French Drain Installation Guide: Complete Step-by-Step Instructions

A French drain is one of the most effective solutions for managing groundwater and surface water problems. Named after Henry French, who popularized the design in 1859, this simple but effective system intercepts water and redirects it away from problem areas.

What Is a French Drain?

A French drain consists of:

• A trench filled with gravel
• A perforated pipe that collects water
• Filter fabric that prevents soil infiltration
• An outlet that discharges water safely

Water enters through the gravel, flows into the perforated pipe, and is carried to a discharge point by gravity.

When to Use a French Drain

Ideal Applications

• Intercepting groundwater before it reaches a foundation
• Draining soggy areas where water pools
• Redirecting surface water away from structures
• Behind retaining walls to relieve hydrostatic pressure
• Along driveways to catch runoff

When NOT to Use

• Active springs - May need professional dewatering
• High water table - Drain will just fill up
• No outlet available - Water must go somewhere
• Heavy clay with no gravel discharge - Dry wells won’t work

Planning Your French Drain

Step 1: Identify the Problem

Before digging, understand your water issue:

Questions to Answer:

• Where does water come from?
• Where does it collect or cause problems?
• Where can it be safely discharged?
• Is this surface water, groundwater, or both?

Observation Tips:

• Watch your property during rain
• Note where water flows and pools
• Check after heavy rain and days later
• Look for wet spots, moss, or erosion

Step 2: Locate Utilities

Step 3: Plan the Route

Route Considerations:

• Start where water collects or enters
• Follow the natural slope of the land
• Avoid tree roots when possible
• End at a safe discharge point

Outlet Options:

• Daylight to lower area of property
• Storm drain (check local codes)
• Dry well (if soil percolates)
• Rain garden or infiltration area

Step 4: Calculate Size

For most residential applications:

Pipe Diameter:

• 4 inches: Most residential applications
• 6 inches: Larger areas or higher flow

Calculate your French drain requirements →

Materials and Tools

Materials List

Material Quantities (per 10 linear feet):

• Gravel: ~0.5 cubic yards (18” wide × 24” deep)
• Pipe: 10 feet + fittings
• Fabric: 20 linear feet (4’ wide roll)

Tools Required

Hand Tools:

• Shovel, spade
• Trenching shovel (narrow blade)
• Rake
• Level, string line
• Tape measure
• Utility knife

Power Tools (Recommended):

• Trencher (rental) - Saves hours of digging
• Plate compactor
• Reciprocating saw (for pipe cutting)

Installation Steps

Step 1: Mark the Trench

1. Use stakes and string to mark trench location
2. Spray paint the route
3. Mark outlet location
4. Verify slope direction

Step 2: Excavate the Trench

Trench Specifications:

• Width: 12-18 inches
• Depth: 18-24 inches (or as calculated)
• Slope: Minimum 1% (1/8” per foot)

Digging Tips:

• Work from outlet toward source
• Check depth frequently with level and tape
• Keep sides fairly straight
• Pile soil on one side for easier backfill

Step 3: Check Grade

The drain must slope continuously toward the outlet:

Example: 100-foot drain needs 1 foot of fall (minimum)

Checking Grade:

1. Drive stakes at start and end
2. Stretch string line
3. Use line level to level the string
4. Measure down from string at intervals
5. Adjust trench depth as needed

Step 4: Line with Filter Fabric

1. Unroll fabric along trench length
2. Leave enough to fold over top later
3. Smooth fabric against trench walls
4. Overlap sections by 12 inches
5. Secure with stakes if windy

Step 5: Add Gravel Base

1. Pour 2-3 inches of gravel in trench
2. Spread evenly with rake
3. Check depth and grade
4. Compact lightly if needed

Step 6: Install Pipe

Pipe Orientation:

• Holes face DOWN for groundwater collection
• Holes face UP for surface water (with gravel to surface)
• Most manufacturers recommend holes down

Installation:

1. Lay pipe on gravel bed
2. Connect sections (snap fit or glue)
3. Maintain consistent slope
4. Install clean-out fittings at direction changes
5. Connect to outlet pipe (solid, not perforated)

