How does a French drain work?

A French drain is a trench filled with gravel around a perforated pipe. Water in the surrounding soil takes the path of least resistance into the gravel and through the pipe's holes, then the pipe — sloped slightly downhill — carries the water by gravity to a discharge point or a sump pit. There are no moving parts; it works purely on gravity and the permeability of gravel.

What is the difference between an interior and exterior French drain?

An interior French drain sits inside the basement, in a channel cut into the slab at the footing, and routes water that has already seeped in to a sump pump. An exterior French drain (a footing drain) is buried outside the foundation at the footing to intercept groundwater before it reaches the wall, usually draining by gravity. Interior is cheaper and works year-round; exterior addresses water at the source but requires excavation.

What slope does a French drain need?

A French drain needs a continuous downhill slope so water flows by gravity. A common rule of thumb is at least 1 percent — about 1 inch of drop per 10 feet of run, often cited as a minimum of 1/8 inch per foot. Too little slope lets water sit; the pipe must fall consistently from the collection end toward the discharge or sump.

Do French drains need a sump pump?

An interior French drain almost always needs a sump pump because the under-slab pipe sits below the level where it could drain out by gravity, so the collected water must be lifted out. An exterior footing drain often does not need a pump if the site allows it to daylight downhill by gravity, though some exterior drains also route to a sump.

How long do French drains last, and do they clog?

A properly built French drain — pipe surrounded by clean gravel and wrapped in filter fabric where appropriate — can last for decades. The main failure mode is clogging: fine silt or soil migrating into the gravel and pipe, or roots intruding. Using filter fabric, clean angular gravel, and the correct pipe reduces clogging, and interior systems remain accessible for maintenance.

French Drains Explained: Interior vs Exterior & How They Work

The French drain is one of the oldest and simplest ideas in drainage — a trench full of gravel and a perforated pipe — and it’s the backbone of most basement waterproofing systems. The name is a little misleading: it has nothing to do with France. It’s named for Henry French, a 19th-century farmer who popularized the design for draining farmland. The concept has barely changed since, because it works.

This guide explains exactly how a French drain moves water, the crucial difference between interior and exterior systems in a basement, the materials and slope specs that make one work, and what each costs.

What a French drain is and how it works

In short

A French drain is a gravel-filled trench containing a perforated pipe. Water in the surrounding soil flows into the loose gravel and through the holes in the pipe, which is sloped slightly downhill so gravity carries the water away to a discharge point or a sump pit.

The genius is that there are no moving parts. It relies on two principles:

Water takes the path of least resistance. Loose, clean gravel is far more permeable than packed soil, so groundwater migrates toward the trench rather than building pressure against your foundation.
Gravity does the work. Once water enters the perforated pipe, a consistent downhill slope carries it away — to daylight on a hillside, to a dry well, or to a sump pit where a pump finishes the job.

That’s the whole machine: gravel to gather the water, a perforated pipe to channel it, and slope to move it.

A French drain: a perforated pipe in clean gravel collects water and carries it by gravity to a sump pump or daylight outlet.

Interior vs exterior: the critical distinction

In a basement, “French drain” can mean two very different installations. Confusing them is the most common source of mismatched quotes.

Interior French drain (sub-floor / drain tile)

An interior French drain sits inside the basement, in a channel cut into the concrete slab around the perimeter, against the footing:

A channel is cut into the slab at the wall-floor joint.
A perforated pipe is laid in gravel in that channel.
Water that seeps in at the cove joint — the most common entry point — drips down into the pipe.
The pipe slopes to a sump pit, where a pump lifts the water and discharges it outside.

Because the under-slab pipe sits below any gravity outlet, an interior system almost always needs a sump pump to lift the water out. It’s reactive — it manages water that has already entered — but it’s reliable, works year-round in any weather, and doesn’t disturb the yard.

Exterior French drain (footing drain)

An exterior French drain — properly a footing drain — is buried outside the foundation, down at the footing:

The soil is excavated to the footing along the affected walls.
A perforated pipe is bedded in gravel beside the footing.
The pipe is sloped to drain by gravity, ideally to daylight downhill, sometimes to a sump.
The trench is backfilled, often with a dimpled drainage board against the wall.

This intercepts groundwater before it reaches the wall, addressing the cause rather than the symptom. The trade-off is excavation: it’s expensive, slow, and disruptive to landscaping. Many homes have an original footing drain that has silted up over the decades and no longer functions.

