Curtain Drain vs. French Drain: Intercepting Hillside Water

Same Trench, Different Job

A curtain drain and a French drain look almost identical in the ground — a gravel-filled trench with a perforated pipe at the bottom. People use the names interchangeably, and contractors sometimes do too. But they're built to do two different jobs, and on a sloped Oregon lot, knowing which one you actually need can be the difference between a drain that solves the problem and one that's installed in the wrong place entirely.

The short version: a curtain drain intercepts water before it reaches the area you want to protect — it's a barrier across the path of incoming water. A French drain collects water within an area that's already wet. On hillside and sloped properties, where water travels downhill through the soil toward your house and yard, that distinction matters a lot. This guide draws the line clearly. For the broader context, start with property & site drainage in Oregon.

What a French Drain Does

A French drain is a collection system. It's placed within an area that has a water problem — a soggy lawn, a wet zone along a wall — and it gathers groundwater seeping up and through the surrounding soil, carrying it away through perforated pipe. The classic French drain is the answer to "my yard here is wet, collect that water and move it out."

The defining trait is that it works inside the wet area. It's reactive to water that's already present where you don't want it. Our French drain cost in Oregon guide covers how these are built and priced. A French drain is the right tool when the water problem is essentially local — the wet area is producing or holding water and you need to drain that spot.

What a Curtain Drain Does

A curtain drain is an interception system. Instead of sitting in the wet area, it's installed upslope of it — across the path that water takes coming downhill. As groundwater and near-surface water move down the slope toward your house or yard, the curtain drain catches them first and routes them away to the side, so they never reach the area you're protecting.

Think of it as a "curtain" hung across the hill. Water flowing downhill hits the gravel trench, drops into the perforated pipe, and is carried off laterally to an outlet, leaving the ground below it drier. The curtain drain is proactive — it stops the water before it becomes a problem downhill. This is the right tool when the source of your wet yard isn't the yard itself but water arriving from higher ground.

The Core Difference: Where the Water Comes From

The whole choice comes down to diagnosing where your water originates:

French Drain	Curtain Drain
Purpose	Collect water that's already in the area	Intercept water before it arrives
Placement	Within the wet area	Upslope, across the water's path
Best for	Local wet spots, soggy lawns	Hillside water flowing toward you
Approach	Reactive	Proactive

On a flat lot with a self-contained wet spot, a French drain fits. On a sloped lot where the wet area is being fed by water coming downhill from above, a curtain drain placed upslope often solves it more effectively, because it cuts off the source. Installing a French drain in the wet spot when the real problem is hillside water arriving from above can mean the drain is constantly fighting a flow it can't keep up with.

When You Have Hillside Water

Sloped and hillside properties are common across Oregon — the West Hills, the valley's rolling terrain, foothill lots. On these properties, a frequent pattern is:

• The lower part of the yard or the uphill side of the house stays wet.
• The wetness appears even when that area itself isn't the obvious source.
• Water is traveling downhill through the soil from higher ground and surfacing where the slope flattens or meets the structure.

This is the textbook case for a curtain drain. Placed across the slope above the wet area — or above the house — it intercepts the downhill flow and diverts it, drying out the ground below. It's also a key technique for protecting a foundation on a slope, working alongside footing drains; see our foundation drain installation cost guide. The curtain drain handles the water heading toward the foundation; the footing drain handles what reaches the foundation base.

The Outlet Requirement

Both drains share one non-negotiable: they need a viable outlet lower than the drain so the collected water can flow out by gravity. A curtain drain on a slope usually has this advantage — there's downhill terrain to daylight to. But the outlet still has to be planned: the intercepted water has to go somewhere safe, not just dumped where it re-enters the problem or floods a neighbor.

Without a proper outlet, neither drain works. This is one of the most common installation failures — a perfectly built trench with nowhere for the water to go. On sloped lots, identifying the daylight point is part of the design, and it's a key reason to involve a professional rather than guess.

Diagnosing Which You Need

Picking correctly comes down to answering: is the water arriving from uphill, or originating in the area itself?

• Walk the slope during or after rain. See where water travels and where it surfaces.
• Note whether the wet area is at the base of a slope or fed from higher ground — a strong sign for a curtain drain.
• Check whether the wet spot is self-contained and flat — more likely a French drain candidate.
• Look for seeps and springs higher up that feed the lower wet area.

Because the systems look so similar but serve different purposes, this diagnosis is where the value is. Our standing water drainage solutions guide helps read the clues, and a site assessment confirms whether interception or collection is the right strategy.

Get the Right Drain in the Right Place

A curtain drain and a French drain are close cousins built for different jobs. On Oregon's many sloped and hillside lots, getting the diagnosis right — intercept the hillside water, or collect the local water — determines whether your drainage actually works. Placement, depth, and the outlet all flow from that one decision.

Our excavation services include the slope evaluation needed to choose between interception and collection and to locate the outlet correctly. Request a free assessment and we'll determine which drain your lot needs. Every slope is different, so treat this as general guidance and get a site assessment before installing.