Perimeter Drain Installation: French Drains, Costs, and What to Expect

A perimeter drain — commonly called a French drain — is a gravel-filled trench with a perforated pipe that intercepts water and routes it away from structures or wet areas. In Seattle’s wet climate, French drains are one of the most effective tools for managing yard drainage, protecting foundations, and keeping water away from crawl spaces and basements. Here’s how they work, what they cost, and when they’re the right solution.

How Much Does It Cost to Install a French Drain?

Seattle area (2026):

What drives cost variation:
– Length of the drain run
– Depth required (Seattle’s frost depth is 12–18 inches; deeper runs cost more)
– Soil conditions (hard clay requires more excavation effort)
– Outlet point (daylight vs. storm drain vs. dry well)
– Whether existing landscaping or hardscape must be removed and replaced
– Interior vs. exterior installation

Use the cost estimator for current rates in your area.

Does a French Drain Really Work to Stop Yard Flooding?

Yes — for the right problem. A French drain works by intercepting water before it reaches the area it would otherwise flood. The effectiveness depends on matching the drain location and design to the specific flooding cause.

Works well for:
– Water running off a slope into a flat yard area
– Surface water collecting in a low area with a clear outlet path available
– Water running along a property line from a higher neighboring property
– Foundation perimeter drainage to prevent water from pooling against the foundation

Works less well for:
– A high water table rising from below the surface — a French drain that’s shallower than the water table doesn’t intercept the water; a sump system is needed
– Surface flooding where there’s no feasible outlet at a lower elevation than the problem area
– Very short seasonal flooding that stops within hours — the installation cost may exceed the nuisance level

The outlet requirement: A French drain needs somewhere to send the water. Common outlets: a lower yard elevation (daylight at the yard edge), a catch basin connected to the storm drain, or a dry well (underground percolation area). If no downhill outlet exists, the drain collects water but can’t drain it.

Can I Install a French Drain Myself?

Yes — with realistic expectations:

DIY-appropriate for:
– Straightforward runs with no obstacles, available outlet, and accessible soil
– Short runs (20–40 feet) in a yard with soft or loamy soil
– Simple installation with no need for landscape restoration

What’s involved:
1. Dig a trench 12–24 inches deep, 6–12 inches wide, along the planned drain path
2. Line the trench with filter fabric (prevents soil from entering the gravel)
3. Add 2–3 inches of gravel at the bottom
4. Lay the perforated pipe (holes down) on the gravel bed
5. Fill with clean drainage gravel (3/4-inch crushed rock) to within 4–6 inches of the surface
6. Fold filter fabric over the top of the gravel
7. Top with topsoil and seed or lay sod

Seattle-specific challenges for DIY:
– Clay soil is harder to dig than loam — rental trenching equipment helps significantly
– Disposing of excavated clay soil — it’s not suitable as fill elsewhere in the yard and may need to be hauled away
– Finding a proper outlet — routing water to an appropriate outlet location (not toward a neighbor, not into a sewer) requires planning

When to hire a contractor: For runs longer than 40 feet, in tight spaces, near the foundation, or where outlet routing is complex.

Exterior vs. Interior French Drain — Which Is Better?

These are two different applications:

Exterior French drain (around the foundation perimeter):
– Installed outside the foundation wall, typically 2–4 feet from the wall
– Intercepts surface water and subsurface water before it reaches the foundation
– Requires excavating around the foundation exterior
– Most effective when combined with exterior waterproofing membrane on the foundation wall
– Disrupts landscaping during installation
– Long-term: prevents water from reaching the foundation in the first place

Interior drain tile system:
– Installed inside the basement perimeter, below the floor slab
– Collects water that has already entered through the walls or floor and routes it to a sump
– Requires cutting the basement perimeter slab (demolition and repair)
– Less disruptive to exterior landscaping
– Manages water entry — doesn’t prevent it from entering the foundation system
– Often the only practical option when exterior access is limited

Which to choose:
– If exterior access is available and the problem is approaching but not yet severe: exterior drain
– If the foundation already has water entry and interior work is more practical: interior drain tile + sump
– If the water table is the issue (water rising from below): interior drain tile + sump is the appropriate solution

How Deep Does a French Drain Need to Be?

For yard drainage (intercepting surface runoff):
– 12–18 inches deep is typically sufficient to intercept surface and near-surface runoff
– The drain pipe should be at the depth where water is accumulating

For foundation perimeter drainage:
– The drain should be at or below the footing level to capture water before it reaches the foundation wall
– Typical Seattle residential footings are 18–36 inches below grade
– Foundation perimeter drains: 24–48 inches deep

For intercepting subsurface water (high water table):
– The drain must be at or below the seasonal high water table depth
– In flat Seattle neighborhoods, water table may be only 3–4 feet below grade in the wet season
– Drains at 18–24 inches may be above the water table and ineffective — needs deeper installation or a sump system

Frost depth consideration: Seattle’s frost depth is approximately 12 inches. Drain pipes installed above frost depth may freeze in extreme cold, though Seattle’s mild climate makes this uncommon. Standard installation at 12–18 inches is below frost depth for most Seattle winters.

