French Drain vs. Dry Well: Which Drainage Solution Does Your Portland Home Need?

Portland gets over 40 inches of rain a year. If you're dealing with standing water in your yard, a damp crawlspace, or water pooling near your foundation, you've probably started researching drainage solutions and landed on two terms: French drain and dry well.

Both solve water problems. But they work differently, they're designed for different situations, and choosing the wrong one means spending money on a system that doesn't fix the actual problem. This guide explains what each one does, when to use it, and how to decide which one your property needs.

What Is a French Drain?

A French drain is a gravel-filled trench with a perforated pipe at the bottom. It collects water along its entire length and carries it — by gravity — to a discharge point somewhere else on the property or into the storm system.

Think of it as an underground gutter for groundwater. Water in the surrounding soil seeps into the gravel, enters the perforated pipe through small holes, and flows downhill through the pipe to wherever you direct it — a dry well, a storm drain connection, a lower area of the property, or the street.

French drains are linear. They're installed in trenches that can run along a foundation perimeter, across a yard, or down a slope. A typical residential French drain trench is 12-24 inches wide and 18-48 inches deep, depending on the application.

French drains are best at: intercepting and redirecting groundwater that moves through the soil. They work along a line, collecting water over their entire length, which makes them ideal for situations where water is coming from a broad area — like a hillside above your house, a saturated yard, or soil around a foundation.

What Is a Dry Well?

A dry well is an underground chamber — usually a perforated plastic barrel, concrete ring, or gravel-filled pit — that collects a specific volume of water and lets it slowly soak (percolate) into the surrounding soil.

Think of it as an underground holding tank that releases water gradually. Water is piped into the dry well from a downspout, sump pump, or French drain, and then slowly disperses into the ground over hours or days. A typical residential dry well is 3-4 feet in diameter and 4-6 feet deep.

Dry wells are point solutions. They handle water at a single location, which makes them ideal for managing roof runoff from downspouts, receiving discharge from a sump pump, or handling concentrated water flow from a specific source.

Dry wells are best at: absorbing and dispersing a known volume of water from a specific source. They work at a single point, storing water temporarily and releasing it slowly so it doesn't cause erosion, flooding, or foundation damage at the surface.

Key Differences

How they handle water: A French drain collects and moves water horizontally along its length. A dry well collects and disperses water vertically into the ground at a single point.

What they connect to: A French drain needs somewhere to discharge — a dry well, storm drain, or lower elevation. A dry well IS the endpoint — it's where water goes to be absorbed.

Where they're installed: French drains run along foundations, across yards, and down slopes. Dry wells are installed in a single location, typically in the yard away from the foundation.

Soil dependency: Both depend on soil conditions, but dry wells are more sensitive. A dry well relies entirely on the surrounding soil's ability to absorb water (percolation rate). If your soil is heavy clay — common in parts of East Portland, Gresham, and Milwaukie — a dry well may fill up faster than it can drain, making it ineffective. French drains are less dependent on percolation because they move water to a discharge point rather than relying on absorption alone.

Volume capacity: A French drain can handle a continuous flow of water along its entire length — the longer the drain, the more water it manages. A dry well has a fixed capacity determined by its size and the soil's percolation rate. For heavy, sustained rain events (which Portland has regularly from November through March), French drains generally handle the load better.

When to Use a French Drain

Your yard has standing water or soggy areas after rain. Water that pools in low spots or saturates the lawn is groundwater that needs to be collected and redirected. A French drain installed across the wet area collects this water and carries it to a discharge point.

Water is seeping into your crawlspace or basement through the foundation walls. An exterior French drain installed around the foundation perimeter intercepts groundwater before it reaches the foundation. This is often paired with foundation waterproofing for a complete solution.

Your property slopes toward your house. If the lot grade sends water toward your foundation instead of away from it, a curtain drain (a type of French drain) installed uphill from the house intercepts the subsurface flow.

