How to Add a French Drain Around a Wet Foundation (2024)

TL;DR: A French drain is a gravel-filled trench with a perforated pipe that intercepts groundwater and carries it away from your foundation before it seeps through basement walls or footings. DIY materials run $10-$25 per linear foot, while a professional install costs $50-$100 per foot depending on soil and depth. The pipe needs at least a 1% slope (1 inch of drop per 8 feet) to drain properly, and the trench should sit below the frost line and against the footing for a true interior or exterior perimeter system.

_Last reviewed: July 2026 · 8 min read_

Water stains on the basement wall, a musty smell that won't go away, or soil that stays soggy for days after it rains — these are signs your foundation is fighting a losing battle against groundwater. A French drain fixes the root cause instead of just moving the puddle.

Okoniq Property Hub logs drainage work and photos by property so you have a record of what was installed and when, useful if a buyer or inspector asks later.

What is a French drain and how does it actually work?

A French drain is a trench dug alongside or around a foundation, lined with landscape fabric, filled with gravel, and fitted with a perforated pipe that carries water away by gravity. Water enters through the perforations, gets guided along the pipe, and exits at a daylight point, dry well, or storm drain far enough from the house to stop it from cycling back.

The system only works if it slopes downhill the whole way. Most contractors aim for a 1% grade, meaning the pipe drops 1 inch for every 8 feet of run. Anything flatter and water sits in the pipe instead of moving, which defeats the purpose and can lead to silt buildup inside the perforations within a few years.

There are two versions worth knowing. An exterior French drain sits against the foundation footing, outside the wall, and catches water before it ever touches the concrete. An interior version runs along the basement floor's edge, inside the house, and catches water that has already gotten through the wall, directing it to a sump pump. Exterior systems prevent the problem; interior systems manage it after the fact. If you're also dealing with downspouts dumping water in the wrong place, fix that first, since a French drain can't keep up if a downspout is adding hundreds of gallons right at the same spot.

How deep and how far from the foundation should the trench go?

The trench should run at least 12 inches below grade and ideally down to the footing, roughly 18-24 inches deep for most residential foundations, with the pipe set 6-12 inches out from the wall. Depth matters more than most people expect. A shallow trench catches surface water fine, but it misses the groundwater pressing against the footing, which is usually the actual source of basement seepage.

Width is simpler: an 8 to 12-inch trench gives enough room for gravel on both sides of a 4-inch perforated pipe. Use ¾-inch washed gravel, not crushed stone with fines, because fines clog the drain over time. Wrap the whole trench in landscape fabric before backfilling so soil doesn't migrate into the gravel and choke the system within a few seasons.

If you're seeing water inside rather than just wet soil outside, check for signs of a slab leak first. A slab leak is a plumbing problem, not a drainage problem, and a French drain won't touch it.

DIY or hire a pro — what's the real cost difference?

DIY French drain material costs run $10-$25 per linear foot, while a fully professional install with excavation, gravel, and finish grading runs $50-$100 per linear foot, sometimes more if a concrete patio or walkway needs to be cut and replaced. For a typical 100-foot perimeter, that's a $1,000-$2,500 DIY project versus $5,000-$10,000 professionally.

| Factor | DIY | Professional | |---|---|---| | Cost per linear foot | $10-$25 | $50-$100 | | Time | 2-4 weekends | 1-3 days | | Equipment | Rented mini-excavator or hand digging | Owned excavation equipment | | Risk | Wrong slope, hitting utilities | Warranty on workmanship |

The deciding factor is usually soil and access. Clay-heavy soil is exhausting to hand-dig and often needs a mini-excavator rental at $200-$400 per day. If your yard has mature trees, buried utility lines, or a septic system nearby, call 811 before digging and strongly consider a pro, since hitting a gas or electric line turns a weekend project into an expensive emergency.

Where should the water actually go once it leaves the pipe?

Water needs a discharge point at least 10 feet from the foundation, either daylighting downhill, feeding into a dry well, or tying into a storm drain where local code allows. Dumping the outflow 3 feet from the house just relocates the wet spot without solving anything, and in freeze-thaw climates it can create ice buildup right next to the wall.

A dry well is a gravel-filled pit, usually 3-4 feet deep and lined with fabric, that lets water percolate slowly into the soil away from the house. They work well on flat lots where there's no natural downhill slope for daylight drainage. Check with your city before tying into a municipal storm drain, since many jurisdictions restrict or meter this connection, and an unpermitted tie-in can cause a headache if the property sells.

While you're regrading for drainage, it's worth confirming your gutters aren't part of the problem. Downspout mistakes that send water into the basement are a common reason a brand-new French drain still doesn't fully dry things out, because the drain is fighting two water sources instead of one.

FAQ

How long does a French drain last?

A properly installed French drain with washed gravel and fabric typically lasts 20-30 years before the pipe needs cleaning or the fabric needs replacing due to silt intrusion.

Can I install a French drain myself in a weekend?

A short 20-30 foot section along one problem area is realistic in a weekend, but a full perimeter system around a house is usually a 2-4 weekend project once you account for slope-checking and gravel hauling.

Will a French drain fix a wet basement on its own?

Often not by itself. Grading, gutters, and downspout extensions need to be addressed together, since a French drain intercepts groundwater but can't handle roof runoff that's still being dumped near the foundation.

Do I need a permit to install a French drain?

Many jurisdictions don't require a permit for a standard exterior French drain, but tying into a municipal storm drain or working near a property line often does. Check with your local building department before digging.

What's the difference between a French drain and a curtain drain?

A curtain drain is a shallower, simpler version placed upslope of a house to intercept surface water before it reaches the foundation, while a true French drain sits at the footing depth and handles groundwater pressure directly against the wall.

This is educational information, not engineering advice. Call a licensed foundation or drainage contractor if you're seeing structural cracks, active water intrusion, or aren't sure how deep your footing sits.