So, how deep should a French drain be? For most yards, 18 to 24 inches deep and about 12 inches wide is the right starting point. A drain protecting a foundation usually goes deeper, toward the footing, and here in Cincinnati our clay soil and 32-inch frost depth change the math more than most online guides admit.
The honest answer is that it depends on what you’re draining. After more than 40 years of building yard drainage solutions in Cincinnati, we’ve learned that the right depth comes from the problem, the soil, and where the water can go. Below, we’ll cover French drain depth by job, plus the slope, pipe, and gravel specs that decide whether a drain lasts.
How Deep Should a French Drain Be for Your Specific Problem?
A French drain is a gravel-filled trench with a perforated pipe at the bottom, wrapped in filter fabric. Water in the surrounding soil moves into the gravel, drops into the pipe, and flows by gravity to a safe outlet. Every French drain system works this way, so depth matters: the trench has to sit where the water actually is.
You’ll often see 12 to 18 inches quoted for lawns and 24 to 36 inches for foundations, with trenches 9 to 12 inches wide. Here’s how we size French drain depth for the situations we see most in Greater Cincinnati.
What you’re draining | Typical depth | Typical width |
Surface water, puddling, soggy lawn | 12 to 24 inches | 8 to 12 inches |
Groundwater near the house (curtain drain) | 24 to 36 inches | 12 inches |
Foundation protection (footing drain) | At or just below the footing | 12 inches or more |
Surface Water and Soggy Lawns
If you’re dealing with standing water after a heavy rain, pooling water in a low spot, or a lawn that stays spongy for days, the water is near the surface. An 18- to 24-inch trench intercepts it before it pools. We rarely go shallower than 12 inches, because a pipe that close to the surface is easy to crush and fills with grass roots.
Groundwater Moving Toward the House
On a sloped lot, water often travels downhill through the soil toward your home. A curtain drain set a few feet out and 24 to 36 inches deep cuts across that path, carrying excess water away to prevent water from reaching your foundation walls.
Foundation Protection
To protect your home’s foundation, the drain has to reach the level where water presses against the wall, usually at or just below the top of the footing. Digging next to a house takes care, because going too close or too deep can undermine the footing’s structural integrity. Good French drain design matters here, since poor drainage against a wall is a common cause of foundation damage.
Why Cincinnati Changes the Answer
Most national articles quote 18 to 24 inches and stop there, often assuming sandy soil. But soil type is one of the biggest factors in drain sizing, and ours behaves very differently.
Heavy Clay Soil Drains Slowly
Much of Southwest Ohio sits on heavy clay. Clay holds water tightly, which means slow drainage, so a drain in clay collects water from a smaller area than the same drain in sandy soil. That’s why we often recommend a wider trench and more gravel here, and why saturated soil lingers in so many Cincinnati yards for days after it rains. In some hillside neighborhoods, shallow shale or limestone bedrock can also shift the route or depth of a trench.
The Frost Line and Freeze-Thaw Cycles
The frost line used for footings in the Cincinnati area is about 32 inches. You’ll read that in colder climates, drainage pipes should sit below the frost line to prevent freezing. That’s true for footing drains, which already sit that deep. A yard French drain doesn’t need to. Freeze-thaw cycles punish flat spots and buried outlets, where trapped water freezes, so steady slope matters more here than extra depth.
French Drain Slope: Why Depth Changes Along the Trench
Here’s what most depth guides skip: a French drain isn’t one depth. It gets deeper as it runs, because the pipe has to fall toward the outlet.
The standard French drain slope is 1 percent, or about 1 inch of fall for every 8 feet of pipe. We aim for 1 to 2 percent when the yard allows. Over a 100-foot trench length, 1 percent adds about 12.5 inches, so a drain that starts at 18 inches can end near 30 inches at the outlet.
That’s why the outlet decides so much. Water has to leave the pipe somewhere lower than where it entered: a daylight outlet on a slope, a dry well, a rain garden (the EPA promotes rain gardens for stormwater), or a permitted storm connection. Discharging water in the wrong place causes erosion. A properly installed French drain goes downhill along its entire length.
French Drain Pipe Size and Type
The standard French drain pipe size for residential yards is 4 inches. It handles far more water than a typical yard produces. We rarely use 3-inch pipe, which clogs sooner and carries much less. A 6-inch pipe makes sense for long runs or when downspouts and a catch basin feed the same line.
