Wet Floodproofing
Wet Floodproofing
Wet floodproofing allows unoccupied portions of a building to be flooded during a storm. It can greatly reduce damage and recovery time. Older buildings may not be designed to withstand the hydrostatic pressure that occurs with dry floodproofing (blocking water from entering the building). With these buildings, or when dry floodproofing is prohibited by code, it is important to allow water to flow through a building in a controlled way. The space can then be dried after flood water has receded.

Description + function

FEMA defines wet floodproofing as “permanent or temporary measures applied to a structure or its contents that prevent or provide resistance to damage from flooding while allowing floodwaters to enter the structure or area. Generally, this includes properly anchoring the structure, using flood resistant materials below the Base Flood Elevation (BFE), protection of mechanical and utility equipment, and use of openings or breakaway walls.” 

2 www.fema.gov/floodplain-management/wet-floodproofing

Strategy into action

Wet floodproofing vents should be located below the BFE and no more than 1 ft. above the ground. (See Strategy: Elevated Equipment)


Relocate or protect equipment that cannot be exposed to water.

Electrical panels, mechanical equipment, gas and electric meters and shut-offs should be relocated from flood-prone areas to locations above the DFE. If that is not possible, they should be protected in place. (See Strategy: Dry Floodproofing.)

Provide floodwater entry and exit points.

To avoid structural damage due to hydrostatic pressure during flooding, provide multiple vent openings through which floodwater can enter and exit.

To satisfy NFIP requirements, the entry points should be flood openings that meet NFIP requirements for number, size and location. Blockage of entries by flood debris must also be taken into consideration.

Use water-resistant building materials below the Design Flood Elevation (DFE).

Building materials installed in floodable spaces—including framing, wallboard, flooring and ceiling paneling—should be able to survive water exposure without major damage, promoting mold or mildew, or absorbing contaminants. Building materials under the DFE should be able to withstand contact with flood waters for up to 72 hours without requiring more than cosmetic repairs.

In coastal areas with salt water, corrosion of metals may be a problem. Materials that might dry out and be usable after exposure to fresh water may be damaged beyond repair by salt water.

Operations + maintenance

Before flooding.

Items used or stored in flood-prone basements or ground-floor spaces should be moved out of the building or to higher floors in advance of a flood. These include:

  • Vehicles.
  • Mechanical equipment including lawn mowers and snow blowers.
  • Furniture.
  • Area rugs.
  • Residents’ belongings.
  • Cleaning supplies and toxic chemicals.

How to quickly remove such items out of floodable spaces before a predicted flood should be clearly laid out in an emergency plan. (See Strategy: Developing an Emergency Management Manual).

After flooding.

Polluted floodwater poses a health risk to facility operators. Engage professional cleaning teams who have been trained and have equipment to mitigate exposure risk. Use commercial fans and dehumidifiers to dry out affected areas to prevent mold growth.

Hydrostatic pressure refers to the pressure exerted by still or slowly-moving floodwater or groundwater against building walls and floors. Hydrostatic pressure increases as the depth of the floodwater increases.

If soil is saturated during flooding, pumping out basements too quickly can lead to structural problems in foundations. If buildings share common foundation or party walls, and one flooded basement is pumped out while the adjacent basement is not, hydrostatic pressure on the common foundation wall can cause damage or collapse. See Resources for FEMA advice on pumping out basements.

Prevent mold growth.

Mold growth is a common problem after flooding. All materials in a flooded space should be dried as soon as possible. The space should be well-ventilated. Porous materials such as drywall, wood or carpeting will need to be discarded. Hardened, non-porous materials including plastic and glass may be cleaned with a commercial cleaner.

Floodwater is often polluted and carries the risk of exposure to pathogens, pollutants, asbestos and heavy metals, as well as danger of electrocution caused by submerged wires. Contaminants can soak into unsealed concrete floors or walls. With paper drywall, wicking action may pull contaminants into the core of walls on upper floors, even if this is not visible from the outside.


Mold growth can lead to serious health problems for building occupants.
Image: Jewish Women’s Archive


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Case Study:
Installing Floodwater Vents

Case StudyInstalling Floodwater Vents


During Superstorm Sandy, Hoboken, NJ suffered heavy damage to its buildings and infrastructure, including this six unit multifamily building, 132 Jackson St., located in the AE flood zone. After the storm, the property faced escalating insurance costs.


To mitigate future risk and reduce insurance premiums, the owner chose a wet floodproofing strategy, which added nine Smart Vents on the first floor and used 9 inches of gravel and concrete fill to raise the floor to ground level. To minimize heat loss during cold weather, the owner chose insulated Smart Vents.


Total cost of the renovation, including installation of Smart Vents and the first floor fill, was $25,000. The one-panel Smart Vents cost $200 to $250 each. the retrofit, the building experienced an 83 percent reduction in the cost of its flood insurance policy. Originally, the owner paid $12,000 for $300,000 worth of coverage; after the retrofit, their premium fell to $2,000 and coverage rose to $820,000. The owner experienced a return on investment in just two and a half years.

Images: www.yourfloodrisk.com
132 Jackson St., Hoboken, NJ
6 Units
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