French Drain Installation in Lake Oswego, Oregon: Cost & Process

Why Lake Oswego Properties Need French Drains

Lake Oswego is built on hills. Homes climb the slopes around the lake and across Clackamas County's wooded ridges, where steep grades, mature trees, and heavy clay soil combine to make drainage a constant concern. Through the wet season — late October into April — Pacific rain soaks these hillsides, and water runs downhill toward homes built into the slope. The result is a familiar set of problems: water collecting against the uphill side of a foundation, saturated lawns below grade, and seepage into hillside crawlspaces.

A French drain is one of the most reliable answers. It is a gravel-filled trench with a perforated pipe at the bottom that intercepts subsurface water and carries it to a lower, safer exit point. On a Lake Oswego hillside lot, the most valuable version is often a curtain drain — a French drain positioned across the uphill side to intercept water before it reaches the house. Used the right way, it gives that downhill water a path to follow before it pools where you do not want it.

This guide explains how a French drain is installed on local hillside properties, what the process looks like step by step, and the factors that drive your cost. For statewide pricing ranges, see our French drain cost in Oregon guide, and for the bigger picture, our overview of property and site drainage in Oregon.

How a French Drain Works on Lake Oswego Hillsides

Water takes the easiest path. A trench packed with clean drain rock is far more permeable than the surrounding clay, so subsurface water flows into the gravel, drops to the perforated pipe, and runs downhill to daylight.

Lake Oswego's conditions shape the design more than almost anywhere in the metro area. The soil is clay-heavy and slow-draining, and the steep terrain means water is constantly moving downhill below the surface as well as on top. A plain French drain at the foundation deals with water that has already arrived — but on a hillside, the smarter move is usually to intercept it higher up. A curtain drain across the uphill side cuts off the flow before it loads the foundation.

The steep grade is also an advantage in one respect: hillside lots usually have plenty of fall, so finding a gravity outfall to daylight the drain downhill is rarely the problem it is on flat valley lots. The challenge in Lake Oswego is more often access, tree roots, and the depth required to intercept water moving through the clay.

The French Drain Installation Process

1. Site Assessment and Outfall Planning

A good installation starts with a site visit, not a phone quote. A contractor walks the property, finds where water collects, traces where it comes from on the slope, and identifies where the drain can daylight downhill. On Lake Oswego hillsides, mapping the incoming water and planning the interceptor line is the most important step.

2. Trenching

With the line laid out, the trench is excavated to the planned depth and slope. A curtain drain on a hillside often goes deeper than a simple yard drain to catch water moving through the clay. Steep terrain, tree roots, and limited machine access can make this the most demanding part of a Lake Oswego dig.

3. Fabric and Gravel Base

Non-woven filter fabric lines the trench to keep fine clay and silt from migrating into the gravel and clogging the system. In Lake Oswego's clay soils, this fabric is essential — skipping it is the most common reason a hillside drain fails within a few years. A base layer of clean, washed drain rock goes in first.

4. Perforated Pipe

A perforated pipe is set on the gravel base with the holes oriented to collect water, bedded to maintain consistent fall downhill to the outfall. Solid pipe is used for any section that only conveys water — such as a tie-in carrying roof runoff — so it does not leak collected water back into the slope.

5. Backfill and Restoration

More drain rock is placed around and over the pipe, the fabric is wrapped over the top to fully encase the gravel, and the trench is topped with soil, sod, or decorative rock. On a wooded hillside lot, restoring the slope and protecting against erosion at the outfall matters as much as the drain itself.

What Affects French Drain Cost in Lake Oswego

Local factors that drive the final number on a Lake Oswego installation:

• Terrain and access — Steep, wooded hillside lots with limited machine access push work toward hand digging and raise cost.
• Depth — Intercepting hillside water through clay often requires a deeper trench than a flat-lot yard drain.
• Tree roots and rock — Mature trees and buried rock slow excavation and can require careful work around root zones.
• Length — A curtain drain spanning the full uphill side of a lot adds trench and pipe.
• Restoration and erosion control — Stabilizing the slope and the drain outfall adds to the total.

For statewide pricing ranges and how each factor moves the number, see our French drain cost in Oregon guide. Because every hillside lot differs, the only accurate figure comes from an on-site assessment of your slope, soil, and outfall options.

When a French Drain Is the Right Call in Lake Oswego

On a Lake Oswego hillside, a French drain in the form of a curtain drain is frequently the single most effective drainage measure, because it stops water before it loads the foundation or saturates the lawn below. A plain French drain still has its place for a chronically wet low spot or a saturated lawn, and sometimes the right system pairs an uphill interceptor with a footing drain at the house.

What rarely works on a steep clay lot is a shallow drain installed without regard to where the water is coming from. A contractor who assesses your slope can tell you whether you need an interceptor, a foundation drain, or both, and at what depth to actually catch the water moving through the hillside.

Get Your Lake Oswego French Drain Quote

Cojo Excavation & Asphalt installs French drains, curtain drains, and full drainage systems for Lake Oswego and Clackamas County hillside properties. We start with a site assessment — checking your slope, soil, incoming water, and outfall options — then deliver a clear, no-obligation quote based on what your lot actually needs.

Request a free drainage estimate and we will respond within 24 hours. Learn more about our excavation services and how we help Lake Oswego homeowners manage water on hillside lots.