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Why sewers back up during heavy rain
San Francisco largely uses a combined sewer system, where wastewater and stormwater share the same pipes, according to the SF Public Utilities Commission. During intense storms the system carries much more flow, and low-lying fixtures or properties with damaged laterals can be more vulnerable to backups or slow drainage.
Before the rainy season
- Have recurring slow drains cleared and camera-checked.
- Make sure downspouts and yard drains don't feed into cleanouts or damaged pipes.
- Ask a plumber whether a backwater valve is appropriate for ground-floor fixtures.
- Keep the cleanout accessible and capped.
During a storm
- Limit water use if drains are already slow.
- If several neighbors have backups, report it to the city via SF311 as well as calling a plumber.
Source: SFPUC, "Sewer System" overview (sfpuc.gov). Confirm current guidance there.