For a full cost-and-disruption comparison of the two whole-basement approaches, see interior vs exterior waterproofing.

Side-by-side

Factor	Interior French drain	Exterior footing drain
Location	Inside, under the slab at the footing	Outside, at the footing
What it does	Manages water that gets in	Intercepts water before the wall
Needs a sump pump	Almost always	Often no (gravity to daylight)
Disruption	Concrete cut & patched inside	Major excavation, landscaping
Works in any weather	Yes	Install limited by weather/season
Addresses	Symptom	Cause
Relative cost	Lower	Higher

Materials and specs that make it work

A French drain is simple, but a few specifications separate a system that lasts decades from one that clogs in a few years.

Key French-drain specifications (confirm against local code and the pipe manufacturer)
Element	Typical spec	Why it matters
Slope	≥ 1% (about 1 in. per 10 ft)	Gravity moves water; flat pipe lets it sit and silt up
Pipe	Perforated PVC or corrugated HDPE	Holes let groundwater enter the pipe
Gravel	Clean, angular ¾-in. washed stone	High permeability; resists packing and clogging
Filter fabric	Wrap around gravel (exterior)	Keeps fine soil from migrating in and clogging
Trench width	Roughly the pipe diameter + gravel envelope	Enough gravel to collect and convey water

A few specifics worth understanding:

Slope is non-negotiable. Without a consistent downhill fall, water pools in the pipe and the drain fails. A common minimum is about 1 percent — roughly an inch of drop per ten feet of run.
Pipe orientation matters. For corrugated perforated pipe used in footing drains, manufacturers such as ADS publish guidance on perforation placement; the goal is to let water in along the run and carry it to the outlet.
Clean, angular gravel resists packing far better than rounded pea gravel or dirty fill, keeping the water paths open.
Filter fabric around an exterior gravel envelope is the main defense against the drain’s worst enemy: fine silt migrating in and clogging the system over time.

What it costs

These are regional ranges to sanity-check a quote, not the quote itself. The huge gap between a shallow yard drain and an interior or footing system is, once again, mostly labor and excavation — cutting concrete or digging to the footing.

When to choose which French drain

Choose an interior French drain if:

Your problem is classic seepage at the wall-floor joint or a damp slab.
You want the most reliable water control per dollar without digging up the yard.
The basement is finished and you want a year-round, weather-independent fix.

Choose an exterior footing drain if:

You’re already excavating for another reason, or the original footing drain has failed.
You want to intercept water before it loads the foundation at all.
The site slopes enough to let the drain daylight by gravity without a pump.

For most homeowners with ordinary seepage, the interior system plus a sump pump is the practical, cost-effective answer. Whichever you install, the system is only as reliable as the pump that empties it — which is why pump choice and a backup matter.

The pump is half the system

An interior French drain hands all its collected water to a sump pump. If that pump can’t keep up — or loses power in the storm that’s filling the drain — the drain backs up and the basement floods anyway. Two decisions matter:

Pump type. A quieter, higher-capacity submersible suits finished basements and busy drains; a pedestal is the budget pick for a narrow pit. We compare them in submersible vs pedestal sump pumps.
A backup. A storm big enough to fill the drain is the storm most likely to cut the power. A battery backup sump pump keeps the system working through an outage.

Why French drains fail — and how to avoid it

The design lasts for decades when built correctly, so most failures trace to a handful of mistakes:

Inadequate slope. Flat pipe lets water and silt sit. Verify a consistent fall to the outlet.
Dirty or rounded gravel. Packs down and loses permeability. Use clean, angular washed stone.
No filter fabric on an exterior run. Fine soil migrates in and clogs the gravel and pipe over time.
A discharge that goes nowhere useful. Routing the outlet back toward the foundation, or to a spot that floods, defeats the drain. Send it well away and downhill.
A neglected sump pump. On an interior system, the best drain in the world fails if the pump that empties it does.

Build it right, point the water somewhere sensible, and keep the pump healthy, and a French drain quietly does its job for the life of the house.

Bottom line

A French drain is gravel, a perforated pipe, and slope — a simple machine that collects water and lets gravity carry it away. In a basement, an interior French drain manages seepage and feeds a sump pump, while an exterior footing drain intercepts water outside before it reaches the wall. Get the slope, gravel, and filter fabric right, send the water somewhere sensible, and back the system with a reliable, backed-up pump, and it’s one of the most dependable fixes in all of basement waterproofing.