Why Does My Sewer Back Up When It Rains?
Because rainwater is getting into a sewer pipe that was only built to carry what goes down your drains.
When that pipe is cracked, full of roots, or sagging, a storm fills it faster than it can drain. The overflow has to go somewhere, and the lowest drain in your house is the easiest way out. Here is how to tell what is happening at your place, and how to stop it before the next storm.
A sewer that only backs up in wet weather almost always has one of four problems:
- Groundwater seeping in through cracks and loose joints in your sewer lateral.
- Tree roots that grew into the pipe over the dry months and now catch everything.
- A downspout, yard drain or sump pump tied into the sewer line.
- The public sewer main surcharging during a heavy storm and pushing back toward your house.
A camera inspection tells you which one it is, usually in a single visit.
Last updated: October 2, 2026
Why does rain affect my sewer at all?
Your sewer and the storm drains on your street are two separate systems. The sewer is only supposed to carry wastewater from your toilets, sinks, showers and washer. Rainwater is supposed to run off into the gutters and storm drains.
The trouble is that older sewer pipes are not watertight. Sonoma Water describes the problem directly: leaky or deteriorating laterals and mains let groundwater and stormwater seep into the pipes. Engineers call it inflow and infiltration, or I&I. On a dry day you never notice it. After three days of steady North Bay rain, when the soil around your pipe is saturated, that extra water can fill the line.
Most of the North Bay’s rain falls between November and March, which is why backups so often show up right after the first real storm of the season.
Which part of the pipe is my problem?
The pipe that runs from your house to the public sewer main is called the sewer lateral, and in Sonoma, Marin and Napa counties it belongs to you.
Sonoma Water’s rule is that the property owner is responsible for the lateral “from the building to the point of connection with the sewer main.” Santa Rosa’s city code says the same thing and adds that owners must maintain the cleanouts, backwater valves and overflow devices on it. If the backup is caused by your lateral, the repair is on the homeowner. If it is caused by the public main, the city or sanitation district handles it.
Quick way to tell the difference: if only your house is backing up, the problem is almost always your lateral. If neighbors on the same street are backing up at the same time, call your city or sanitation district first. Their main is likely overloaded.
What is causing the backup?
Each cause leaves its own clues. Use this table to narrow it down before you call anyone.
| What you notice | Most likely cause | Typical fix |
|---|---|---|
| Slow drains and gurgling only during or after rain | Groundwater seeping in through cracked pipe or loose joints | Camera inspection, then a spot repair or lateral replacement |
| Backups that come back every winter, often in the same spot | Tree roots inside the lateral | Hydro jetting to clear roots, then a repair or replacement so they cannot grow back |
| Water rises in the lowest drain the moment it starts pouring | A downspout, area drain or sump pump connected to the sewer | Disconnect it and send that water to the yard or storm drain |
| Neighbors backing up at the same time | Public sewer main surcharging | Report it to the city or district, and add a backwater valve or overflow device to protect your house |
| Backups in a basement, garage or lower-level bathroom | Fixtures sitting below the street sewer level | A backwater valve on the lateral |
Homes built before the 1980s often still have clay or cast iron laterals, which are the most likely to crack and let roots in.
Why does it get worse every year?
Roots are the big one. Through the dry summer, tree roots go looking for moisture, and a hairline crack in a sewer pipe is a steady source of it. The roots work their way in, grow into a net inside the pipe, and catch paper and grease. You do not notice until the first storm sends a wave of extra water down the line and it has nowhere to go.
Clearing the roots buys you a season. It does not fix the crack they came in through, so they come back. If you have had your line cleared more than once in the last few years, the pipe itself is the problem.
How do I stop it before the next storm?
Here is the order we recommend, cheapest and most useful first.
- Get a camera inspection. We run a waterproof camera through your lateral and show you the footage. You see exactly where the cracks, roots or sags are, and you stop guessing. It is the step that keeps you from paying for a repair you do not need.
