French Drain Installation: A Complete Guide for Pacific Northwest Homeowners

If your yard has standing water after rain, a soggy basement, or a slope that sends water toward your house — a French drain is almost certainly the fix. It's one of the most effective drainage solutions for western Washington's clay-heavy soils and heavy rainfall.

This guide covers everything: how French drains work, what materials you need, how to calculate drainage rock quantities, and how to install one yourself (or brief a contractor).

What Is a French Drain?

A French drain is a gravel-filled trench that redirects groundwater and surface water away from problem areas. At its simplest: a perforated pipe wrapped in filter fabric, buried in gravel, directing water downhill to a safe outlet.

French drains solve:

• Standing water that won't drain after rain
• Wet basements and crawl spaces
• Soggy lawn areas that stay wet all winter
• Water pooling against foundations
• Slope runoff heading toward structures
• Retaining wall drainage (weeping walls)

How a French Drain Works

Water always moves toward lower pressure — and toward air space. A French drain creates a preferred path:

1. Water saturates soil and reaches the gravel-filled trench
2. Water flows through the loose gravel (higher permeability than surrounding soil)
3. Water enters the perforated pipe through small holes
4. Water travels down the pipe to the outlet (daylighting on a slope, or into a dry well/drain)

The filter fabric wrapped around the pipe and gravel prevents soil from migrating into the system over time.

Materials You'll Need

Drainage Rock (The Most Important Material)

The backbone of a French drain. You need clean, washed drain rock — no fines (dust/sand) that would clog the system over time.

Best choices:

• 3/4" Washed Gravel or Pea Gravel: Most common. Good void space for water movement. Available for delivery from Harbor Soils.
• 1-1/2" Clean Crushed Rock: Better for heavy flow areas. More void space.
• Drain Rock (specifically labeled): Pre-washed, sized for drainage applications.

Avoid: Any gravel with fines, sand, or "minus" material (e.g., 3/4" minus). These clog drain systems. You want clean, angular rock.

Perforated Pipe

• 4-inch perforated ADS pipe (flexible corrugated) for most residential applications
• 4-inch solid PVC for outlet runs where you don't want water to exit
• 6-inch diameter for high-volume applications (large catchment areas, heavy clay)

Filter Fabric (Geotextile)

• Wraps around the gravel to prevent soil migration
• Use non-woven geotextile fabric rated for drainage
• Some perforated pipes come pre-wrapped in a sock — skip the extra fabric layer if so

Other Supplies

• Stakes or landscape pins to hold filter fabric
• Couplings and elbows for pipe connections
• Outlet cover or splash block at the terminus
• Shovel, trenching spade, or rented trencher

How Much Drainage Rock Do You Need?

Calculate based on your trench dimensions:

Trench Width (ft) × Trench Depth (ft) × Length (ft) ÷ 27 = Cubic Yards of Drain Rock

Typical French drain trench: 12 inches wide × 18-24 inches deep

Drainage Rock Calculator by Trench Length

Add 10-15% for waste — you'll lose some rock during trench backfill uneven spots and compaction.

Step-by-Step Installation

Step 1: Plan Your Route

• Water must flow downhill — French drains need a minimum slope of 1% (1 inch per 10 feet). More is better.
• Identify where water will exit. Ideally this "daylights" — comes out on a slope or into a swale. If no downhill option, you need a dry well.
• Call 811 before digging to mark utilities.
• Sketch your route. In western Washington, a perimeter drain around the foundation is common; for yard drainage, a simple trench from low point to outlet often suffices.

Step 2: Dig the Trench

• Standard dimensions: 12 inches wide, 18-24 inches deep
• For foundation drains, go deeper — the bottom of the pipe should be at or below the footing
• Maintain consistent slope throughout (use a level and string line)
• A rented electric trencher makes this job dramatically faster for long runs

Step 3: Line with Filter Fabric

• Lay filter fabric in the trench, draping up the sides
• Leave enough overhang to fold over the top of the gravel later (you'll fold it over like a burrito)
• Use landscape pins to hold fabric in place on walls

Step 4: Add a Gravel Bed

• Pour 3-4 inches of drain rock into the bottom of the trench
• Level it roughly — doesn't need to be perfect

Step 5: Lay the Pipe

• Place perforated pipe on the gravel bed, holes facing down (this is a debated point — some say up, some say down; down allows water to enter from below where it collects)
• Connect sections with couplings; use elbows for turns
• Run solid pipe at the outlet end to direct water to the exit point

Step 6: Fill with Drain Rock

• Pour drain rock around and over the pipe, filling to within 3-4 inches of the surface
• Leave room for the fabric fold-over and topsoil cap

Step 7: Fold and Cap

• Fold the filter fabric flaps over the top of the gravel — this prevents soil from migrating down
• Cover with topsoil or sod to restore the surface
• If you want visible access, use decorative rock as the surface layer instead

Pacific Northwest–Specific Notes

Clay Soils

Kitsap and Pierce County clay soils make French drains both more necessary and more challenging. Clay sheds water rather than absorbing it, causing surface runoff. Your drain needs to intercept this runoff before it pools.

In heavy clay, consider using a larger drain rock size (1.5" clean crushed) and a wider trench (18 inches) to improve capacity.

High Water Table Areas

Near Puget Sound or in low-lying areas of Gig Harbor and Port Orchard, water tables can be seasonally very high. A French drain alone may not fully solve the problem — you may need a sump pump system in addition.

When to Call a Pro

DIY is fine for yard drainage issues. Call a contractor if you're dealing with foundation drainage, basement waterproofing, or a problem that involves more than 100 feet of trench.

Frequently Asked Questions

How deep should a French drain be?

For surface water and yard drainage, 18-24 inches is standard. For foundation drainage, the pipe should sit at or below the footing (often 3-5 feet deep). The deeper the trench, the more groundwater you can intercept.

What type of gravel is best for a French drain?

Use clean, washed drain rock — 3/4" or 1-1/2" angular crushed rock with no fines. Avoid "3/4 minus" or any material with sand or fine particles. Pea gravel is acceptable but round gravel has less void space than angular crushed rock.

How long do French drains last?

A properly installed French drain with quality materials and filter fabric typically lasts 30-40 years. Systems without filter fabric can clog with soil migration within 5-10 years in clay soils.

Can I install a French drain myself?

Yes — for yard drainage issues, this is a manageable DIY project. The main challenge is the physical labor of trenching. Rent a trencher to make it much faster. The material cost is mainly the drain rock and pipe.

Need drainage rock delivered? Harbor Soils delivers clean drain rock, 3/4" washed gravel, and crushed rock throughout Gig Harbor, Port Orchard, and Kitsap County. Same-day delivery, no minimum order. Order gravel → | See our drainage solutions guide →