Every drainage contractor has seen jobs fail because of avoidable design and installation mistakes. This guide covers the most common errors we see in the field and how to prevent them.
Mistake 1: Undersized Pipes
The problem: The pipe is too small for the design flow, causing surcharging, backup, and flooding at inlets.
Why it happens: Using rule-of-thumb sizing without calculating actual flows. Assuming a 4-inch pipe is always sufficient for residential work.
How to avoid it:
- Calculate the actual peak flow using the Rational Method (Q = CiA)
- Size the pipe for the design storm, not for average conditions
- Use the 10-year storm minimum for residential; 25-year for commercial
- Always verify pipe capacity with Manning’s equation
| Drainage Area | Peak Flow (10-yr, C=0.5, i=4”/hr) | Minimum Pipe |
|---|---|---|
| 2,500 sq ft | 0.11 cfs | 4” |
| 5,000 sq ft | 0.23 cfs | 6” |
| 10,000 sq ft | 0.46 cfs | 8” |
| 1 acre | 1.83 cfs | 12” |
Mistake 2: Insufficient Pipe Slope
The problem: Pipe is installed too flat, causing sediment buildup, reduced capacity, and eventual blockage.
Why it happens: Following the grade of the existing ground instead of establishing a proper pipe gradient. Not checking invert elevations at each end before trenching.
How to avoid it:
- Establish a minimum self-cleaning velocity of 2 ft/s
- Use minimum slopes: 1% for 4”, 0.5% for 6”, 0.4% for 8”, 0.2% for 12”
- Check the available fall between the upstream and downstream inverts before starting the job
- Use a laser level to set grade, not a string line on long runs
Field check: If the available fall divided by the pipe run length gives less than the minimum slope, you either need to go deeper at the outlet or shallower at the inlet.
Mistake 3: No Filter Fabric on French Drains
The problem: Soil migrates into the gravel and pipe, clogging the drain within 2 to 5 years.
Why it happens: Skipping the fabric to save time and material cost. Using the wrong type of fabric.
How to avoid it:
- Always wrap the trench (gravel and pipe) with non-woven geotextile filter fabric
- Use fabric rated for the soil conditions (check the AOS/apparent opening size)
- Overlap fabric seams by at least 12 inches
- Do not use woven landscape fabric — it clogs faster than non-woven
- In silty soils, use a fabric with smaller openings to prevent fine migration
Cost of the mistake: A failed French drain costs 100% of the original installation to redo. The fabric costs $30 to $60 for a 50-foot roll.
Mistake 4: Connecting to a System Without Checking Capacity
The problem: Adding flow to an existing storm drain or pipe system that is already at capacity, causing flooding upstream or downstream.
Why it happens: Assuming the existing system has spare capacity without verifying. Not checking as-built drawings or measuring the existing pipe.
How to avoid it:
- Identify the existing pipe size, slope, and condition before connecting
- Calculate the existing flow in the system
- Calculate the additional flow you are adding
- Verify the total flow does not exceed the pipe capacity
Mistake 5: Ignoring Tailwater Conditions
The problem: The pipe outlet is submerged or partially submerged during storms, reducing the system’s effective capacity to zero.
Why it happens: Designing the system in dry conditions and not considering what happens when the receiving channel, ditch, or storm drain is full during the design storm.
How to avoid it:
- Check the tailwater elevation at the outlet for the design storm
- If the outlet will be submerged, size the system for the net head differential, not free discharge
- Consider a flap gate on the outlet to prevent backflow
- Design the system to function even if the outlet is partially submerged
Mistake 6: Wrong Pipe Material for the Application
The problem: Using corrugated pipe where smooth-wall is needed (or vice versa), leading to insufficient capacity, structural failure, or excessive cost.
Why it happens: Using whatever pipe is on the truck rather than selecting the right material for the application.
How to avoid it:
| Application | Recommended Pipe | Avoid |
|---|---|---|
| Under driveways/roads | Smooth-wall PVC or HDPE | Single-wall corrugated |
| French drains | Perforated PVC or dual-wall HDPE | Solid pipe |
| Deep burial (> 4 ft) | SDR 35 PVC or dual-wall HDPE | Single-wall corrugated |
| Connections to structures | Smooth-wall PVC with couplings | Corrugated (poor seal) |
| Temporary or shallow yard | Corrugated (acceptable) | Overkill with RCP |
See our full Material Comparison Guide for detailed pipe material selection.
Mistake 7: No Cleanout Access
The problem: When the drain clogs (and all drains eventually need maintenance), there is no way to access and clean the pipe without digging it up.
Why it happens: Cleanouts cost $20 to $40 each and add time. They get omitted to save cost.
How to avoid it:
- Install cleanouts every 100 feet on straight runs
- Install a cleanout at every change in direction greater than 45 degrees
- Install a cleanout at the upstream end of every pipe run
- Use pop-up or flush-mount cleanouts in landscaped areas
- Mark cleanout locations on the as-built drawing
Mistake 8: Discharging Onto Neighbor’s Property
The problem: Concentrating flow and discharging it onto an adjacent property creates legal liability and neighbor disputes.
Why it happens: The path of least resistance for the pipe outlet is toward the neighbor’s lot.
How to avoid it:
- Always discharge to a public right-of-way, storm drain, or natural drainage way
- If the only option crosses a neighbor’s property, get a written drainage easement
- Diffuse concentrated flow at the property line using a level spreader or splash block
- Check local codes for discharge requirements — many prohibit concentrated discharge onto adjacent property
Mistake 9: Not Accounting for Construction Settling
The problem: The trench settles after backfilling, creating low spots where water pools instead of draining.
Why it happens: Backfill is not properly compacted. Gravel settles around the pipe.
How to avoid it:
- Compact backfill in 6-inch lifts
- Over-excavate and use clean gravel bedding around the pipe (4 inches minimum under the pipe)
- Crown the backfill slightly (1 to 2 inches above grade) to account for settling
- Wait 2 to 4 weeks before final grading and seeding to allow initial settlement
Mistake 10: Designing for Average Conditions Instead of Worst Case
The problem: The system works fine for typical rain events but fails during the design storm it was supposed to handle.
Why it happens: Testing or observing the system during light rain and declaring success. Not calculating performance under design conditions.
How to avoid it:
- Always design for the required design storm (typically 10-year for residential, 25-year for commercial)
- Verify that the overflow path during larger storms does not cause damage
- Consider what happens when the system exceeds capacity — where does the water go?
- Document the design storm and overflow path for the client
Quick Checklist: Before You Leave the Job Site
- All pipes drain freely with no standing water at inlets
- Pipe slopes verified with laser level
- Filter fabric in place on all French drains
- Cleanouts installed and accessible
- Outlet discharges to an appropriate location
- Backfill compacted and crowned
- Grade slopes away from structures in all directions
- Client briefed on maintenance requirements
- As-built photo documentation complete
Related Calculators
- Manning’s Pipe Flow Calculator — Verify pipe capacity and velocity
- Rational Method Calculator — Calculate design flows
- French Drain Calculator — Size French drains correctly
- Catch Basin Calculator — Size catch basins
- Swale Calculator — Design swales properly