French Drain Installation in San Jose, CA

A French drain doesn't pull water out of the ground like a straw. Water in saturated soil moves toward the easiest path, and a trench full of open rock is far easier to flow through than clay. So the drain intercepts groundwater and shallow seepage, gives it a low-resistance route, and carries it to an outlet by gravity. That's the whole trick, and it's why slope and outlet matter so much.

Surface water behaves differently. Water running over the ground doesn't always sink into the drain rock fast enough, especially if there's a layer of sod, mulch or soil above. For surface runoff, you want a channel drain, catch basin or regrading that moves the water into a solid pipe on its way out. We often combine a French drain for the seepage and surface drains for the runoff, because most Valley yards have both.

A French drain also won't fix a roof that dumps ten gallons a minute next to your foundation or a patio that slopes toward the house. We look at the source of water first. Sometimes the fix is a gutter extension, a grade correction and one short drain, not a 100-foot trench.

We prefer rigid perforated pipe, like perforated SDR 35 or a similar rigid pipe, set with holes down. The rigid wall keeps its shape and holds slope without sagging, and it can be flushed or snaked if it ever needs cleaning. Corrugated pipe costs less and is flexible, and it's fine in some situations, but it can sag between supports, collect silt in its corrugations and make it harder to keep a consistent fall. If the pipe has a sock, we skip it in favor of wrapping the entire stone package with fabric, since socks clog with fine clay.

Rock matters just as much. We use clean, washed 3/4-inch drain rock, angular and uniform in size, so there are open voids for water to flow through. Pea gravel is round and finer, and it packs differently, so it moves less water. Crushed rock with fines, like 3/4-inch minus road base, sets up almost like concrete and holds water rather than moving it. Washed means the dust is gone, and the dust is what clogs fabric.

Non-woven geotextile fabric keeps native soil out of the stone while letting water through. It wraps the sides and top of the stone package, with generous overlap. We don't use woven landscape fabric or plastic sheeting here, since they either clog or block flow. A properly lapped wrap keeps the silt out. A skimpy one lets it in, and the stone slowly fills.

Every drain is a slope. At least 1 percent is our minimum for pipe, and steeper is better when the site allows. The ground itself needs slope too. Building codes typically call for hard surfaces to slope at least about 2 percent (1/4 inch per foot) away from the structure, and for soil next to a foundation to fall about 6 inches over the first 10 feet, which is 5 percent. Many Valley yards from the 1950s to 1970s have settled flat or even back toward the house, and a few hours with a shovel and a laser level can fix more than a long trench can.

Where does the water go? That's the question we ask first. The best outlet is daylight on your own property, a lower corner of the yard with a pop-up emitter, a sloped swale or a spot where it can spread without eroding. Next best is a curb drain through the curb to the street, which many cities allow with rules. Connecting to a storm drain or sewer line is often restricted, and we check with the city rather than assume. The one rule that never changes: don't send water onto a neighbor's property. That's a legal problem and a human one.

When gravity can't do it, a sump pump can. A sump basin collects water from the drain, a pump lifts it to a discharge line, and a check valve stops it from running back. Pumps need power, a battery backup if the site calls for it, and regular testing. They're a solution for low sites, not a first choice.

A slotted channel drain is a long grate-covered trough, usually 4 or 5 inches wide, set at the edge of a patio, doorway or driveway apron. It catches sheet flow before it reaches the structure. Channels need their own slope or an internally sloped body, a solid outlet and a spot to clean them. They look neat when they're set flush and tied to a solid line.

Catch basins and area drains sit at low spots. A catch basin has a sump below the outlet so sediment drops out instead of rolling down the line. An area drain is a simple grate inlet. We use them where the grade can't be fixed by shaping alone, like a side yard that funnels water between two fences. These connect to solid pipe, not perforated, so the water moves in a closed line to the exit.

Gutters and downspouts are a big share of the water. A roof can deliver an enormous volume in a heavy atmospheric-river storm. Downspouts should drop into solid pipe with a cleanout and run to the outlet. Plugging a downspout into a perforated French drain near the foundation is a common mistake, because the water simply soaks out and saturates the soil you were trying to dry.

A dry well is a buried chamber or rock-filled pit that holds water and lets it soak into the ground. It works when the soil underneath drains well, like sandy or gravelly soil. On heavy Valley clay, percolation is poor, so a dry well can fill and stay full for days. Sizing needs a real percolation test and an honest look at the soil, and in some spots we'd recommend a different solution entirely.

Perched water is another clay issue. When a clay layer sits above deeper permeable soil or the water table, rainwater collects on top of it and creates a perched water table, even in places where deeper groundwater is far below. That water sits right at footing depth and won't drain down. French drains intercept perched water well, but it still needs an outlet.

In parts of the Valley, groundwater can also rise seasonally near creeks and low areas such as parts near the Guadalupe River and Coyote Creek. We hedge on this because it varies by block and year. A site with a high water table needs a deeper approach, sometimes a sump, and sometimes a conversation with a geotechnical engineer if there's a slab or structure involved.