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San Francisco plumbing can sit almost idle through months of foggy, dry weather, then be pushed to its limits the first time an atmospheric river storm hits. The same pipes that handled a summer’s worth of showers and dishwashing without complaint can suddenly struggle when the ground is saturated, the sewer system is full, and every fixture in the house is working overtime.
What many homeowners don’t realize is that our city’s plumbing risks in a storm are different from almost anywhere else. San Francisco is the only coastal city in California with a combined sewer system, which means stormwater and sewage share the same pipes. When that system is overwhelmed, backups can come up through toilets, tubs, and floor drains, not just through manholes in the street. That is not something a generic “rainy season” checklist is built for.
At Atlas Plumbing & Rooter, we have been working on San Francisco homes since 2005, through dry years and record-breaking winters. We have seen how a little preparation in October or early November can be the difference between a quiet, rainy night and a flooded bathroom. This plumbing checklist before rainy season in San Francisco is built from that local experience, so you can focus on the weather report, not on emergency plumbing.
Why San Francisco Homes Face Unique Storm Season Plumbing Risks
San Francisco’s rainy season usually runs from November through March. December is typically the wettest month, averaging around 4.4 inches of rain over about 13 rainy days. In a combined sewer system, that amount of water, concentrated into a few strong storms, all moves through the same network that carries household wastewater.
In low-lying parts of the city, like stretches of the Mission near 17th and Folsom or parts of the Marina built on sandy fill, that can translate into a real risk of sewer backups into homes when storms outpace system capacity. In uphill neighborhoods, the visible flooding risk is often lower, but the pressure in buried pipes and the shifting of wet soil can put a different kind of stress on older plumbing.
Under many Victorian and Edwardian homes, the pipes are every bit as old as the architecture. About 30 percent of San Francisco’s sewers are more than 100 years old, and roughly 60 percent are over 80 years old. A lot of laterals that serve these houses are original clay tile and cast iron pipes. Over decades, tiny root intrusions and minor cracks invite groundwater infiltration. Once the rainy season saturates the soil, those hairline defects can turn into significant leaks, blockages, or even collapses.
Our hills add another layer of complexity. Water pressure in San Francisco varies by elevation, so homes at lower elevations often live with higher static pressure pushing on every joint, valve, and fitting. When heavy rain drives everyone indoors, more showers, laundry loads, and dishwashing combine with that baseline pressure and an already stressed sewer network. A plumbing system that seemed “fine” in October can quickly reveal its weak spots in January.
Check Your Sewer Lateral Before the First Big Storm
One of the most important pieces of a San Francisco plumbing checklist before rainy season is the part you never see: your sewer lateral. Under San Francisco Plumbing Code, property owners are responsible for the upper sewer lateral, from the building to the curb. Many homeowners do not find this out until there is a backup in the lowest bathroom and they learn the city is not responsible for that section of pipe.
There are usually warning signs before things get to that point. Slow drains throughout the house, gurgling sounds when more than one fixture is running at the same time, or faint sewage odors near the foundation or in a basement or garage are all clues that the lateral may be partially blocked. In dry weather, the system might manage. Under the high flows that come with major storms, that same restriction can cause waste to back up into the home.
A sewer camera inspection before November gives us a clear look at what is going on underground while conditions are still dry and repairs can be planned instead of rushed. We can see root intrusion in clay tile joints, grease and scale inside cast iron, or cracked and offset sections that invite groundwater infiltration. Catching those problems early lets us recommend focused cleaning, like hydro jetting, or targeted repair before the first big storm tests the system.
Inspect and Service Your Backwater Valve
For homes in the parts of San Francisco that see the most repeated flooding, a backwater valve can be one of the most important defenses. A backwater valve is a device installed in the building sewer line that allows wastewater to flow out, but closes if sewage from the public main starts to move back toward the house. It is a simple concept, but it only works properly if the valve is maintained.
The San Francisco Public Utilities Commission recommends servicing backwater valves before every rainy season. Debris, grease, or small objects in the line can wedge the valve open or prevent it from sealing completely. The result is that when the combined sewer system backs up during a big storm, the valve does not do its job, and wastewater can still find its way into tubs, showers, and floor drains.
Guidance on whether a particular property should have a backwater valve is available through the San Francisco Department of Building Inspection Plumbing Division. For homeowners who need to add flood protections, the SFPUC Floodwater Grant program can provide up to $100,000 for eligible projects, including backwater valve installation and other resiliency upgrades. For many houses in low-lying Mission blocks or Marina properties close to street grade, backwater valve inspection or installation belongs at the top of the rainy-season prep list.
When we inspect a backwater valve, we confirm that the flapper or gate moves freely, that seals are intact, and that there is no debris obstructing the mechanism. We also look at the surrounding piping so that the valve is not being asked to protect against problems a little upstream, like a partially collapsed section of lateral.
