How much slope does a French drain need?

The fall a 4-inch pipe needs

Slope and fall mean the same thing here: how far the pipe drops along its run. The fall a pipe needs depends on how rough its wall is and how fast the water has to move to carry silt out.

Fall a 4-inch pipe needs, in percent, to reach each water speed
Pipe and wall roughness	2.0 ft/s	2.5 ft/sFDOT minimum, full flow	3.0 ft/sADS, commonly accepted
Smooth-wallroughness 0.009 (ADS)	0.40	0.63	0.91
Smooth-wallroughness 0.010	0.50	0.78	1.12
Smooth-wallroughness 0.012 (FDOT)	0.72	1.12	1.61
Corrugatedroughness 0.015 (ADS)	1.12	1.75	2.52
Corrugatedroughness 0.024 (FDOT)	2.87	4.48	6.45

How to read it: find the pipe on the left and the water speed across the top. The number where they meet is the percent of fall a 4-inch pipe needs to reach that speed. Roughness is the Manning n figure each source publishes.

What the percentages mean on the ground

1%1 foot of drop per 100 feet of pipe. About 1/8 inch per foot.

2%2 feet of drop per 100 feet of pipe. About 1/4 inch per foot.

Half of one percent, 0.5%, is about 1/16 inch per foot.

Why corrugated needs more

Corrugated pipe has a ribbed wall. The ribs make the inside rougher, and a rougher wall slows the water. For the same pipe size and the same water speed, the fall needed goes with the square of the roughness.

By the ADS roughness figures (0.015 for corrugated against 0.009 for smooth-wall), corrugated pipe needs about 2.8 times the fall. By the FDOT figures (0.024 against 0.012), it needs about 4 times. So the same water speed needs about 2.8 to 4 times the fall in corrugated pipe.

Put the other way: at 1% of fall, a 4-inch smooth-wall pipe runs at about 2.4 to 3.2 feet per second, and a 4-inch corrugated pipe runs at about 1.2 to 1.9. At 2% of fall, a 4-inch corrugated pipe runs at about 1.7 to 2.7 feet per second.

If the lot is flat

Use smooth-wall pipe. It carries silt out on about a quarter to a third of the fall corrugated pipe needs.

A larger pipe needs less fall.

Where gravity cannot do it, a sump and discharge line moves the water. See sump and discharge systems.

How it fails

Silt settles in the low spots, and the line clogs. A pipe that is too flat, or laid with uneven fall, drops its silt where the water slows down. This is silting, not scouring. Scouring is a problem of water moving too fast, not too slow.

Check your own numbers

Our pipe slope calculator works out the fall for your run of pipe.

Open the pipe slope calculator

Method

The numbers on this page come from Manning's equation for a round pipe flowing full or half full.

V = (1.486 / n) × R^2/3 × S^1/2V is water speed in feet per second, n is the roughness of the pipe wall, R is one quarter of the pipe's inside diameter in feet, and S is the fall.

Roughness comes from the ADS Drainage Handbook (Table 3-1) and the FDOT Drainage Manual (section 3.6.4). The target water speeds are 2 to 3 feet per second: FDOT's minimum for a storm drain flowing full is 2.5, and ADS calls 3 commonly accepted for self-cleaning.

The 2% figure for corrugated pipe is a calculation, not a rule from a code book or a pipe maker. The FDOT manual is written for roadway storm drains, and we use its figures as a benchmark.

General guidance, not a design for your property.

For your property, fall is the second point of the FAS Drainage Standard: we measure it on site before design.

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Sources

Where the numbers come from

ADS Drainage Handbook, Hydraulics section.Table 3-1, pipe roughness (Manning n): single-wall corrugated pipe 3 to 6 inches 0.015; smooth-wall sewer and drain pipe 3 to 6 inches 0.009. Section 3-4: 3 feet per second as a commonly accepted self-cleaning speed.https://assets.adspipe.com/m/4f6d410de4cd761f/original/Hydraulics-Section-of-Drainage-Handbook.pdf
FDOT Drainage Manual, January 2021.Section 3.6.4, pipe roughness (Manning n): single-wall polyethylene 0.024; smooth-interior PVC 0.012. Section 3.6.1: at least 2.5 feet per second when the storm drain is flowing full.https://fdotwww.blob.core.windows.net/sitefinity/docs/default-source/roadway/drainage/files/drainagemanual2021.pdf

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How much slope does a 4-inch French drain pipe need?

A 4-inch smooth-wall pipe needs about 0.5% to 1% of fall, which is 1/16 to 1/8 inch per foot. A 4-inch corrugated pipe needs about 2%, which is 1/4 inch per foot, because its ribbed wall slows the water.

Is 1% enough slope for a French drain?

For a 4-inch smooth-wall pipe, yes. At 1% of fall it runs at about 2.4 to 3.2 feet per second. A 4-inch corrugated pipe at 1% runs at about 1.2 to 1.9 feet per second, which is too slow to carry silt out.

Why does corrugated pipe need more slope than smooth-wall pipe?

Its ribbed wall is rougher, and a rougher wall slows the water. For the same water speed, corrugated pipe needs about 2.8 to 4 times the fall of smooth-wall pipe, by the ADS and FDOT roughness figures.

What if my yard is too flat for a French drain?

Use smooth-wall pipe, which needs less fall. A larger pipe also needs less fall. Where gravity cannot do it, a sump and discharge line moves the water.