- Clear the line with hydro jetting. High-pressure water cuts out roots and flushes grease and debris from wall to wall. It is a better short-term fix than a cable snake, which punches a hole through a clog and leaves the rest behind.
- Disconnect anything that sends rainwater into the sewer. Downspouts, area drains and sump pumps should never discharge into the sanitary sewer. Most North Bay cities and districts prohibit those connections, and they can flood your own line in a storm.
- Add a backwater valve or overflow device. If any of your drains sit below the next upstream manhole on your street, the California Plumbing Code requires a backwater valve to protect them. A one-way flap lets wastewater out and stops sewage from coming back in.
- Repair or replace the damaged section. If the camera shows broken pipe, trenchless sewer repair can often fix it without digging up your driveway or garden. See our sewer services for the options.
Are there local lateral rules I should know about?
Yes, and they vary by district. A few to know:
- Sonoma Valley County Sanitation District requires video inspections of laterals on properties 30 years or older. Sonoma Water estimates about two-thirds of the laterals in Sonoma Valley are at least that old. When significant defects are found, owners have one year from the report to repair them.
- Santa Rosa puts the full lateral on the owner, including the cleanout and any backwater valve or overflow device.
- Marin County has no single countywide rule. Several Marin sanitary districts require a lateral inspection or certificate when a home is sold or remodeled. Check with your district before you list your home.
Your city or sanitation district can tell you exactly what applies to your property.
What should I do if it is backing up right now?
- Stop using water. No flushing, no showers, no dishwasher or washer. Every gallon you use has nowhere to go but back up.
- Check your cleanout. If you can find the outdoor cleanout cap and it is safe to do so, loosening it can let sewage spill outside instead of inside.
- Keep people and pets away from the water. Sewage carries bacteria, so do not try to clean it up without gloves and boots.
- Call a plumber. We answer 24/7, including during storms.
Our step-by-step guide on what to do when your sewer line backs up walks through cleanup and what to tell your insurance company.
Questions homeowners keep asking us
Why does my toilet gurgle when it rains?
Gurgling means air is being pushed back up through the drain because the sewer line is filling with water faster than it can empty. In wet weather, that usually points to groundwater getting into a cracked lateral, roots slowing the flow, or the public main running full. It is an early warning. Get it looked at before it turns into a backup.
Who is responsible for a sewer backup, me or the city?
It depends on where the blockage is. In Sonoma, Marin and Napa counties, the homeowner owns the sewer lateral from the house to the public main. If the problem is in your lateral, the repair is yours. If the public main is blocked or overloaded, the city or sanitation district is responsible for the main.
Does homeowners insurance cover a sewer backup?
Often not on a standard policy. Many insurers only cover sewer and drain backup if you added a water backup endorsement. Call your agent before the rainy season and ask whether you have it.
Do I need a backwater valve?
If any drains in your home sit lower than the next upstream manhole cover on the public sewer, the California Plumbing Code requires one. It is common in homes with basements, garages or lower-level bathrooms. Even when it is not required, it is cheap protection for a house that has backed up before.
Can I connect my downspouts or sump pump to the sewer?
No. Rainwater belongs in the yard or the storm drain, not the sanitary sewer. Most North Bay cities and sanitation districts prohibit those connections because they overload the system in a storm, and they can back up your own house first.
Will clearing the roots fix the problem for good?
Not usually. Hydro jetting or snaking clears the roots that are in the pipe today, but the crack they grew through is still there, so they come back. If your line has been cleared more than once in the last few years, a camera inspection will show whether the pipe needs a repair.
Get your sewer line checked before the next storm
We will run a camera through your lateral, show you what we find, and tell you whether it needs a fix or just a cleaning. No pressure and no scare tactics.
Sources: Sonoma Water, Private Sewer Laterals and Sonoma Valley lateral inspection program · City of Santa Rosa, Water & Sewer · California Plumbing Code §710.1 (backwater valves) · West Coast Plumbing, License CA #1115191.