Which pipe is better for a French drain?
It turns on fall: how far the pipe drops along its run. Corrugated pipe needs far more of it to stay clean, and 2% over a 100-foot run is 2 feet of drop.
So we lay smooth-wall PVC, chosen for the fall the ground actually has, as the FAS Drainage Standard sets out. In our owner's words, from a note to a customer in July 2026:
“The downspouts as well are connected with corrugated pipe, which we don't use since it regularly clogs”
Why does corrugated pipe clog?
Single-wall corrugated pipe is ribbed inside. The ribs make the wall rough, and a rough wall slows the water. ADS, which makes both kinds of pipe, gives its small single-wall pipe a roughness (Manning n) of 0.015 and its smooth-wall sewer and drain pipe 0.009.1 The FDOT Drainage Manual gives single-wall polyethylene 0.024 and smooth-interior PVC 0.012.2
ADS notes that sediment can cut a pipe's capacity and that inside texture affects how much debris collects. In its own worked examples, ADS picks smooth-interior pipe over single-wall pipe of about the same capacity, because only the smooth one reaches a self-cleaning speed.1
Michigan State University Extension sees the same in farm drains: fine sand or silt builds up until the pipe clogs, and sand is a soil it lists as a risk.4 UF/IFAS Extension calls North and Central Florida soils typically very sandy.5
This is silting, not scouring. A drain laid too flat fails because the water is too slow, not too fast.
How do corrugated and PVC pipe compare?
| Corrugated, single-wall | Smooth-wall PVC | |
|---|---|---|
| Wall roughness (Manning n) | 0.015 to 0.024ADS and FDOT1,2 | 0.009 to 0.012ADS and FDOT1,2 |
| Fall needed to carry silt out | About 2%1/4 inch per foot | About 0.5% to 1%1/16 to 1/8 inch per foot |
| Water speed at 1% fall | 1.2 to 1.9 ft/sbelow the 2 to 3 ft/s target | 2.4 to 3.2 ft/swithin or above the 2 to 3 ft/s target |
| How it fails | Silt settles in the low spots and the line clogs | The same, if laid too flat; it needs far less fall to stay clean |
| Cleaning | Water jetting at no more than 2,000 psi; no bladed cutters, by the maker's manual3 | At the right fall, the flow carries silt out on its own1 |
| Where it fits | Runs with plenty of fall; farm drains wrapped in a sock against silt6 | Flat yards, French drains and downspout lines. What we lay. |
Fall and speeds by Manning's equation for a 4-inch pipe flowing full or half full, against a target of 2 to 3 feet per second (FDOT minimum 2.5, ADS 3).1,2
Is 1% of fall enough?
For smooth-wall pipe, yes. We design for at least 1%, about 1/8 inch per foot, measured on site before design. At 1% a 4-inch smooth-wall line runs at about 2.4 to 3.2 feet per second; a corrugated one is too slow to carry silt out.
1% is also a common building-code minimum: the International Plumbing Code (2015, section 1106.3) sets it for horizontal storm drain branches.7 The 2% for corrugated pipe is not a code or a pipe maker's rule. It is a calculation: for the same water speed, corrugated pipe needs about 2.8 to 4 times the fall of smooth-wall pipe.1,2 The full table is in How much slope does a French drain need?
What about dual-wall pipe?
Some corrugated pipe is ribbed outside and smooth inside. ADS calls that dual wall; pipe corrugated inside is single wall.3 The inside wall sets the fall: ADS and FDOT both give smooth-lined pipe 0.012, the same figure FDOT gives smooth-interior PVC.1,2 Single-wall pipe needs the extra fall.
What does the swap look like on a job?
Both films are from our own jobs; more are in our work.
Can a clogged corrugated line be cleaned?
It can be. ADS recommends water jetting for deposited sediment.3 Cleaning does not change the fall, though, so a line too flat collects silt again. Where the fall is short, we replace it with smooth-wall PVC on a measured fall and test it with water, as in the second film. See our yard drainage and French drain service.
General guidance, not a design for your property.
The offer
Drainage assessment of your property.
