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Why drains back up after heavy rain in older Westchester neighborhoods

Updated October 30, 2026 · 2-minute read · Old House Sewer Lines

A sewer backup after heavy rain usually happens because rain and groundwater have leaked or been piped into sewers designed to carry only sewage, and the system fills faster than it can drain. When that happens, sewage rises up the laterals and comes out at the lowest drain in the house, typically a basement floor drain.

The extra water is called inflow and infiltration. Inflow is water deliberately or carelessly connected: the codes in Rye and the Village of Mamaroneck list roof leaders, sump pumps, cellar drains, yard drains, foundation drains and catch basins among the sources (Village of Mamaroneck Code ch. 282). Infiltration is groundwater seeping through cracked pipe and leaky joints, which EPA lists among the defects common to aging laterals (EPA SSO FAQ). More detail is in inflow and infiltration explained.

The rules on storm water

State and county law both keep storm water out of sanitary pipes. The state plumbing code, in §1101.3, bars rainwater from any sewer built to carry sewage alone. The Westchester County Sewer Act, Chapter 824 of the county's laws, prohibits groundwater, stormwater and excessive inflow and infiltration in the county trunk sewers, and sets the "excessive" line at 150 gallons a day for each resident served (Westchester County model agreement). The county has been under a 2004 state consent order, amended in 2008, that requires it to reduce that extra flow. See roof leaders and sump pumps in the sewer.

Yonkers and combined sewers

Yonkers is different. Some of its neighborhoods drain to combined sewers, built to move sewage and street runoff together in one pipe. The county's 2024 annual report to the state says the combined sewers are owned by the City of Yonkers, with county-owned overflow regulators built on them (NYS DEC, Yonkers Joint CSO report). The 2024 tally appears in the table below. Where the public sewer is combined, the state code requires a building's sanitary sewer to connect to it independently (§703.6). Combined sewers in other southern Westchester towns were not confirmed from an official source.

Yonkers Joint combined sewer overflows, 2024
Measure2024 figure
Overflow outfalls13
Overflow events in the year31
Combined overflow volume1,888.17 million gallons

What protects a basement

The state code requires a backwater valve where fixtures sit below the rim of the next upstream public manhole cover (§714.1). FEMA describes the valve as letting wastewater out while stopping sewage from backing in when municipal mains are overloaded (FEMA). FEMA adds that while the valve is closed, water used in the house cannot drain, so toilets, showers, the washer and the dishwasher should sit idle during heavy storms. See backwater valves. Installation is permitted plumber work.

Insurance is a separate question. FEMA's flood program covers a sewer backup only when flooding caused it, not clogged pipes, and the NY Department of Financial Services describes sewer backup cover on a homeowners policy as an optional endorsement (NY DFS). Policy details are in the New York sewer backup insurance item.

Key facts

  • Rainwater may not be piped into a sanitary-only sewer under §1101.3 of the state plumbing code (NYS DOS, 2025 code).
  • The county Sewer Act treats extra flow above 150 gallons per resident per day as excessive (Westchester County).
  • The Yonkers Joint plant's 2024 report lists 13 overflow outfalls on the Yonkers combined system (NYS DEC).
  • §714.1 requires a backwater valve for fixtures below the next upstream manhole cover (ICC).
  • FEMA flood insurance covers a sewer backup only when flooding caused it (FloodSmart).

Sources