You need to move water a significant distance. French drains can run 50, 100, or 200+ feet, carrying water from a problem area to a safe discharge point far from the house.

When to Use a Dry Well

Your downspouts dump water right next to your foundation. Connecting downspouts to underground drain lines that empty into a dry well 15-20 feet from the house is one of the most common and effective uses. The water that was pooling at your foundation now soaks harmlessly into the ground in the middle of your yard.

Your sump pump needs somewhere to discharge. If your crawlspace has a sump pump, the discharge water needs to go somewhere that isn't right next to the foundation. A dry well gives it a place to absorb into the ground.

Portland stormwater code requires on-site management. Portland's stormwater regulations often require that rainwater be managed on your property rather than directed into the street or storm system. Dry wells are one of the approved methods for meeting this requirement, especially for new construction and major renovations.

Your soil drains well. If your soil is sandy, loamy, or has good percolation (common in parts of SW Portland, Tigard, and Sherwood), a dry well can be a simple, cost-effective solution for managing roof runoff.

When You Need Both

In many Portland homes, the best solution is a French drain that discharges into a dry well. This is especially common when there's no gravity outlet to a storm drain or street, and the French drain needs somewhere to send the water it collects.

For example: your crawlspace is damp because groundwater is moving through the soil toward your foundation. You install an exterior French drain around the foundation perimeter to intercept that water. The French drain needs to discharge somewhere — and if the nearest storm drain connection is too far away or uphill, the answer is a dry well in the yard. The French drain collects the water, the dry well absorbs it.

This combination is one of the most effective drainage solutions for Portland homes, especially in neighborhoods with heavy clay soil and high water tables during the wet season.

Cost Comparison in Portland

French drain installation: $25 – $50 per linear foot

A standard exterior foundation drain (50-80 linear feet) typically costs $3,000 to $6,000. A yard French drain for a soggy area runs $2,000 to $5,000. Interior crawlspace drains with a sump pump system cost $3,000 to $8,000.

Dry well installation: $2,000 – $5,000

This covers excavation, the dry well chamber, connecting drain lines, gravel, and backfill. Most residential dry wells receiving downspout drainage fall in the $2,000 to $3,500 range.

Combined French drain + dry well system: $4,000 – $10,000

The combined cost depends on the length of the French drain and the size of the dry well. For a typical residential project, this is the range.

For a detailed breakdown of excavation costs by project type, see our complete guide: How Much Does Excavation Cost in Portland, OR?

How to Tell What Your Property Needs

If you're not sure which system — or which combination — is right for your property, here's a simple way to start narrowing it down.

Where is the water coming from?

• Roof runoff (downspouts) → dry well is likely the primary solution
• Groundwater in the soil → French drain is likely the primary solution
• Both → you probably need both

Where is the water showing up?

• Pooling in the yard away from the house → yard French drain
• Wet crawlspace or basement → exterior foundation French drain, possibly with waterproofing
• Pooling at the base of the foundation near downspouts → downspout drain lines to a dry well
• General sogginess everywhere during rain → could be either or both — needs a site assessment

What does your soil look like?

• Sandy or loamy (drains quickly when you pour water on it) → dry wells work well
• Heavy clay (water sits on the surface for hours) → French drains are more reliable; dry wells may struggle

The most reliable way to determine the right solution is to have someone who understands Portland's soil and drainage patterns look at your specific property. Every lot is different, and the right system depends on grade, soil, water sources, and where you can discharge.

Get a Professional Assessment

At A&N Construction and Excavation, we install French drains, dry wells, and complete drainage systems across the Portland metro area. We'll walk your property, identify where the water is coming from and where it needs to go, and recommend the solution that actually solves the problem — not the most expensive option, the right one.

Call (503) 983-6984 or request a free estimate. We serve Portland, Beaverton, Hillsboro, Tigard, Lake Oswego, Gresham, Oregon City, Happy Valley, and 15+ surrounding cities.