Credited toward your project.
A field tech runs your property through a 25-year storm simulation in Water MRI, builds the game plan, and then we come install it.
- A field tech runs your property through a 25-year storm simulation in Water MRI, from the roof to the street
- You see where water stands, how deep and for how long
- A field check in person when we need more accuracy, with no second fee
- The game plan: written recommendations and a preliminary drainage design
- A quote for us to install it; if you go ahead, the $89 is credited in fullOnce, within the project's initial one-third deposit.
How payment works. Requesting is free. FAS confirms the details with you first, then collects the $89.
Book my $89 drainage assessmentNothing is charged when you request. Water MRI is a visualization. A planning model.
Sources
Where the numbers come from
- ADS Drainage Handbook, Hydraulics section.Table 3-1, roughness (Manning n): single-wall 3 to 6 inches 0.015; smooth-wall sewer and drain 3 to 6 inches 0.009; smooth-lined N-12 0.012. Sections 3-2 and 3-4 and examples 2 and 4: texture, sediment, and 3 feet per second for self-cleaning.https://assets.adspipe.com/m/4f6d410de4cd761f/original/Hydraulics-Section-of-Drainage-Handbook.pdf
- FDOT Drainage Manual, January 2021.Section 3.6.4, roughness: single-wall polyethylene 0.024; smooth double-wall polyethylene and smooth-interior PVC 0.012. Section 3.6.1: at least 2.5 feet per second flowing full.https://fdotwww.blob.core.windows.net/sitefinity/docs/default-source/roadway/drainage/files/drainagemanual2021.pdf
- ADS Pipe Operation and Maintenance Manual.Single wall or dual wall, told apart by the inside; water jetting for deposited sediment, at no more than 2,000 psi; no bladed cutters.https://assets.adspipe.com/m/1432f0dd1654e04a/original/Pipe-Operation-Maintenance-Manual.pdf
- Michigan State University Extension, Bulletin E3455 (revised July 2025).Section 2: fine sand or silt builds up in drain pipes and clogs them; sand is a problem texture.https://www.canr.msu.edu/field_crops/uploads/files/E3455_DrainSedimentationTool_Rev3.pdf
- UF/IFAS Extension, Working in Your Florida Soil.North and Central Florida soils are typically very sandy.https://gardeningsolutions.ifas.ufl.edu/care/planting/florida-soil/
- Texas A&M extension bulletin B-6229, Understanding and Installing Drainage Systems (2009).Farm drains use perforated corrugated pipe in a nylon sock to keep silt and sand out.https://texaslocalproduce.tamu.edu/files/2023/08/B-6229.-Understanding-and-Installing-Drainage-Systems.pdf
- International Plumbing Code, 2015 edition, section 1106.3, on UpCodes.1/8 inch per foot (1 percent) minimum for horizontal storm drain branches. The model code; the Florida text was not read.https://up.codes/viewer/north_carolina/ipc-2015/chapter/11/storm-drainage
Questions
About drain pipe.
Talk it through
(407) 490-0441Tell us where the water collects and what it has reached. We will tell you what the $89 assessment will show.
Book my $89 drainage assessmentIs corrugated or PVC pipe better for a French drain?
Smooth-wall PVC, unless the run has plenty of fall. A 4-inch corrugated line needs about 2% of fall to carry silt out. Smooth-wall pipe does the same job on about a quarter to a third of that fall.
Why does corrugated drain pipe clog?
Its ribbed wall slows the water. When the water is too slow to carry silt out, the silt settles in the low spots and the line clogs from there. It fails by silting, not by scouring.
How much slope does corrugated pipe need?
About 2%, or 1/4 inch per foot, for a 4-inch line. It is a calculation from the ADS and FDOT roughness figures, not a pipe maker's rule.
Is dual-wall corrugated pipe the same as single-wall?
No. Dual-wall pipe is smooth inside, and ADS and FDOT give that inside wall a roughness of 0.012, the same as smooth-interior PVC in the FDOT manual. Single-wall pipe, ribbed inside, needs the extra fall.