For the pipe itself, we prefer rigid perforated PVC, which holds its slope and can be cleaned with a jetter. Corrugated pipe is cheaper and flexible, but it sags, traps sediment, and is hard to clean. Lay perforated pipe with the holes facing down so less sediment settles inside. A traditional French drain was a pipeless French drain, just a trench filled with stone, but pipe moves water faster in clay.
French Drain Gravel Size and Filter Fabric
Gravel does most of the work, so the type matters. We use clean, washed, angular stone between 3/4 inch and 1 1/2 inches, usually #57 stone in Ohio. Angular stone leaves open pockets for water movement, while pea gravel packs tight and slows the drain down.
Yes, you need gravel under the pipe. We put 2 to 3 inches of stone at the bottom of the trench, set the pipe on it, then fill around and over it. Line the trench with non-woven filter fabric (sometimes sold as landscape fabric) first and fold it over the top of the gravel. In clay, that fabric is the difference between a drain that works for decades and one that silts up in a few seasons.
To estimate gravel volume, multiply trench width by depth by length. A larger trench needs more stone. A 100-foot trench that’s 12 inches wide and 24 inches deep takes about 7.5 cubic yards, so plan the gravel needed before you start. Depth is one of the biggest drivers of French drain cost, and we break down what a French drain costs in Cincinnati in a separate guide.
Common French Drain Mistakes We Fix
Many of the drainage issues we’re called to fix come from drains that were installed, just not installed right:
- Too narrow or shallow in clay: the trench collects very little water.
- Flat spots or reverse slope: water sits in low sections, then freezes or clogs.
- Missing filter fabric: soil washes into the stone and slowly chokes the drain.
- Pea gravel or dirty stone: fine material packs down and blocks water flow.
- No real outlet: a drain that ends at a single point with nowhere lower to go just stores water.
Each of these quietly cuts the drain’s effectiveness. Done right, a French drain can last 30 to 40 years with basic maintenance, mostly keeping the outlet clear. If you’re weighing options, our guide on French drain vs. catch basin explains when a surface inlet is the better fit.
Where Not to Put a French Drain
Choosing the best location matters as much as the right depth. We avoid placing a French drain:
- Over underground utilities. Ohio law requires you to contact OHIO811 at least two working days before you start digging so underground lines get marked. Your local utility company won’t mark private sprinkler lines.
- Next to large trees, whose roots will find their way into perforated pipe.
- Where it discharges onto neighboring properties, which just moves the problem next door.
- Behind a retaining wall without a wall-specific design. We cover that in our guide to drainage behind a retaining wall.
When to Call a Drainage Contractor
A short, shallow drain across a side yard can be a reasonable DIY project. Deeper drains, long runs, and anything tied into downspouts are different, because slope, outlet, and soil conditions all have to line up.
When we plan a French drain installation, we start with where the water comes from, check how the soil drains, and plan the outlet before we start digging. Often the right answer combines a French drain with grading, a catch basin, or a dry creek bed, so your yard drainage and other drainage systems work as one. Learn more about landscape drainage in Cincinnati, or contact us to schedule a consultation.
Getting the Depth Right the First Time
How deep should a French drain be? Plan on 18 to 24 inches for surface water, 24 to 36 inches for groundwater near the house, and footing depth for foundation protection. In Cincinnati’s clay, gravel, fabric, and a steady slope to a real outlet matter just as much, and we’re glad to help you get it right.
French Drain Depth FAQs
What's the minimum depth for a French drain?
We don’t recommend going shallower than 12 inches for a piped French drain. Most yard drains work best at 18 to 24 inches deep, and drains that protect a foundation go deeper, toward the footing.
Do you need to put gravel under a French drain?
Yes. Put 2 to 3 inches of clean, washed, angular gravel (3/4 to 1 1/2 inches) under the pipe, then surround and cover it with the same stone. The gravel under the pipe keeps it on a steady slope and gives water a path into the pipe from below.
Does a French drain pipe need a slope?
Yes. A French drain needs at least a 1 percent slope, or about 1 inch of fall for every 8 feet of pipe. Without steady slope, water sits in the pipe and the drain loses capacity.
What type of pipe do you use for a French drain?
Most residential French drains use 4-inch perforated pipe. We prefer rigid perforated PVC because it holds its slope and can be cleaned, while corrugated pipe is cheaper but sags and traps sediment more easily.
How deep should you bury a French drain pipe in Cincinnati?
For most Cincinnati yards, we bury the pipe 18 to 24 inches deep, with the trench getting deeper as it falls toward the outlet. The local frost depth is about 32 inches, but a yard drain doesn’t need to go below it as long as the slope is steady and the outlet stays clear.
