet; each reading should grow by 1.2 inches. Check it again after the bedding stone goes in, since that is the surface the pipe actually follows.

Step by step: building the drain

1. Find the water and the outlet. Watch where water stands after rain and pick a discharge point lower than the trench bottom by the fall in the table. Keep yard water out of the sanitary sewer and off neighbors: Lake Zurich, Ill. sets drains at least 3 feet from property lines and, where no storm structure exists, ends discharge at least 10 feet from them.
2. Call 811. The national 811 service is free and wants notice a few business days ahead. Florida’s law, for example, requires at least 2 full business days and has utilities mark within 24 inches of each side of a line; dig by hand inside that zone.
3. Mark and dig from the outlet uphill. WisDOT 612.3.1(2) starts at the outlet so water drains away while you work. Dig 12 inches wide, 18 to 24 inches deep at the top end, deepening 1.2 inches every 10 feet. Pile sod and topsoil apart from subsoil.
4. Line the trench with nonwoven fabric. Leave enough on each side to fold over the top with a 12-inch lap: about 5.3 feet of fabric width per foot of an 18-inch trench, 6.3 feet for 24 inches.
5. Lay 2 inches of washed stone and re-check grade with the string line before any pipe goes in.
6. Lay 4-inch perforated pipe, holes down. WisDOT 612.3.3(3) specifies perforations on the underside. Cap the upstream end so stone cannot enter, per 612.3.3(4).
7. Switch to solid pipe where the drain leaves the wet area. FDOT Index 440-001, note 9 uses nonperforated pipe for outlets and turns corners with 45 degree elbows; a 90 degree turn is two 45s at least 1 foot apart.
8. Cover with stone to 6 inches over the pipe (12 inches for the deeper stack), keeping 3 to 4 inches below grade for soil.
9. Fold the fabric over the stone with its 12-inch lap, then backfill with topsoil and reset the sod.
10. Finish the outlet. Screen or grate the discharge end per WisDOT 612.3.3(5), or fit a pop-up emitter set below the trench bottom. Run a hose into the top end and confirm water reaches the outlet.

Do the pipe holes go up or down?

Down. Wisconsin DOT Standard Specification 612.3.3(3) says to lay perforated drainage pipe with the perforations on the underside unless the engineer directs otherwise. Water in a French drain does not drip in from above: rain soaks through the cap and stone, collects at the trench floor and rises. Holes set low on a 4-inch pipe, at 4 and 8 o’clock, take that water in once it is about 1 inch deep, and the pipe then carries it away at 1 percent.

With holes up, the trench has to fill to near the top of the pipe, about 4 inches, before any water gets in, and the pipe holds that water in the trench instead of draining it. The question matters most for rigid PVC drain pipe with 2 or 3 rows of holes. Single-wall corrugated tubing is usually slotted all the way around; check the slot pattern on the 10 feet you buy before deciding it has no top.

Fabric: does it clog, and where does it go?

The worry is real but the evidence is thin. FAO Irrigation and Drainage Paper 60 reviewed drain envelopes worldwide and says laboratory experiments, field trials and practical experience do not give clear evidence of a clogging risk from properly selected, properly installed fine geotextiles. The same paper says sandy, cohesionless soils need an envelope in all circumstances, and soils with at least 20 to 30 percent clay usually do not.

Your soil	FAO guidance	What to do in a yard drain	Source
Sand or loamy sand (falls apart when squeezed)	Envelope required	Wrap the whole trench in nonwoven fabric; a socked pipe adds a second filter	FAO Paper 60, ch. 6 summary
Silt, silt loam or very fine sand	Clogging-prone textures; envelope advised	Wrap the trench; a sock is optional once the trench is wrapped	FAO Paper 60
Clay of 20 to 30 percent or more	Usually not needed for the pipe	Still line the trench so topsoil cannot wash into the stone	FAO Paper 60; WSU Extension
Rust-colored iron deposits in wet spots	Iron ochre risk	FAO notes fine-textured envelopes are argued to raise ochre clogging; plan a cleanout at the upper end	FAO Paper 60

Line the trench, not just the pipe. WSU Extension says the fabric wrapped over the gravel keeps soil from filling the spaces between stones and so keeps water moving through them. A sock alone keeps sand out of the pipe but lets the stone silt up. Lake Zurich’s permit rules make the same choice: pipe surrounded by washed stone and the trench wrapped in geotextile.

