What is a French drain? How it works and when it helps

1. Filter fabric, where the soil needs to be kept out of the stone
2. Clean, washed stone
3. Perforated drainage pipe
4. With rigid two-row pipe, holes facing down and to the sides (about 4 and 8 o’clock)
5. Soil or stone cover

SoilConcrete / masonryWoodInsulationWater

How a French drain works

The trench is filled with clean stone around a perforated pipe. Water in the surrounding soil finds it easier to move into the stone than to keep going through the ground, gets into the pipe and runs downhill to an outlet.

For me, the name doesn’t matter much. What matters is which water you’re trying to intercept and where you’re going to send it.

When a French drain helps, and when it doesn’t

It helps when water is moving through the ground and can be intercepted before it gets where you don’t want it:

• shallow groundwater;
• water moving down a slope;
• a wet area next to the foundation;
• water seeping just under the surface of a hillside.

It isn’t my first fix for a short downspout, a clogged gutter or a small low spot next to the house. Those have cheaper answers in yard grading and downspout drainage. And it doesn’t do much if there’s nowhere for the water to go. I’ve seen too many French drains installed because a French drain was what the contractor sold.

Where the water goes

• Daylight: the simplest outlet, if there’s a lower point on the property.
• A sump pump: when the water can’t drain by gravity. See sump pumps.
• An approved storm drain: when there is one and it’s allowed.
• A dry well: only when the soil, the amount of water and the site let that water soak in.

A dry well doesn’t make water disappear; it stores it for a while as it soaks in. If the soil is already saturated or doesn’t drain well, it may not be a good solution. If a French drain has no outlet and isn’t designed to let water soak into the ground, what you have is a trench full of gravel and water.

French drain, curtain drain, footing drain, swale or dry well

They aren’t five names for the same thing.

Depth, slope, stone and fabric

I don’t have one depth for every French drain. It depends on which water you’re trying to catch. A curtain drain has to be deep enough to cut off the flow you’re after. Next to a footing, the relationship with the footing and the foundation is much more delicate. For slope, my field rule is a continuous downhill fall with no sags, and I wouldn’t publish a single slope as the rule for every French drain. The stone should be clean, washed stone, not material full of fines that ends up clogging everything. The pipe should be perforated pipe made for drainage.

On holes up or down: with rigid pipe that has two rows of holes, I usually set them facing down and to the sides, at about the 4 and 8 o’clock positions, so the water can enter the pipe from below before it fills the whole trench. That’s an installation practice for that kind of pipe, not a universal rule. Some pipes have holes all the way around or their own instructions, and I follow the system I’m installing. I use filter fabric when I need to keep the surrounding soil out of the drainage stone, but I don’t want to wrap everything in a way that turns the wrong fabric into the next place it clogs.

Next to the foundation

Daniel Mercer stops treating it as a simple landscaping job when the trench gets near a footing. Before anything else, call 811: Virginia 811 says homeowners must contact it for all digging projects, at least three working days before they dig.

Then he wants to know where the footing really is and how deep it goes, what kind of wall it is, whether the soil next to the wall is backfill, whether the wall has cracks or bowing, how much of it will be opened up, and what loads sit above or next to the trench. His rule is simple: don’t dig below the bottom of a footing right next to it without a design that accounts for how it will stay supported. He wouldn’t use a universal 45-degree rule to say any excavation is safe, because soil conditions vary too much. A shallow trench for a downspout several feet from a healthy foundation is a very different situation from digging down to the footing along a basement wall.

He would ask for an engineer’s evaluation first if:

• the trench reaches the level of the footing or goes below it;
• a whole basement wall has to be dug out;
• the foundation is old stone or masonry;
• the wall is already bowing;
• there’s significant cracking;
• there’s settlement;
• nobody knows how the footing was built;
• there’s a driveway, a retaining wall or another load next to the excavation;
• the contractor wants to open a long, deep stretch all at once.

Why French drains clog

• soil fines and silt;
• roots;
• iron deposits, in some areas;
• crushed pipe;
• a blocked outlet;
• too little fall, so sediment settles in the pipe.

Signs that it’s clogged:

• the wet area comes back;
• almost no water comes out of the outlet;
• the sump gets much less water than it used to;
• there’s standing water in a cleanout;
• the drain works in one part but not another.

If there are cleanouts, I try flushing the line or running a camera first. Roots or sediment in one spot can be cleaned out. A crushed pipe, a trench completely full of silt or a system built with no outlet usually has to be opened up and redone.

Interior and exterior French drains

An exterior drain intercepts or drains water before it ends up inside. An interior drain collects the water once it has reached the foundation and carries it to a sump or another outlet. The two are compared in detail in interior vs. exterior basement waterproofing.