Brooklyn Co-Op Faces Flooding Crisis From Foundation Cracks, Rising Water Table

Michel Monteiro
Senior Project Engineer,
Cowley Engineering

“Vibrations from traffic and a subway station compromised the foundation wall and left the  building vulnerable during heavy rains”

As storm intensity increases in New York City, failing to identify and address leaks quickly can lead to disastrous consequences. The buildup of rust and corrosion on steel can weaken the structural integrity of a building and cause major problems. Michel Monteiro, senior project engineer at Cowley Engineering, recently worked with a Brooklyn co-op where cracks in the foundation wall left the building vulnerable to flooding during heavy rains.

Inundated by Hurricane Ida 

The first time the subcellar at 10 Plaza St. East flooded was in September 2021, when the remnants of Hurricane Ida hit the city — and the flood damage wasn’t contained. The storage room, boiler room, laundry room and bike room were all inundated. “The water even went out into the parking garage along Flatbush Avenue,” Monteiro says. The building is at the corner of Grand Army Plaza, and over the years vibration from traffic and the nearby subway station had resulted in hairline cracks in the foundation wall. Monteiro says this is fairly normal. However, a rise in the groundwater table due to an increase in the intensity and volume of major rain events was making the building vulnerable. “Once the water table rises, surrounding the foundation walls, the water infiltrates through these cracks,” he says. When another intense storm took place the following September and the subcellar was again flooded, Cowley was brought in to address the problem, which involved some triage. The building was involved in a $1 million facade project, so the decision was made to tackle the flood remediation in stages. 

Improving the Drainage System

The laundry room was the main culprit. Because it only had a sump pit, it was the location where the flooding always began, triggering the cascade of problems down below. “It was collecting discharge from the laundry machines and from the drainage in the parking garage, but there was nothing to collect groundwater,” Monteiro explains. To reduce the impact of the rising groundwater table, the decision was made to install a French drain — a trench filled with gravel or rock — along the Flatbush Avenue foundation wall in the laundry room. “This would collect the groundwater as it rises and then discharge it into an existing sump pit with two sump pumps in the laundry room,” he says. Small openings known as weep holes were also drilled around the sump pit to help collect groundwater and redirect it to the sump pit. 

The installation, which cost $45,500, worked at first, but its success was short-lived. When more September rains pummeled the city, the subcellar was submerged once more.

“The drain was working, but sand was coming in through the cracks in the foundation wall,” Monteiro says. This clogged the drain, so water could not be discharged. The flooding even caused the bike room floor slab to crack due to hydrostatic pressure, creating another hazard for the co-op’s cyclists. 

Creating an Impenetrable Waterproof Foundation

The board had to move quickly to avert another inundation. That involved exposing the foundation walls along Flatbush Avenue to repair all the visible cracks and eliminate any paths for groundwater along the building’s foundation walls. “The walls were repaired via concrete patching and injections,” Monteiro says. Once the walls were repaired, a negative-side waterproof coating was applied to the interior of the foundation wall as an added water barrier. Additional weep holes were also drilled around the sump pit basin to collect groundwater through capillary action, and the sump pumps were replaced with higher-horsepower versions. “This was to help account for the additional water the sump pit would be taking on,” he explains. The bike room slab was also replaced and reinforced. This work increased the cost of the repairs to around $200,000. Monteiro says he is confident that rains will no longer penetrate the subcellar.

Staying Vigilant About Cracks and Leaks Down Below

The takeaway: If you see something, say something. Subcellars and basements are not places where residents typically spend a lot of time, and in many cases, they are ignored. “That’s where the issue lies,” Monteiro says, adding that it’s important for the building’s resident manager as well as residents who use the laundry room or parking garage to pay attention to their surroundings and look for signs of infiltration or leaks. “If you see any cracks on the foundation wall, anything that appears hazardous, it should be brought up to the property manager,” he says.