A French drain is a trench with perforated pipe, washed rock and filter fabric that collects water underground and carries it away. Built right, it keeps groundwater and seepage away from foundations, crawlspaces and soggy lawns. Built wrong, it silts up or holds water against the house.
French Drain Installation in Boise
The name comes from Henry French, a Massachusetts farmer who wrote about the method in the 1800s, not from France. The idea hasn't changed much since. Water in the soil takes the easiest path. A trench of clean rock with a pipe at the bottom is a much easier path than clay, so water moves into it and the pipe carries it off.
We install French drains in Boise, Meridian, Kuna, Nampa, Eagle and Caldwell, mostly along foundations, at the bottom of slopes and through lawns that stay wet. The layout comes from where the water shows up and where it can leave the lot.
When a French Drain Is the Right Fix
French drains handle water that's moving through the ground, not water sheeting across the top. Signs of underground water include a spot that stays wet for days after the lawn around it dries, water seeping out of a slope or cut bank, and damp soil along the stem wall even with the gutters working. They're a good fit for:
- Seepage into a crawlspace from saturated soil around the stem wall
- A yard at the bottom of a slope where water comes up out of the ground
- Lawns that stay soggy long after the surface has dried
- The back side of a retaining wall
They aren't the answer to everything. If water is running across the surface, regrading or a swale moves more water for less money, and our drainage excavation page covers those. A French drain under sod only takes water as fast as it can soak down through the turf. If the problem is roof water, buried downspout lines fix it more directly. If you're not sure which you have, our drainage solutions page explains how we figure that out.
How We Build a French Drain
Trench depth and slope
Most yard drains run about 18 to 24 inches deep. Drains along a foundation may go deeper to catch water before it reaches the footing. The trench bottom is graded to fall the entire way, at least about 1 percent, which is roughly an inch every 8 feet. Flat spots hold water and collect silt.
Filter fabric
We line the trench with non-woven filter fabric before any rock goes in. It lets water through and keeps fine soil out. In clay, fabric is the difference between a drain that keeps working and one that clogs in a few seasons.
Washed rock and perforated pipe
A bed of washed drain rock goes in first, then 4-inch perforated pipe with the holes facing down, then more rock over the pipe. The fabric folds over the top before backfill. The rock has to be washed. Rock with fines in it packs tight and slows the water down.
Backfill and finish
The top gets soil and sod, or it's left as rock if you want the drain to catch some surface water too. We settle the soil over the trench so the line doesn't sink into a new low spot.
Where the Water Goes
A French drain is only as good as its outlet. The outlet sets the low end of the whole drain, so we find it before we decide on trench depth. We check the fall from the wettest spot to where the water can leave. If there isn't enough fall to reach daylight, the plan shifts to a dry well or a shallower drain.
- Daylight. The pipe comes out at a lower spot on your lot, with a screen to keep rodents out and rock to stop erosion. It's the simplest option when the lot has enough fall.
- Dry well. On flat lots, which describes much of Meridian and Nampa, the pipe ends in a rock-filled pit or chamber where water soaks into the ground. Small ones can be augered.
Drainage water shouldn't be tied into your sewer line, and it shouldn't be pointed at a neighbor's yard. If your lot has no good place for the water, a dry well or a different layout is the better answer.
French Drains in Treasure Valley Clay and Caliche
Valley soil makes French drains harder than the how-to videos suggest. We look at the soil early, often with a quick test hole along the planned line, because it changes how long the dig takes, how the trench walls behave and how much water the outlet can take.
- Clay smears when a bucket cuts through it wet, which seals the trench walls. We'd rather dig clay sites after they've dried out some, and fabric is not optional.
- Caliche in Meridian, Nampa and Kuna is a cemented layer that water can't get through, so water perches on top of it. A drain set at that layer catches the perched water, but a dry well outlet has to break through it or it fills and stays full.
- River cobble near Eagle and Garden City drains on its own, but groundwater rises in irrigation season. The outlet has to sit below that water.
- Basalt close to the surface in parts of Southeast Boise and Kuna can limit how deep the trench goes and adds cost.
French Drain Cost in Boise
Price depends mostly on length and depth, the outlet, soil and rock, and access. A short run along one side of the house that daylights into the backyard costs much less than a drain wrapping the whole foundation into a dry well. Concrete, decks and mature landscaping in the path add work. Sprinkler lines are the most common thing in the way, and Digline doesn't mark them, so tell us where yours run.
Getting Ready for the Install
A few decisions up front keep the job moving. Choose sod or exposed rock on top of the trench, clear anything stored along the wall where the drain will run, and tell us how wide your side gate is, since that decides what equipment fits. After the drain is in, watch the outlet during the next storm or irrigation cycle. Water coming out the end means it's working.
Why Hire Us for Your French Drain
A French drain is buried the day it's finished. You'll never see whether the fabric was lapped or the pipe has fall, so it matters who digs it. Our reviews describe us as "professional, efficient and highly skilled," and we're an Idaho registered contractor (RCE-65836) with a 5.0 rating on Google. Call (208) 697-2429 or request a free estimate.
