French Drain Installation

Why this matters

A French drain is the engineered solution for redirecting subsurface and surface water away from a problem area (foundation, yard, retaining wall). The basic concept is simple (perforated pipe in a stone-filled trench), but the details (slope, discharge, fabric placement, depth) determine whether it works. A poorly designed French drain holds water and doesn't redirect; a well-designed one carries 5 to 50 gallons per minute away from the problem area.

When to install

• Surface water flowing toward foundation
• Subsurface water table at problem level
• Yard wet area (low spot)
• Retaining wall pressure issue
• Sub-grade water management

When NOT to recommend (different solution):

• Single crack with leak (inject the crack)
• High water table affecting basement floor (sump pump system)
• Sewer or storm drain backup (different issue)

Tools and materials

Tools

• Excavator (mini-excavator typical for 50 to 200 ft drains)
• Shovel
• Tamper
• Level
• Measuring wheel
• Smartphone with mapping (for measuring discharge route)

Materials

• Perforated drainage pipe (4 inch typical residential; 6 inch for larger applications)
• Drainage fabric (geotextile, 4 oz typical)
• Drainage stone (3/4 inch clean stone)
• Optional: rigid pipe (non-perforated) at the discharge end
• Rigid couplings and fittings

Procedure for typical foundation French drain

Step 1: Plan the route

• Identify the start (where water collects or pressure is highest)
• Identify the discharge (must be lower elevation; typically 50 to 200 ft from start)
• Route between them should slope continuously (no high points)
• Avoid utilities (call before you dig)
• Mark the route on the ground

Step 2: Verify discharge location

The discharge must be at a lower elevation:

• Daylight (the drain emerges at a slope or in a swale)
• Storm drain connection (where permitted by code)
• Dry well (an underground stone-filled pit for slow release)
• Other appropriate discharge point

The slope from start to discharge should be at least 1 percent (1 ft drop per 100 ft).

Step 3: Excavate the trench

• Width: typically 12 to 18 inches (wide enough for the pipe with stone around it)
• Depth: depends on application
  • At foundation: typically 6 inches below the footing
  • For yard drainage: typically 12 to 24 inches below grade
  • Below frost line in cold climates

The trench must maintain consistent slope. Use a string line with a level to verify.

Step 4: Compact the trench bottom

• After excavation, the soil at the bottom of the trench may be loose
• Light tamping
• Verify the bottom is uniform and slopes correctly

Step 5: Install drainage fabric

• Lay drainage fabric in the trench
• Extend fabric up the trench walls (will fold over the stone later)
• Fabric prevents fine soil from clogging the stone

Step 6: Add bedding stone

• Add 2 to 4 inches of drainage stone in the bottom of the trench
• This is the bedding layer for the pipe

Step 7: Install drainage pipe

• Lay perforated pipe on the bedding stone
• Holes should face DOWN (counter-intuitive but standard)
• Why holes down: water enters the pipe from below; the bedding stone delivers water to the pipe; the pipe carries it away

Slope verification:

• Use a level to confirm the pipe slopes consistently toward the discharge
• Any sag in the pipe collects water and reduces flow

For drains over 100 ft, install cleanout access at the high end (a rigid pipe rising to the surface with a removable cap).

Step 8: Cover with stone

• Add drainage stone over and around the pipe
• Continue until stone covers the pipe by 6 to 12 inches
• The stone provides the water-carrying capacity

Step 9: Fold fabric over the stone

• Fold the drainage fabric over the top of the stone
• The fabric envelopes the stone-pipe system
• Prevents soil migration into the stone

Step 10: Backfill with native soil

• Add native soil over the fabric
• Compact gently
• Restore grade

Step 11: Restore surface

• Replace grass, mulch, or other surface material
• Walk the line; verify nothing sinks

Step 12: Test (if possible)

• During the next rain, observe the discharge
• Water should emerge from the discharge during heavy rain
• Verify water flows; doesn't pool

Specific applications

Foundation French drain

• Excavated alongside the foundation
• Captures water hitting the foundation
• Carries water away to a lower elevation
• Often combined with exterior waterproofing of the foundation wall

Driveway / sidewalk French drain

• Captures water flowing across paved surfaces
• Installed at the base of slopes
• Discharges to storm drain or daylight

Yard French drain

• Captures wet areas in lawns
• Installed at the low point
• Discharges to a lower elevation or dry well

Retaining wall French drain

• Installed behind the wall
• Reduces hydrostatic pressure on the wall
• Discharges through wall (weep holes) or away

Common issues

Drain doesn't work after install

Causes:

• Reverse slope (water flows backward)
• Stone too small or too compact
• Fabric placement wrong
• Pipe holes facing up
• Discharge blocked

Fix: dig up and redo with correct method.

Slow drainage

Causes:

• Pipe inadequate diameter for the volume
• Insufficient drainage stone
• Discharge restricted

Fix: enlarge or improve.

Drainage stone clogged

Causes:

• Fabric not installed correctly; fines migrated into stone
• Time degradation of fabric

Fix: dig up and replace fabric or re-clean stone.

Pipe collapse

Causes:

• Inadequate pipe wall thickness
• Heavy load over the drain
• Soil settling

Fix: replace damaged section.

Customer expectations

• "The drain redirects water; doesn't seal anything"
• "Water still enters the soil; we just direct it away from your foundation"
• "Annual inspection: ensure discharge doesn't get blocked"
• "Service life: 25 to 50 years typical"

Maintenance

• Annual visual inspection of discharge
• Clear any debris from discharge
• Verify no surface settling along the drain line
• If drain seems less effective: cleanout access (if installed) to flush

References

• IRC R406 (Foundation Drainage).
• ASTM C33 (Concrete Aggregates).
• AASHTO M288 (Geotextile Specification).
• Manufacturer literature: drainage pipe and fabric manufacturers.
• Manuall internal: Foundation Repair Basement Waterproofing Systems.