French Drains: When a Montana Yard Actually Needs One

What is a French drain, exactly?

The Building America Solution Center, the Department of Energy's building guidance site run by Pacific Northwest National Laboratory, describes it as a perforated pipe wrapped in rock and landscape fabric, installed under turf, mulch, pavers or stone at low points in a yard to reduce surface ponding. The pipe carries collected water to a lower part of the site, to a drywell for underground dispersal, or off site to a storm system or retention pond.

It works on water that is already in the ground or soaking into it. Water moves through soil slowly, but it moves through clean rock and into a pipe very quickly, so a trench of rock acts like a low-resistance lane that pulls water toward it and carries it away. That is the key distinction. A French drain is excellent at intercepting water moving through the soil. It is a poor substitute for fixing a surface that slopes the wrong way.

When does a yard need a French drain instead of regrading?

Grading should be the first answer. The residential code (section R401.3, as published for Montana) asks for 6 inches of fall in the first 10 feet away from a foundation, and our guide to grading away from the foundation shows how to check it. The code's own exception says that where lot lines, walls, slopes or other barriers prevent that fall, drains or swales shall be built instead. That is the first case for a French drain: no room to slope.

The second case is water arriving underground. On a hillside lot, water from uphill can move through the soil and surface at the base of a slope, a wall, or the uphill side of a house. Regrading the surface does not touch it. A drain trench across the slope, uphill of the problem, catches it before it arrives. The third case is a persistent soggy area where surface water collects and the soil beneath does not drain, and where a swale would cut through a lawn or patio. If your problem is roof water pouring out at the base of a wall, start with downspout extensions instead.

How is a French drain built so it lasts?

The Building America guide gives a step-by-step build. Dig a trench at least 22 inches deep and 14 inches wide, sloping toward the outlet. Lay landscape filter fabric in the trench wide enough to wrap over the top. Put down 4 inches of clean rock in the 3/4 inch to 2 inch range. Lay 4 inch perforated PVC pipe with the perforations down. Cover the pipe with another 4 to 6 inches of clean rock, fold the fabric over and lap it, then finish with 9 inches or more of rock, topsoil or mulch.

Three details matter most. The pipe has to fall continuously toward the outlet; a sag holds water and silt. The fabric keeps fine soil out of the rock, and without it the gaps fill and the drain slowly becomes a buried gravel line that does nothing. And the guide prefers smooth, non-corrugated pipe because corrugated pipe is more likely to collect sediment and clog. The same source says to avoid planting shrubs or trees over or near the drain, because roots go where the water is.

Where can the water from a French drain go?

Somewhere lower that can take it. The Building America guide lists a lower part of the site, a drywell, a storm sewer, or a community swale or pond. If the drain empties to daylight, it should end on a slope that carries water away from the building, with a splash block, stones or a dispersal fitting at the outlet to stop erosion. The residential code adds that surface drainage must go to a storm sewer conveyance or other approved point of collection that does not create a hazard. Sending a pipe's worth of water onto a neighbor's lot is not that.

A drywell is the usual answer on a flat lot with no daylight outlet. The guide describes a hole about 4 feet by 4 feet and 4 to 6 feet deep, lined with fabric and filled with clean 3/4 inch to 1.5 inch rock with no fines, with the inlet pipe entering at half height or higher, capped with fabric and at least 8 inches of soil. Whether that works depends entirely on the soil underneath. Gravelly valley-floor ground can swallow water fast. Tight, silty or clay pockets cannot, and a drywell in those just fills up.

Are dry wells regulated around Missoula?

They are a regulated category of well under federal rules. The EPA classifies stormwater drainage wells, including dry wells and infiltration galleries, as Class V underground injection wells, and its stormwater well page explains that infiltration systems deeper than they are wide, or with perforated pipe distributing water underground, can meet that definition. Private citizens are among the owners it lists.

Missoula sits on top of its drinking water. The Missoula Valley Water Quality District describes the valley aquifer as an EPA-designated sole-source aquifer, and the City's Stormwater Utility says most city stormwater already reaches it through more than 6,000 dry wells. An October 2025 Missoula Current report on the City's stormwater plan quoted the city's stormwater superintendent saying most of the valley has a high infiltration rate, and that it also means stormwater reaches the aquifer rapidly with no water quality treatment. Clean roof and yard water is the point of a French drain. Keep wash water, oil, fertilizer spills and anything else out of it, and check with your local building or public works office before building a new drywell.

Can a French drain freeze in a Montana winter?

The parts near the surface can, and the outlet is the usual weak point. A daylight outlet that sits in a shady low spot can ice over or get buried under plowed snow, and then the drain backs up during the March melt, exactly when you need it. Keep outlets where the sun reaches them, mark them before the first snow so nobody piles snow on them, and do not run the outlet into the bottom of a depression where meltwater refreezes.

Depth helps. The Building America guide says a French drain should sit 18 inches or more below ground, and its build method uses a trench at least 22 inches deep. What a drain cannot do is take in water from frozen soil at the surface. During melt, grading and swales carry the surface water; the drain handles what soaks in as the ground opens up. Our guide to standing water in a lawn covers how to tell those two problems apart.

French drain, swale or dry creek bed?

Use a swale when the water is on the surface and you have room for a shallow channel; it is cheaper, easy to inspect, and has nothing to clog. The Building America guide adds that a French drain can sit under the bottom of a swale where larger volumes are expected, which is a common combination on bigger lots. Use a French drain when water is moving through the soil, when the route crosses lawn or patio you want to keep, or when there is no room for a channel.

A dry creek bed is a swale lined with rock so it looks like a feature rather than a ditch, and it is often the best-looking answer in a front yard. On steeper ground, the bigger question is usually erosion on the slope rather than a drain. And in a spot that stays soggy but has nowhere to send water, a planted rain garden may be the practical choice.

What goes wrong with French drains?

Most failures are installation shortcuts. Pipe laid without consistent fall, so water sits in low spots. Fabric skipped or torn, so silt fills the rock. Corrugated pipe that collects sediment. An outlet that was never built, so the drain just fills and overflows. A roof drain line tied into the perforated foundation drain at the footing, which the Building America gutters guide warns against because it soaks the foundation instead of protecting it. Trees planted on top. Any of these can turn a significant project into a buried pipe that does nothing.

Before anyone digs, call Montana 811. Montana 811 says you can dig two full business days after you call and get a ticket number, and recommends calling at least three days ahead. A French drain trench runs right along the foundation where utility lines come in, and customer-owned lines past the meter may not be located, so know where your sprinkler mains and lighting cable run.

How do you know if a French drain is working?

Watch it during a real event. After a heavy rain or during the spring melt, the outlet should run, and the area the drain was meant to dry should drain noticeably faster than it used to. If the outlet stays dry while the yard stays wet, either water is not reaching the pipe or the pipe is blocked. If the outlet runs muddy, fabric has failed somewhere. A cleanout or an accessible outlet makes it possible to flush the line instead of digging it up.

We handle grading, surface drainage and dry creek drainage features as part of our landscaping design and hardscaping work in Missoula and the Bitterroot Valley. Tell us what the yard is doing and we will tell you if a simpler fix will do. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide. Call (406) 215-7319 or request a free estimate.

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