Basement waterproofing in NYC: keeping the next flood out

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Start with where the water came in

The Department of Environmental Protection names four kinds of flooding in New York City. Each points to a different kind of waterproofing, so the first step is to match your basement to one of them.

How the kind of flooding points to the fix

Kind of flooding (DEP)	How it gets in	Where waterproofing starts
Sewer backup	Heavy rain beyond the pipes’ capacity overflows through basement sewer connections	Downspouts off the sewer; a backwater valve, chosen by a plumber or engineer
Groundwater	Saturated ground pushes water through foundation cracks or a leaking floor drain	Sealing cracks, drainage around or under the slab, the drain itself
Street flooding	Rain pools over blocked catch basins and runs down areaways, stairwells and ramps	Report the catch basin to 311; raise or protect low entrances
Coastal flooding	High tide and surge flood the shore and push water back into the sewer	Know your flood zone; backwater valves; plan for water you cannot keep out

Notice the marks the last flood left. As general practice: water that rose from the drain and left the walls dry above a line often points to the sewer. Damp patches, white mineral streaks or seepage at the joint between wall and floor often point to groundwater. A wet trail from the stairs or the door often points to street water.

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The downspout rule

The first step DEP asks of every homeowner is not optional. “It is mandatory for homeowners to ensure their downspouts (rain gutters) are disconnected from the City’s sanitary sewer system,” DEP says, because those connections “can overwhelm the sewer with rainwater and cause sewer back-ups in basements.” Its guidance is to redirect the water at least 6 feet away from the foundation, and it notes that non-compliance may result in fines.

Sending water 6 feet out also helps with groundwater: water dumped at the foot of a wall soaks straight down beside the foundation. Make sure the ground there slopes away from the building rather than toward it, and that the new outlet does not send water onto a neighbor’s property or a sidewalk where it will freeze.

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Backwater valves

A backwater valve sits on the building’s sewer line and closes when sewage tries to flow back in. FloodHelpNY, a flood information site for New Yorkers, lists the common types as gate valves, ball valves and flapper valves, also called check valves or duckbill valves. It leaves the choice to a professional: a licensed plumber or engineer, it says, can tell you which type, if any, suits your home. In some buildings a valve is not the answer at all.

FloodHelpNY has run a Residential Backwater Valve Installation Program and lists these communities as eligible: Lower Manhattan, Rockaway East, Southeast Brooklyn Waterfront, Gravesend and Bensonhurst, Howard Beach, Gerritsen Beach and Sheepshead Bay, and the Southern Brooklyn Peninsula. Whether the program is taking applications now is something to confirm with FloodHelpNY directly; this site cannot say.

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Groundwater: sealing and drainage

Where water comes through the walls or slab, waterproofing contractors use a few broad approaches. These are general practice, not rules from a city agency, and a contractor who inspects the basement decides which fits:

• Sealing cracks in a poured or block foundation from the inside, for small, localized seepage.
• Interior drainage: a channel cut around the edge of the floor that collects water at the wall and carries it to a sump pit, where a pump lifts it out.
• Exterior waterproofing: digging down outside the wall to apply a membrane and drainage, which is a bigger job and often hard on attached row houses with no side yard.
• The floor drain itself: a drain that lets groundwater up may need repair, a plug or a valve.

Ask any contractor to explain which kind of flooding their proposal addresses. A sump pump is not designed to stop a sewer backup, and a backwater valve is not designed to stop groundwater through a cracked wall.

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Street and storm water

Where water runs in from outside, the fixes are about the openings: areaway and stair drains kept clear, window wells covered or raised, door thresholds sealed, and the slope of a driveway or ramp checked. DEP cleans and maintains about 150,000 catch basins and asks residents to report a clogged one to 311, which is worth doing before the next storm rather than during it. DEP’s “Rainfall Ready: A Homeowner’s Guide,” published in 12 languages, gathers its tips for protecting a home from flooding and sewer backups.

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Know your flood risk first

If the home is in or near the coastal high-risk area, waterproofing has limits. NYC Flood Maps describes the high-risk area as one with a one percent chance of flooding in any given year and a twenty-six percent chance over the life of a thirty-year mortgage. NYC Health also reports a 200% increase in high tide flood days between 2000 and 2021. Inland, it notes that rainfall flooding can happen anywhere in the city. The NYC flood zones guide explains how to look up an address.

Some owners consider filling in a basement altogether. FloodHelpNY notes that this needs an engineer and a permit from the NYC Department of Buildings. It is a structural change, not a waterproofing product.

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Waterproofing and mold

“The key to mold control is moisture control,” says the EPA. A basement that stays damp between floods grows mold on stored boxes, drywall and wood framing. Waterproofing, a dehumidifier and keeping stored things off the floor all serve the same purpose. If the basement has already flooded, make sure it was dried properly before any new walls or finishes go in: sealing in damp materials traps the problem.

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Basement waterproofing in Brooklyn, Staten Island and Manhattan

Brooklyn and Staten Island both have long shorelines, and several FloodHelpNY program communities sit on the Brooklyn shore: Southeast Brooklyn Waterfront, Gerritsen Beach and Sheepshead Bay, Gravesend and Bensonhurst, and the Southern Brooklyn Peninsula. In Manhattan the basement is often a building’s cellar with its boiler, meters and storage, and decisions go through the owner, board or managing agent. In the Bronx, inland basements see rain and sewer flooding more than the sea.

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Order of work after a flood

1. Clean up and dry the basement fully. See flood cleanup in NYC.
2. Work out which kind of flooding it was, using the table above.
3. Disconnect downspouts from the sewer and send the water 6 feet out.
4. Have a plumber or engineer look at the sewer line and say whether a backwater valve suits the building.
5. Have a waterproofing contractor look at cracks, seepage and drainage.
6. Rebuild finishes only after the space reads dry. See flood damage repair.

For the prevention steps in a shorter form, read the post on preventing basement flooding in NYC. If the basement is wet right now, start with basement flooding in NYC.

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Questions people ask