How Long Does a French Drain Last Before It Needs Replacing?

Well-installed French drain lifespan:
– The perforated pipe itself: 25–50 years for corrugated HDPE pipe, longer for smooth interior
– The gravel and filter fabric: the filter fabric is the failure point — it can silt up over 10–25 years if soil fines infiltrate past the fabric
– Functional lifespan with minimal maintenance: 20–30 years

What causes French drains to fail:
– Filter fabric silting up (soil gradually enters gravel and pipe, reducing permeability)
– Root intrusion into the perforated pipe
– Gravel compaction reducing void space
– Outlet blockage (leaves, sediment) at the terminus

Signs a French drain is no longer working:
– Areas that were dry after installation are flooding again
– Outlet pipe isn’t discharging during rain events
– Camera inspection of the pipe shows sediment or root intrusion

Maintenance to extend life:
– Flush the pipe periodically (hose from the inlet end to the outlet)
– Keep the outlet clear of debris
– Inspect the outlet for root intrusion every few years

Does a French Drain Require Maintenance?

Minimal but not zero:

Annual:
– Clear debris from any inlet (catch basin, surface inlet) connected to the drain
– Confirm outlet is open — stick a hose down the outlet and confirm flow during dry weather

Every 5–10 years:
– Flush the drain pipe from inlet to outlet with a hose to remove accumulated fine sediment
– Inspect the outlet for root intrusion

When it stops working:
– Camera inspection of the pipe identifies whether the blockage is in the pipe (root, sediment) or at the outlet
– Hydro-jetting clears sediment and minor root intrusion in the pipe

What maintenance can’t address: Filter fabric that has completely silted up requires excavation and replacement of the gravel and fabric. At that point, a full reinstallation may be the practical option.

Will a French Drain Fix My Wet Basement?

Partially — it depends on the water source.

French drain helps when:
– Surface water and near-surface subsurface water are entering through the foundation walls
– An exterior perimeter drain intercepts this water before it reaches the foundation

French drain doesn’t help when:
– The basement floods from water rising from below the floor through hydrostatic pressure — this is a water table issue, not a surface water issue. A French drain shallower than the water table doesn’t intercept it.
– Water enters through a specific crack that isn’t addressed by the drain location

The realistic Seattle basement answer:
– Exterior foundation drain (12–24 inches deep) + exterior waterproofing membrane: addresses surface water entry through walls
– Interior drain tile + sump pump: addresses water table pressure from below
– Both systems together: comprehensive solution for basements with multiple water entry sources

If basement flooding occurs only during heavy rain: Surface water is likely the cause — exterior French drain may resolve it. If it floods when the water table is high (mid-winter even without recent rain), a sump is needed regardless of what surface drainage is installed.

Do I Need a Permit to Install a French Drain?

Typically no for standard French drain installation on private property.

Exceptions that may require a permit or approval:
– Work in the public right-of-way (planting strip, near the curb) — SDOT may require a permit
– Connecting a French drain to the public storm drain system — Seattle Public Utilities (SPU) permit required
– Work near wetland buffers or sensitive areas — King County or City permit may be required
– Significant grading changes that alter drainage patterns to neighboring properties

Discharge location matters: Seattle prohibits discharging French drain water into the sanitary sewer. Discharge should be to a storm drain (with appropriate permit), to a dry well, or to daylight in the yard at a safe distance from the foundation and property lines.

How Long Does French Drain Installation Take?

Typical project timeline:

What extends the timeline:
– Rock or debris encountered during excavation
– Need to route pipe through or around obstacles (tree roots, utilities)
– Outlet pipe requiring significant distance to reach a drainage point
– Landscape restoration (sod, replanting) after installation

FAQ

Q: How much does it cost to install a French drain?
A: $2,500–$8,000 for a typical 50–80 foot residential French drain in Seattle, including excavation, gravel, filter fabric, perforated pipe, and outlet. Shorter runs are proportionally less; interior basement drain tile systems cost $6,000–$15,000.

Q: Does a French drain really work to stop yard flooding?
A: Yes — for surface water and near-surface subsurface water. A French drain intercepts water before it reaches the problem area and routes it to an outlet. It doesn’t work if the water source is a high water table rising from below the drain depth — that requires a sump pump.

Q: What’s the difference between an exterior and interior French drain?
A: An exterior French drain (around the outside of the foundation) intercepts water before it reaches the foundation. An interior drain tile system collects water that has already entered through the foundation walls and routes it to a sump pump. Interior systems are more common for basement flooding because they’re less disruptive to install.

Q: How long does a French drain last?
A: 20–30 years with minimal maintenance. The gravel and filter fabric can silt up over time; periodic flushing of the pipe extends the useful life. The perforated pipe itself lasts 25–50 years.

Q: Do I need a permit to install a French drain?
A: Generally no for standard yard installation. Permits may be required for work in the public right-of-way or for connections to the public storm drain system. Discharge must not connect to the sanitary sewer — route to storm drain, dry well, or daylight.