French Drain Guide: Cost, Installation & When You Need One

Definition: A French drain is a trench filled with gravel and a perforated pipe that redirects surface water and groundwater away from a specific area. The pipe collects water and carries it by gravity to a discharge point (the street, a storm drain, a dry well, or daylight away from the house).

How it works: A trench is dug (typically 12-24 inches wide and 18-24 inches deep). The trench is lined with landscape fabric (to prevent soil from clogging the gravel). A perforated pipe (4-inch rigid PVC or flexible corrugated drain pipe) is laid in the trench with holes facing down. The trench is filled with clean gravel (3/4-inch crushed stone). Water from the surrounding soil flows through the gravel and enters the pipe through the perforations. The pipe carries the water by gravity to the discharge point.

Two types: Exterior French drain (yard/perimeter): installed outside around the foundation or in the yard to intercept surface water and groundwater before it reaches the house. Interior French drain (basement perimeter): installed inside the basement along the perimeter of the foundation to collect water that seeps through the foundation walls and direct it to a sump pump.

Yard/exterior problems: Standing water in the yard after rain (soggy, swampy areas). Water pooling against the foundation. A yard that slopes toward the house (water flows toward the foundation instead of away). Driveway or walkway flooding. A retaining wall that needs drainage behind it (hydrostatic pressure without drainage can push the wall over). A soggy area that prevents you from using part of your yard.

Basement/interior problems: Water seeping through the basement walls (especially at the wall-floor joint). Wet or damp basement after rain. Efflorescence (white mineral deposits) on foundation walls (indicates water migration through the concrete). Mold or musty smell in the basement. Cracks in the foundation that leak. A high water table (the ground is naturally saturated near the foundation).

When a French drain alone won't solve it: If the grading around your home is severely wrong, regrading the soil (sloping it away from the foundation) may be needed in addition to a drain. If water is entering through cracks in the foundation wall, crack injection ($300-$600 per crack) may be needed alongside the drain. If the water volume is very high, a sump pump (in addition to the interior French drain) is essential.

Exterior French drain (yard): Per linear foot: $15-$30 (DIY), $30-$60 (professional). Typical project (50-100 LF): $1,500-$6,000. Short yard drain (25-30 LF): $750-$1,800. Full perimeter drain (150-200 LF around the house): $4,000-$12,000.

Interior French drain (basement perimeter): Per linear foot: $40-$80 (professional, includes cutting through the slab). Typical basement (100-150 LF perimeter): $4,000-$12,000. Includes: sump pump ($500-$1,500) and discharge line. Full basement waterproofing (interior drain + sump + vapor barrier): $5,000-$15,000.

DIY exterior drain: Materials per linear foot: $5-$10. Materials for a 50 LF drain: $250-$500. Materials include: landscape fabric, 4-inch perforated pipe, 3/4-inch gravel (1-2 tons for 50 LF), pipe fittings, and a pop-up emitter or discharge point. Tool rental (trencher): $150-$300/day. Total DIY (50 LF): $400-$800.

Interior French drains are NOT DIY: Cutting through the basement slab requires a concrete saw. The work is messy (concrete dust, demolition). The sump pump installation requires electrical work. Proper slope and connection are critical. Hire a waterproofing professional for interior drains.

Exterior French drain (DIY-friendly): Step 1: Plan the route. The drain should intercept water before it reaches the problem area. The pipe needs at least 1% slope (1 inch per 8 feet) toward the discharge point. Mark the route with spray paint. Step 2: Call 811 (dig line) to mark underground utilities. Step 3: Dig the trench. 12-18 inches wide, 18-24 inches deep. The bottom of the trench should slope toward the discharge point. Step 4: Line the trench with landscape fabric. Leave enough fabric on the sides to fold over the top of the gravel later. Step 5: Add 2-3 inches of gravel to the bottom of the trench. Step 6: Lay the perforated pipe on the gravel (holes facing down). Step 7: Fill the trench with gravel to within 2-3 inches of the top. Step 8: Fold the landscape fabric over the top of the gravel (this prevents soil from entering the gravel and clogging the system). Step 9: Cover with topsoil and sod (or leave gravel exposed if preferred). Step 10: Connect the pipe to a discharge point: pop-up emitter (a spring-loaded lid that opens when water pressure builds), dry well (a buried chamber that allows water to percolate into the soil), connection to a storm drain (if allowed by local code), or daylight (the pipe exits on a slope away from the house).

Interior French drain (professional installation): The contractor cuts a channel in the concrete slab along the basement perimeter (using a concrete saw). Excavates 12-18 inches below the slab. Installs perforated pipe in gravel. Connects the pipe to a sump basin at the lowest point. Installs a sump pump in the basin. Routes the discharge pipe to the exterior. Pours new concrete over the trench (restoring the slab). The system collects water that seeps through the foundation walls and directs it to the sump pump for removal. Timeline: 2-5 days for a full basement perimeter.

Exterior French drains: Inspect the discharge point annually (clear debris, ensure the pop-up emitter opens freely). Check for standing water over the drain route (may indicate a clog). Avoid planting trees near the drain (roots can infiltrate the pipe). Don't drive vehicles over the drain route (can crush the pipe). A properly installed exterior French drain should last 20-30+ years with minimal maintenance.

Interior French drains: Maintain the sump pump (quarterly testing, annual cleaning — see our sump pump guide). The drain itself is largely maintenance-free (sealed under the slab). Monitor for water seepage (if the drain is working, you should see no water on the basement floor after rain).

Signs of drain failure: Standing water returns in previously drained areas. Water backing up at the discharge point. Gurgling sounds from the drain system. The sump pump runs excessively (may indicate a drain clog directing more water to the pump).

Prevention: Maintain proper grading around the house (soil should slope away from the foundation at 1 inch per foot for at least 6 feet). Extend downspouts at least 6-10 feet from the foundation. Keep gutters clean (overflowing gutters dump water right at the foundation). These three actions alone can prevent many drainage problems.