Clear Your Drain Lines Before Storms Arrive
If a kitchen sink or shower drain has been slow all summer, it is unlikely to clear up on its own just because the rainy season starts. In fact, the opposite is true. Cooler weather and more time spent at home usually mean more cooking, more showers, and more laundry. At the same time, the ground around your home is taking on water, which increases the load on the entire drainage system.
Simple snaking can punch a hole through a clog, but it often leaves a layer of grease, soap scum, and fine roots clinging to the pipe walls. Hydro jetting uses high-pressure water to scour the full inside diameter of the pipe, removing that buildup and restoring more of the original capacity. Our Residential Hydro Jetting Maintenance Program is set up to keep drains clean and clog-free so they are ready for heavy seasonal use rather than barely keeping up.
Indoor fixtures are only part of the picture. Outdoor area drains, patio drains, and basement floor drains should be tested before the storms begin in earnest. We want to see that water poured into these drains disappears quickly and does not bubble back or sit in place. When these drains are clogged, the first sign is often water creeping across a garage slab or into a downstairs room during a prolonged rain. Clearing them in dry weather is far easier than trying to open them up in the middle of a downpour.
Inspect Exposed Pipes, Shut-Off Valves, and Water Pressure
In many San Francisco homes, the most vulnerable plumbing is in the places we rarely visit: a crawl space under an old Victorian, a garage under a flats building, or a basement utility room. Before the rainy season, it helps to take a slow walk through these spaces and look closely at any exposed pipes.
We look for corrosion on copper, rust and scaling on steel or cast iron, and any signs of old patch jobs, like rubber sleeves or makeshift clamps. These are all hints that a pipe has had trouble before. Seasonal pressure swings, combined with saturated soil and more frequent use, can turn a small weep into a significant leak when storms move in.
Shut-off valves deserve special attention. The main shut-off, usually near the water meter or just inside the front of the building, should turn smoothly and close fully. So should important secondary valves, like the one feeding the water heater. A valve that is frozen open is only an inconvenience in dry weather. During a storm, if a pipe fails and the only way to limit damage is to turn the water off quickly, that stuck valve can mean the difference between a small puddle and a major cleanup.
Finally, we think about pressure itself. Because of San Francisco’s hills, homes at lower elevations often live with higher incoming water pressure. That might feel great at the shower head, but over time it is hard on pipe joints, flex connectors, and appliances. If a home has never had its pressure checked or does not have a working pressure regulator, a pre-rainy-season plumbing visit is a good time to check and, if needed, adjust that. Keeping pressure in a safer range reduces the chance that a stressed pipe will fail just when access for repair is most difficult.
Get Your Water Heater Ready for Cold-Weather Demand
As the rainy season and cooler weather arrive in San Francisco, water heaters quietly take on more work. Incoming water from the city mains is colder, and everyone tends to take longer, hotter showers. If there is sediment built up in the bottom of the tank, the heater has to work harder and longer to deliver the same amount of hot water.
In summer, a sluggish water heater can be easy to ignore. In January, when guests are visiting or everyone is inside more often, slow recovery times and inconsistent hot water become a real frustration. Flushing sediment, checking the anode rod, and testing the temperature and pressure relief valve before November helps the unit handle the heavier winter load more comfortably.
The location of the water heater matters for rainy-season prep too. In many San Francisco homes, the heater is in a garage, under the front steps, or in a below-grade utility area. In neighborhoods that see surface water during storms, that puts the heater close to potential floor-level intrusion. As part of a storm-readiness check, we look at whether the heater is elevated appropriately, whether there is any sign of past water reaching the base, and whether the surrounding drains are clear so that minor intrusions do not become major damage.
Putting Your San Francisco Rainy-Season Plumbing Checklist Into Action
Getting ready for our rainy season is about more than clearing a few leaves. In San Francisco, the combined sewer system, aging clay tile and cast iron laterals under Victorian and Edwardian homes, and the sharp shift from dry months to concentrated winter storms all create plumbing stresses that generic checklists miss.
A focused look at your sewer lateral, backwater valve, drain lines, exposed piping, shut-off valves, water pressure, and water heater before the first big storm gives you a clear picture of how your system is likely to perform when the rain really arrives. A pre-season visit from a local plumber who understands these city-specific issues is often the easiest alternative to calling for help in the middle of a January downpour.
At Atlas Plumbing & Rooter, we have been helping San Francisco homeowners prepare for and recover from storms since 2005, with free estimates, 24/7 emergency availability, and a Spanish-speaking team for families who prefer to communicate in Spanish. If you would like help putting this checklist into practice before the next rainy season, you can reach us at (415) 849-2346.