Why Marin hillsides push water into your foundation
Most of Marin is built on the Franciscan Complex: fractured sandstone, shale, chert, and greenstone, with bands of serpentinite running across the ridges from Mount Tamalpais to the Tiburon Peninsula. Near the surface, these rocks weather into dense, plastic clay and slick serpentine soils. Rain soaks through a few feet of looser topsoil and colluvium, then hits that layer and stops.
That is how a perched water table forms. It is a sheet of saturated soil sitting on top of clay or bedrock, well above the true groundwater table, and it flows downhill along the contact. When a winter atmospheric river parks over the North Bay, the upper slopes of Mount Tam and the ridges above Mill Valley, Ross Valley, and San Rafael can take several inches of rain in a day. Storm sequences like October 2021 and the 2022–23 winter can stack one on top of another. The perched layer thickens, and every foot of saturated soil adds weight and pressure.
An upslope perimeter foundation sits right in the path of that flow. Water builds up against the uphill stem wall and creates hydrostatic pressure. It seeps through cold joints, wicks into framing, ponds in the crawlspace, and on expansive clay it heaves and cracks the footing as the soil swells. The symptoms usually show up on the uphill side first, often days after the rain stops.
Shallow landscape drains vs. deep interceptor curtain drains
The most common failed system on Marin hillsides is a shallow landscape drain. It is a 12- to 18-inch trench with corrugated pipe and pea gravel, sometimes with no fabric at all. In sandy soil that might work. In Marin clay it skims surface water at best. The perched groundwater flows right underneath it, and within a few winters, clay fines migrate into the gravel and seal it.
| Factor | Shallow Yard Drain | Deep Hillside Curtain Drain |
|---|---|---|
| Trench depth | 12–18 inches | Down to hardpan, clay, or bedrock, often 3–6+ feet |
| What it catches | Surface runoff and puddling | Perched groundwater flowing along the impermeable layer |
| Location | Wherever water puddles | Across the slope, uphill of the foundation and outside the footing's load zone |
| Pipe | Corrugated flex pipe, often holes up | 4-inch rigid SDR-35 perforated PVC, holes down at 4 and 8 o'clock |
| Aggregate | Pea gravel or mixed rock | Washed 3/4-inch crushed drain rock |
| Filter fabric | None, or a woven landscape cloth | Non-woven geotextile (Mirafi 140N) lining the whole trench |
| Maintenance access | None | Cleanouts at the upstream end and at direction changes |
| Typical life in Marin clay | A few rainy seasons before silting | Decades when built and outletted correctly |
| Permits | Rarely | Often for the outlet; sometimes geotechnical review for depth or slope |
How a Marin curtain drain should be built
Find the impermeable layer first
A good estimator digs or augers test holes before quoting. They look for the depth where soil turns from damp, crumbly colluvium into stiff clay, blue-green serpentine, or weathered rock, and for where water weeps out of the hole wall. That contact sets the trench depth. The trench runs across the slope, uphill of the house. It also stays outside a line drawn down and out from the bottom of the footing (commonly 1:1), so the excavation never undermines the foundation it is protecting.
The section, bottom to top
- Geotextile liner. Mirafi 140N non-woven fabric lines the trench bottom and both walls, with enough left over to wrap the top. It lets water through and keeps clay fines out of the rock.
- Bedding. 2–3 inches of washed 3/4-inch crushed drain rock, graded to at least 1 percent fall (1/8 inch per foot), and more where the slope allows.
- Pipe. 4-inch SDR-35 perforated PVC, laid with the two rows of holes facing down at 4 and 8 o'clock. Water rises into the pipe from below and drains out the bottom, so the pipe never holds a standing pool of silty water. Joints are glued, bends use long sweeps instead of hard 90s, and cleanouts are brought to grade.
- Rock envelope. More washed 3/4-inch crushed rock up to within several inches of finished grade. Angular crushed rock locks together and keeps its voids. Rounded pea gravel rolls and packs.
- Fabric wrap and cap. The fabric folds over the top with a generous overlap, the "burrito." It is then capped with rock where you also want to catch surface water, or with compacted native soil where you do not.