How much gravel for a French drain?

A 12-inch trench dug 18 inches deep with a 4-inch soil cap holds 1.17 cubic feet of stone per foot, about 1.06 after the pipe. At 100 pounds per cubic foot, which is what Bedrock Landscaping Materials’ 2026 coverage chart implies for 3/4-inch rock (1 ton covers 120 sq ft at 2 inches), that is about 106 pounds of stone per foot of drain.

Trench length (ft)StoneWashed 3/4-inch or No. 57 stone, about 100 lb per cu ftTrench width12 inches6 inches8 inches10 inches18 inchesStone depth (trench depth minus a 4-inch soil cap)18-inch trench: 14 inches of stone24-inch trench: 20 inches of stone12-inch trench: 8 inches of stone30-inch trench: 26 inches of stone18-inch trench, stone to the surface

Stone (cubic feet)

58.3

Stone (cubic yards)

2.16

Stone (tons at 100 lb per cu ft)

2.92

Bags of 0.5 cu ft

117

Default: 50 ft x 12 in x 14 in of stone = 58.3 cu ft, 2.16 cu yd, 2.92 tons, 117 bags. Length x width x stone depth; the pipe is not subtracted, which leaves about 9 percent for waste. Pipe length equals trench length plus solid pipe to the outlet.

100 ft yard drain, 12 x 18 in	Quantity	How we got it
Soil dug out	150 cu ft (5.6 cu yd) in the ground	12 x 18 in x 100 ft; it fluffs up once dug
Soil to haul or spread	117 cu ft (4.3 cu yd)	All but the 4-inch cap
Washed 3/4-inch or No. 57 stone	106 cu ft, 3.9 cu yd, 5.3 tons	Net of the pipe, at 100 lb per cu ft
Same stone in 0.5 cu ft bags	211 bags	Why bulk delivery wins past about 20 ft
Perforated 4-inch pipe	100 ft plus a cap	Plus solid pipe from the wet area to the outlet
Nonwoven fabric	533 sq ft (5.3 ft wide x 100 ft)	Bottom, sides, top and a 12-inch lap
Fall to build in	12 in	1 percent of 100 ft; outlet trench 30 in deep on a level yard

The calculator’s default, 50 feet at 12 by 18 inches, returns 58.3 cubic feet, 2.16 cubic yards or 2.92 tons, and 117 half-cubic-foot bags. For material and labor prices per foot, see the French drain cost breakdown; for stone by the yard, the river rock cost guide lists bulk prices.

When is a French drain the wrong fix?

When the water is on the surface, the grade is the problem. IRC R401.3 wants the ground to fall at least 6 inches in the first 10 feet away from a foundation, and our lawn leveling guide shows how to build that fall. Runoff cutting a channel is an erosion job, covered in erosion control methods. A drain dug down to a basement footing is a foundation drain under R405.1, and any trench 5 feet or deeper needs sloping, shoring or a trench box under OSHA 29 CFR 1926.652.

1. 1Is the water mostly running or ponding on the surface near the house, where the ground falls less than 6 inches in the first 10 feet?

   YesRegrade first to the IRC R401.3 fall; a drain will not fix a back-sloped yard.

   NoGo to the next question

2. 2Do you need to dig 5 feet or deeper, or down to a basement footing?

   YesThat is a foundation drain under IRC R405.1 and an OSHA 1926.652 excavation. Hire it out.

   NoGo to the next question

3. 3Is there ground you control at least 12 inches below the trench bottom per 100 feet of run?

   Yes

   NoShorten the run, use a pop-up emitter in a lower spot or a dry well, and price the outlet before digging.

4. 4Would the trench sit within 3 feet of a property line or discharge within 10 feet of one?

   YesMove it or check local rules; Lake Zurich, Ill., bars both.

   NoGo to the next question

5. 5Is your soil sandy enough to fall apart when squeezed?

   YesWrap the whole trench in nonwoven fabric and consider a socked pipe too (FAO Paper 60).

   NoCall 811, then dig 12 inches wide and 18 to 24 inches deep at 1 percent fall, with pipe holes down on 2 inches of washed stone.

Source: IRC 2021 R401.3 and R405.1, OSHA 29 CFR 1926.652, Lake Zurich drainage handout, FAO Paper 60

This page is general information. Local codes, HOA rules and drainage law decide where water may go; confirm with your building department before you discharge near a property line or street, and call 811 before any digging.