French drain design: slope, depth, and where the water actually goes

A french drain is deceptively simple — a gravel-filled trench with a perforated pipe that collects groundwater and carries it somewhere it can do no harm. Getting the materials right is arithmetic: trench volume minus pipe volume gives you gravel, and length gives you pipe and fabric. But a drain built to exactly the right spec will still fail if the design is wrong — if it does not slope, if it is too shallow to intercept the water, or if it dumps into a spot that floods right back toward the house. Design first, then calculate quantities.

Slope is the whole point: aim for 1% minimum

Water in a french drain moves by gravity, so the trench bottom must fall continuously from the collection end to the outlet. The widely used minimum is a 1% grade — about 1 inch of drop for every 8 feet of run, or roughly 1 foot over 100 feet. Less than that and water sits in the pipe, silt settles out, and the drain clogs from the inside. More slope is fine and often better. The critical rule is that the fall must be continuous: a trench that dips and rises along its length traps water in the low spots no matter what the average grade is. Run a string line with a level, or shoot the grade with a laser or transit, before you set the pipe — this is the single most common thing DIY drains get wrong.

How deep to dig depends on what you are draining

Depth is a design decision, not a default. A drain meant to keep surface runoff out of a soggy lawn can be relatively shallow, while one protecting a foundation or intercepting a high water table needs to sit below the level of the water you are trying to stop. A footing drain, for instance, belongs down near the base of the foundation footing so it captures water before it reaches the wall — often several feet down. A standard planning depth of a foot and a half is a reasonable middle ground for yard drainage, but the honest question is always "how deep is the water I am trying to intercept?" The drain has to be below it to catch it.

Which way do the pipe's holes face?

Perforated drain pipe has holes on one side, and there is a persistent argument about orientation. For a french drain whose job is to collect groundwater rising from below and around the trench, the conventional guidance is holes facing down. Water enters the pipe from the bottom as the surrounding gravel fills, and the pipe carries it away along the slope. Holes-down lets the pipe start draining before the water level rises high, and keeps the pipe emptying rather than holding standing water. (Pipe used to carry roof or surface water to a drain is a different application and is typically run solid or holes-up — do not confuse the two jobs.)

Landscape fabric: the part that decides its lifespan

The failure mode of an old french drain is almost always the same: fine silt and soil migrate into the gravel, fill the voids, and the trench stops moving water. Wrapping the gravel in a permeable landscape fabric — lining the trench walls before you add stone and folding it over the top like a burrito — lets water pass while keeping soil out. It is a small fraction of the cost and the difference between a drain that lasts a couple of years and one that lasts decades. This is why a materials estimate should include fabric for the full trench surface, not just treat it as optional.

Where the water goes is not optional

A drain has to discharge somewhere lower than its collection end and far enough from the house that the water does not simply seep back. Common outlets are:

• Daylighting — the pipe emerges at a lower point on the property (a slope, a swale, a ditch) and water simply exits. Simplest and most reliable when the grade allows it.
• A dry well — a buried gravel-filled pit or chamber that holds water and lets it percolate into the soil, used when there is no lower ground to daylight to.
• A storm drain or approved discharge point — where local codes permit connection.

What you cannot do is run the drain to a spot that is not actually lower, or one that pools against the foundation, a neighbor's yard, or a walkway. Check local codes too — many areas regulate where you may discharge collected water, and dumping onto a neighbor's property can be a real liability. Plan the outlet first, because it sets the elevation everything else slopes toward.

Design, then quantify

Once the route, slope, depth, and outlet are settled, the materials fall out of the dimensions: trench length, width, and depth give you the gravel (minus the volume the pipe displaces), the length gives you pipe and fabric, and gravel's weight-per-cubic-foot converts volume into the tons a supplier actually sells. Our french drain calculator takes your trench length, width, and depth, your pipe diameter, and local material prices, and returns the gravel in cubic yards and tons, the pipe and fabric, and a total materials cost — so once your design is right, ordering the right amount is one step, not a spreadsheet.