How to Plan a French Drain

A French drain is one of the most useful tools for a soggy yard, but most failures trace back to planning, not digging. Before you rent a trencher, the questions that matter are simple: where is the water coming from, and where can it legitimately go? Get those two answers right and the rest is detail work.

The short answer

Planning a French drain comes down to connecting a wet area to a lower outlet with a continuous downhill path. You dig a sloped trench from the problem spot to a real discharge point, lay perforated pipe in gravel, wrap it in filter fabric so soil does not clog it, and set the depth to match your soil and frost conditions. The single most important element is the outlet. Without a lower place for the collected water to escape, a French drain does not remove water — it just relocates the puddle.

Start with the water source

Before anything else, figure out what you are draining. Walk the yard during or right after a hard rain and watch where water sheets, pools, and lingers. A French drain built for the wrong kind of water underperforms no matter how well it is constructed.

Broadly, you are dealing with one of two situations:

• Surface water running across the ground — from a slope, a downspout, or a neighbor’s higher lot. This often responds to a shallower drain or to grading.
• Subsurface water moving through the soil, showing up as a chronically saturated area or seepage near a foundation. This usually needs a deeper, groundwater-collecting drain.

If you are unsure which you have, our guide on fixing standing water in your yard walks through the diagnosis. Getting this wrong is the most expensive planning mistake you can make.

The outlet is everything

Water flows downhill, and a French drain only works if there is a lower, legitimate place for it to end up. Your outlet options generally include:

• A daylight outlet — the pipe emerges at the surface on a lower part of the property, letting water flow out freely.
• A dry well — a buried gravel-filled pit or chamber that lets water soak back into the soil where conditions allow. See dry wells explained.
• An approved connection to a storm system, where local rules permit it.

If you cannot find a spot lower than your wet area, a gravity French drain may not be the right tool at all, and you should rethink the approach before digging.

Slope, depth, and materials

With a source and an outlet identified, you can plan the trench itself.

Slope. The trench needs to fall continuously from the wet area down to the outlet. What matters is that there are no flat or uphill sections where water and sediment can stall. The right grade depends on your run length and site conditions rather than a single magic number, so plan for consistent, uninterrupted downhill flow.

Depth. Depth should match the water you are collecting and your local conditions — how deep the water sits, your soil type, and your frost line all matter. A drain intended to intercept groundwater near a structure is a different animal from one handling shallow surface runoff. There is no universal depth; check local code and get professional input for anything close to the foundation.

Materials. The classic build is perforated pipe bedded in clean gravel, with the gravel and pipe wrapped in filter fabric to keep fine soil out. Choosing the right pipe matters: our comparison of solid vs perforated drainage pipe explains when each belongs in the system. As a rule, the collection section is perforated and any section that only conveys water to the outlet can be solid.

A planning checklist

Planning step	What you are deciding	Why it matters
Identify the source	Surface vs subsurface water	Sets the type and depth of drain
Find an outlet	Daylight, dry well, or approved connection	Without it, water has nowhere to go
Set the slope	Continuous downhill path	Keeps water and sediment moving
Choose depth	Matched to soil, water table, frost	Determines whether it reaches the water
Select pipe	Perforated to collect, solid to convey	Controls where water enters the line
Add gravel + fabric	Clean stone wrapped in filter fabric	Slows the soil clogging that ends drains
Call for locates	811 before digging	Avoids striking buried utilities

French drain or something else?

A French drain is not always the best answer. If your problem is concentrated runoff from a hard surface or a single low collection point, a catch basin might serve better, and many yards use both together. Our guide on French drain vs catch basin lays out where each shines so you are not forcing one solution onto the wrong problem.

Common mistakes

• No outlet, or an outlet that is not lower than the source. The number one reason French drains fail. The water simply backs up and reappears.
• Skipping utility locates. Never dig before calling 811 or your local one-call service. Buried gas and electric lines are not something to guess at.
• Assuming a single “correct” depth or slope. These depend entirely on your soil, water table, frost line, and local code. Copying a number from an unrelated project is a gamble.
• Using the wrong pipe in the wrong place. Perforated pipe belongs where you want to collect water, not on the conveyance run to the outlet.
• Omitting filter fabric. Fine soil migrating into the gravel is a leading cause of slow, silent clogging over the years.
• Confusing drainage with waterproofing. A French drain manages water in the yard. It is not a substitute for proper foundation waterproofing, which is a separate discipline.
• Discharging without checking the rules. Where the water ends up can be regulated. Assuming you can send it anywhere invites conflict with neighbors or code enforcement.

Bringing it together

Good planning turns a French drain from a hopeful ditch into a system. Map the water, secure a real and legal outlet, keep the slope falling the whole way, and size the depth and materials to your actual conditions rather than a borrowed rule of thumb. Nail those and the digging is the easy part. Guess at them and you will be back with a shovel — moving the same puddle a few feet down the yard.