Step 7: Cover with Gravel

1. Carefully add gravel around pipe
2. Fill to within 4-6 inches of surface
3. Ensure pipe stays in position
4. Don’t overfill—leave room for fabric and soil

Step 8: Wrap Filter Fabric

1. Fold fabric flaps over gravel
2. Overlap by minimum 4 inches
3. Ensure complete coverage
4. Stake if needed

Step 9: Backfill

Options:

• Gravel to surface: Best for surface water collection
• Soil and grass: Conceals drain, less surface collection
• Decorative stone: Good compromise, some surface collection

Backfill in layers, compacting each if using soil.

Step 10: Complete Outlet

Daylight Outlet:

• Extend pipe to discharge area
• Install rodent screen on outlet
• Add splash pad or riprap for erosion control
• Ensure outlet is clear of obstructions

Dry Well Connection:

• See dry well installation requirements
• Ensure adequate percolation
• Size for expected volume

Special Situations

Around Foundations

For foundation protection:

• Install OUTSIDE the footing
• Pipe should be below basement floor level
• Use rigid pipe (not corrugated) for durability
• Connect to sump pump or daylight outlet
• Consider waterproofing membrane on foundation

Behind Retaining Walls

Essential for any wall over 2 feet:

• Install during wall construction
• Place 6-12 inches behind wall face
• Include weep holes through wall
• Collect at base and outlet to side

In Lawns

To maintain grass:

• Use sod-covered trench cap
• Install catch basins at low points
• Consider deeper installation (30+ inches)
• Grass may grow better after drainage improves

Troubleshooting

Drain Not Working

Possible Causes:

• Outlet is blocked
• Insufficient slope
• Pipe is clogged
• Fabric is silted over
• Wrong location for problem

Solutions:

• Check and clear outlet
• Flush with hose
• Add catch basins
• May need to reinstall

Standing Water in Drain

Possible Causes:

• Outlet below water level
• Outlet blocked
• Insufficient capacity
• High water table

Solutions:

• Lower outlet or install pump
• Clear obstruction
• Add parallel drain
• May need professional dewatering

Erosion at Outlet

Causes:

• No energy dissipation
• Concentrated flow
• Steep outlet slope

Solutions:

• Add riprap or splash pad
• Disperse flow with perforated pipe
• Create small settling basin

Maintenance

Regular Maintenance

Annually:

• Inspect outlet for blockages
• Check for settling or sinkholes
• Flush with hose if accessible
• Clear debris from catch basins

After Major Storms:

• Verify proper flow
• Check for new erosion
• Inspect outlet function

Signs of Problems

• Water backing up in catch basins
• Soggy areas reappearing
• Sinkholes along drain route
• Poor outlet flow during rain

Cost Summary

DIY Costs (per linear foot):

50-foot drain DIY cost: $200-400

Professional Installation: $25-50 per linear foot 50-foot professional drain: $1,250-2,500

Equipment Rentals:

• Trencher: $150-250/day
• Plate compactor: $50-75/day

Summary

A properly installed French drain will provide decades of reliable drainage. Key success factors:

1. Adequate slope - Minimum 1%, more is better
2. Proper materials - Clean gravel, quality fabric
3. Good outlet - Water must have somewhere to go
4. Filter fabric - Prevents clogging
5. Sufficient capacity - Size for expected flow

Calculate your French drain requirements →

References

1. French, H. F. (1859). Farm drainage. Orange Judd Company.

2. U.S. Army Corps of Engineers. (2000). Design and construction of levees (EM 1110-2-1913). USACE.

3. Cedergren, H. R. (1989). Seepage, drainage, and flow nets (3rd ed.). John Wiley & Sons.

4. Federal Highway Administration. (2007). Highway subdrainage design (FHWA-NHI-07-021). U.S. Department of Transportation.

5. American Society of Civil Engineers. (1998). Urban subsurface drainage (ASCE Manual of Practice No. 95). ASCE Press.