Why Berkeley Drains Back Up in the First Big Storm

Published October 25, 2026. Facts from EBMUD and the City of Berkeley.

What happens to the sewer in a storm

The City of Berkeley says its sewer system is not intended to handle storm water, and that a sudden influx during storms can cause overflows of untreated wastewater. EBMUD explains how the water gets in: storm water soaks into the ground and infiltrates the sewer through cracks in private sewer laterals, the pipes that connect homes to the street sewer.

The scale is large. EBMUD’s main wastewater plant handles about 50 million gallons a day in dry months, and that can rise to 500 million gallons a day in rainy months. The same pipes have to carry it.

Why your house backs up and the neighbor’s doesn’t

1. Roots in an old lateral. Clay laterals in older homes have joints that roots can enter. A partial blockage passes summer flow but not storm flow.
2. A sag or offset joint. Low spots hold water and debris. More flow means more of that debris moves at once.
3. Grease and wipes. Fall cooking adds grease that hardens in the line and narrows it before the first big rain.
4. Low fixtures. Sewage rises to the lowest opening first, such as a basement shower, floor drain or in-law unit toilet.
5. A missing cleanout cap. An open or broken cap in the yard lets runoff pour straight into your lateral.

If it backs up right now

1. Stop using water everywhere in the house, including the washer and dishwasher.
2. Keep people and pets away. Treat the water as sewage.
3. Check whose side it is. The City maintains the sewer from the curb out to the main. You maintain the lateral from the curb to the house. For backups or overflows from the City side, Berkeley’s number is (510) 981-6620.
4. Call a plumber for the private side. Ask for the line to be cleared and camera-inspected so you learn the cause, not just the symptom.

How to stop it happening again

1. Camera the lateral before the rainy season. See sewer camera inspection.
2. Clear roots and plan the repair if the camera shows cracks or offsets. See tree root removal and sewer line repair.
3. Add a backwater valve if you have fixtures below the street. California Plumbing Code Section 710.1 requires one for fixtures whose flood rim sits below the next upstream sewer manhole cover. See backwater valves and cleanouts.
4. Replace a missing or cracked cleanout cap. It is a small part that keeps runoff out.
5. Fix the lateral for good. A repaired lateral also counts toward the sewer lateral certificate you need to sell or remodel. See sewer lateral certificate.

For the full fall to-do list, read our storm-season plumbing checklist.