Keep roof water out of the trench: solid tightlines
One 1,500-square-foot roof can shed roughly 900 gallons in an inch of rain. If downspouts feed into the perforated curtain or French drain, the pipe surcharges and that roof water leaks out through the perforations into the trench and the soil around your foundation. Downspouts belong in a separate solid PVC tightline with its own cleanouts. It can run in the same trench above the perforated pipe, but it stays a separate system. It joins the subdrain only downstream of the last perforation, at or near the outlet, or it gets its own discharge point.
Discharge: where the water goes in Mill Valley, San Rafael, and Ross Valley
The outlet is where hillside jobs get approved or stalled, and it should be designed before the trench. California follows a reasonable-use rule for surface water. You can improve drainage on your own lot, but if you collect water and dump it onto a downhill parcel in a way that causes damage, you can be liable. Every Marin town also takes part in the countywide stormwater program, so sediment has to stay out of gutters and creeks during construction.
- Curb cores. These are the cleanest outlet for many lots. A 3- or 4-inch outlet is cored through the curb into the street gutter. Work in the public right-of-way normally needs an encroachment permit from the city's public works department. That means Mill Valley, San Rafael, San Anselmo, Fairfax, or Ross, or Marin County for unincorporated areas like Kentfield, Homestead Valley, and Tam Valley.
- Pop-up emitters. These suit flatter lots with lawn and room to spread water. On a slope, an emitter pushes all the flow out at one point. A curtain drain in clay can trickle for weeks after a storm, which keeps the area below soggy and starts rills toward the neighbor.
- Energy dissipators. A rock-lined splash pad or level spreader slows and spreads flow where daylighting onto your own slope is acceptable. Discharge toward Arroyo Corte Madera del Presidio, Corte Madera Creek, San Anselmo Creek, or other creek channels can bring in county flood-control review and state streambed rules.
- No curb, no storm drain. Many hillside lanes in Mill Valley, Fairfax, and Sleepy Hollow have no curb and gutter. Then the design shifts to on-site dispersion, a storm drain connection the city approves, or a pumped outlet.
Seeing water on the uphill side of your foundation? Request a drainage estimate or call (415) 555-0100. If water is reaching the crawlspace, see crawlspace waterproofing and sump pumps.
Frequently asked questions
How deep should a curtain drain be on a Marin hillside?
Deep enough to cut into the impermeable layer that the groundwater is riding on, whether that is dense clay, hardpan, or weathered Franciscan bedrock. On most Marin slopes that falls between 3 and 6 feet, and sometimes deeper. Depth should be set from test pits on your lot, not from a standard spec sheet.
Why not use corrugated black flex pipe? It is cheaper.
Corrugated pipe crushes under backfill, sags between supports so it holds water and silt, and cannot be reliably snaked or jetted. Rigid 4-inch SDR-35 perforated PVC holds its grade for decades and can be cleaned through cleanouts. On a drain buried 5 feet deep, the pipe is the cheapest part of the job and the most expensive part to replace.
Can I connect my downspouts to my French drain?
Not into the perforated section. Roof water should run in a separate solid PVC tightline. If roof water enters a perforated pipe during a storm, it leaks back out into the trench and saturates the soil the drain is supposed to dry out. The two systems can join downstream of all perforations, at or near the outlet, where your town allows it.
Can I just drain the water onto the slope below my house?
Only if it does not concentrate flow onto a downhill neighbor or start erosion. Under California's reasonable-use rule for surface water, an uphill owner who collects water and dumps it onto a lower parcel can be liable for the damage. A pop-up emitter or energy dissipator can work on a large lot with room to spread the water. Many hillside lots need a curb core or a storm drain connection instead.
Do I need a permit or a geotechnical engineer?
Often for the outlet, and sometimes for the trench. Coring a curb or working in the street right-of-way usually needs an encroachment permit from the city's public works department, or from Marin County in unincorporated areas like Kentfield and Homestead Valley. Deep excavation near a foundation, on a mapped landslide area, or on a very steep slope may need geotechnical review.