Hillside Homes and a Wet Winter: What Happens to the Pipes Buried in a Slope
The storm of February 4 to 6, 2024 is the best recent picture of what a long atmospheric river does to the hills around the Valley. The Los Angeles area got 6 to 12 inches of rain in about three days, and the city counted hundreds of mudslides. On Lockridge Road and Fryman Road in Studio City, mud came off the hillside and into houses. Sixteen people were evacuated from that one canyon.
That winter was an El Niño year. NOAA says the one building now is likely to be stronger, and its outlook favors a wetter than normal December through February for Southern California. If you live on the south side of the Valley above Ventura Boulevard, in the Encino and Sherman Oaks hills, or up in Porter Ranch, West Hills, Chatsworth or the hills above Sylmar, your pipes face a different kind of winter than a flat lot in Panorama City.
Three things a slope does that a flat lot does not
It feeds the drains a steady supply of dirt. Water running down a slope picks up silt, decomposed granite, leaf litter and gravel. Area drains and catch basins on hillside lots catch all of it. The drain line behind the grate carries it downhill until the pipe flattens out or turns a corner, and there it settles. After a few dry years and one hard storm, a 4-inch drain line can be packed solid a few feet past a clear grate.
Most of the drain lines on these lots are 3 or 4-inch PVC or ABS, with some older corrugated plastic and the occasional clay line, each fed by a grated inlet with a small sump. The sewer lateral is usually vitrified clay on the older houses and cast iron or ABS on the newer ones, with a cleanout near the foundation.
It moves. Wet soil is heavy, and on a slope it creeps. Retaining walls lean and pipes buried in the slope move with the ground around them. A sewer lateral that runs downhill to a street like Mulholland Drive or Valley Vista Boulevard is anchored at the house and at the main. Pull on the middle and the joints open. Clay cracks. A pipe that was straight for 60 years ends up with an offset joint that catches paper and lets in groundwater.
It concentrates water. On a hillside lot, rain from the roof and the slope above heads for the same few drain inlets. If one inlet is blocked, the water does not spread out and soak in. It runs over the surface toward the house, the garage or the neighbor's yard below.
Burn scars change the math
January 2025 brought two fires to the edges of the Valley. The Kenneth Fire burned more than 1,000 acres around West Hills, and the Hurst Fire burned in the hills near Sylmar the same week. Burned slopes lose the plants that hold soil in place, and the soil itself can turn water-repellent for a while, so rain runs off instead of soaking in. For several winters after a fire, the slopes below send much more ash and sand downhill than they used to.
If your house sits below one of those burn areas, expect your drain inlets to fill faster this winter than ever before, and plan to clear them after every significant storm, not just once a season.
What we check on a hillside property
- Each drain inlet and the first run of pipe behind it. A clear grate does not mean a clear line. We look past it.
- Where each drain line actually comes out. On many hillside lots the outlet is a pipe in the curb face or a spot at the bottom of the property that has been buried by years of slope wash. A line with a buried outlet holds water no matter how clean it is.
- The sewer lateral. A camera run from the cleanout down to the main, with a sonde in the camera head so we can mark the spot from the surface, shows any joint that has pulled apart or cracked. On a slope, a separated joint will leak sewage into the hillside as well as let groundwater in, which is the last thing a wet slope needs.
- Old damage. Hillside homes in Sherman Oaks and Encino took hard shaking in 1994. Lines that have not been looked at since are worth a look now.
Cleaning and repair on a slope
For drain lines packed with silt and gravel, drain cleaning with a jetter usually beats a cable, because the water scours the full width of the pipe and carries the sediment out instead of punching a hole through it. Thin-wall drain pipe gets checked with a camera first, since a cracked line can be made worse by high pressure.
For a sewer lateral with a separated or cracked section, a trenchless repair is often the right call on a hillside, because digging a trench across a slope that is about to get soaked is asking for trouble. We cover how lining and bursting work on steep lots in our posts on trenchless repair in Sherman Oaks and hillside sewer repair in Calabasas.
Slope stability itself, meaning grading and retaining walls, is work for a geotechnical engineer and a landscape contractor. Our part is the buried pipe that carries the water away, and making sure it is clear and whole before the first storm.
More on this winter: the Valley's flood history and our FAQ.
Call BBC Rooter & Plumbing at 818-280-9135, Northridge, CA. CSLB #720343. Open 7 days, 6am to 6pm.
Frequently asked questions
Why do hillside drain lines clog faster than flat-lot lines?
Because water moving downhill carries soil with it. On a slope, every storm washes silt, decomposed granite, leaves and small rock toward the drain inlets, and the drain lines carry it until they reach a flatter stretch or a bend, where it settles. Lines below burned or freshly graded slopes collect even more.
Can a wet winter damage a sewer lateral on a hillside?
Yes. Saturated soil is heavier and can creep downhill, and a lateral that runs down the slope to the street moves with it. Joints pull apart, clay cracks, and a pipe that was straight can develop an offset. Lines that already had root entry or earthquake damage are the most likely to go.
Do the 2025 fires near the Valley matter for drainage?
They can. The Kenneth Fire burned more than 1,000 acres around West Hills in January 2025, and the Hurst Fire burned near Sylmar the same week. Burned slopes shed ash, soil and debris much faster than vegetated ones for several winters after a fire. Homes below those areas should expect more sediment in their drain inlets and lines.
What should a hillside homeowner check before the rain?
Clear every inlet grate and the first few feet of pipe behind it, confirm that each drain line actually reaches its outlet at the street or the bottom of the lot, and get a camera run on any drain or sewer line that has been slow or has not been looked at in years. Cracked or separated sections are much easier to repair before the soil is soaked.
Who handles slope drainage problems, a sewer company or a landscaper?
It depends on what failed. Regrading a slope, building retaining walls and planting are landscape and geotechnical work. Cleaning, inspecting and repairing the buried pipe that carries water off the property is underground pipe work. We do the buried pipe side.