A Downspout Tied Into the Sewer: Why It Floods the House in a Big Storm
A common call after a big storm goes like this. The house drains fine all summer. Then two inches of rain come in overnight, and by morning there is sewage in the shower and the toilet will not flush. The family has never had a sewer problem. A camera run shows the lateral is in fair shape. What it shows instead is a second pipe tied into it a few feet from the house, carrying all the water off the back half of the roof.
That is a cross-connection, and older Valley houses are full of them.
How they get there
Most of the ranch houses in Northridge, Reseda, Granada Hills and Van Nuys went up between 1946 and 1970 with plain gutters that spilled onto the ground. Over the next 50 years owners added patios, room additions, pool decks and concrete driveways, and the water that used to soak into the yard needed somewhere to go. The sewer cleanout was right there at the side of the house, already a pipe in the ground headed for the street. Tying a downspout into it took an afternoon.
The same thing happened with low patios, basement stairwells on the older houses in Sherman Oaks and Studio City, and garage floor drains. A drain was needed, the sewer was the nearest pipe, and nobody pulled a permit.
Why it only fails in a big storm
The plumbing code California uses is plain about this: storm water shall not be drained into sewers intended for sewage only. The reason is size. A house lateral in the Valley is a 4-inch pipe. It carries what comes out of a few toilets, a couple of showers, a dishwasher and a washer, and most of the day it runs nearly empty.
Roof water is a different kind of flow. One inch of rain on 1,000 square feet of roof is about 620 gallons. A 1,800 square foot roof in a two-inch storm puts out more than 2,000 gallons, and in an atmospheric river much of it arrives in a few hours. If even half that roof drains into the lateral, the pipe fills. Now add every other cross-connected house on the block, plus the groundwater leaking into old clay laterals up and down the street, and the 8-inch city main under the street runs full too.
When the main is full and your lateral is full, your sewage has nowhere to go. It comes up at the lowest drain in the house. Your own downspout is helping flood your own shower.
With NOAA calling for a very strong El Niño this winter, and wetter than normal odds for Southern California from December through February, this is the year these connections show themselves.
How to tell if you have one
Start with a walk around the house:
- Follow every downspout to where it ends. A splash block, a pop-up emitter in the lawn or a pipe headed toward the street is fine. A downspout that goes straight into the ground near the sewer cleanout is suspect.
- Look at any drain set in a low patio, stairwell or garage floor. Ask where it goes. If nobody knows, that is worth finding out.
- Think about when trouble happens. A toilet that gurgles or a tub that backs up only in heavy rain points to rain getting into the sewer.
The way to know for sure is a test. We put water, and sometimes a tracing dye, into the downspout or patio drain and watch for it at the sewer cleanout. Or we run a camera up the lateral from the cleanout and look for a side connection that should not be there. We explain the trade-offs between the two in camera versus dye testing. A sewer camera inspection also tells us where the tie-in is and how deep, so the repair is a small hole at a marked spot.
The fix
The roof water has to be cut off from the sewer and sent where it belongs. On a typical Valley lot that means:
- Exposing the tie-in and capping it on the sewer side, so the lateral is back to carrying sewage only.
- Running a solid drain line from the downspout or patio drain out to the curb, or into an existing yard drain line that already reaches the street.
- Running the camera again to confirm the lateral is clean and the tie-in is sealed.
On some lots the drain line can run along the side yard to a curb outlet. On others it has to cross a driveway, and the route is planned around that. Either way it is a dry-weather job, and November is a good month for it.
If the house backs up during a storm before you get to it, our guide to an emergency sewer backup covers what to do in the first hour. More on rain and Valley sewers: El Niño 2026-27 and your sewer line, and our FAQ.
Call BBC Rooter & Plumbing at 818-280-9135, Northridge, CA. CSLB #720343. Open 7 days, 6am to 6pm.
Frequently asked questions
Is it legal to connect a rain gutter to the sewer in Los Angeles?
No. The plumbing code used in California says storm water shall not be drained into sewers meant only for sewage. Rain from roofs and yards is supposed to leave the lot through the surface, a yard drain line, or a pipe to the curb, and reach the street storm drain. A cross-connection usually comes to light during a sewer inspection, a home sale, or a backup.
How much water can one downspout send into the sewer?
A lot more than most people expect. One inch of rain on 1,000 square feet of roof is about 620 gallons. A typical Valley ranch house has 1,500 to 2,000 square feet of roof. In a storm that drops two inches, a roof that drains into the sewer can push well over 2,000 gallons into a 4-inch lateral that was sized for sinks, toilets and a washing machine.
How do you find out if a downspout goes into the sewer?
We run water or a dye down the downspout and watch for it at the sewer cleanout, or run a camera up the lateral and look for a side connection that should not be there. A camera also shows exactly where the tie-in is, so the fix is a small dig at a known spot.
What is the fix once a cross-connection is found?
The roof or patio water gets cut off from the sewer and sent somewhere legal. Usually that means capping the tie-in on the sewer side and running a solid drain line from the downspout to the curb or to an existing yard drain line. The sewer side is then left with only the flow it was built for.
Can a patio or area drain also be tied into the sewer?
Yes, and it is just as common. Low patios, basement stairwells and garage floor drains on some older houses were connected to the sewer because it was the closest pipe. In a storm those drains pour yard water into the sewer, and when the main backs up they can also be where sewage comes out.