Wet Floodproofing


Wet Floodproofing includes permanent or contingent measures applied to a structure or its contents that present or provide resistance to damage from flooding while allowing floodwaters to enter a structure or area.

Federal Emergency Management Agency. “Wet Floodproofing.” Federal Emergency Management Agency. Accessed March 20, 2020. https://www.fema.gov/wet-floodproofing.

Wet floodproofing allows water to enter a historic building during a flood event and drain out as the floodwaters recede.


Jenifer Eggleston, Jennifer Parker, and Jennifer Wellock. “Guidelines on Flood Adaptation for Rehabilitating Historic Buildings.” National Park Service, November 2019. https://www.nps.gov/tps/standards/rehabilitation/flood-adaptation-guidelines.pdf.


Think of a house as a scuba diver. If a diver goes too deep, the water exerts pressure on their body which can make breathing difficult. However, if the diver is in a pressurized suit, then they feel the pressure less. The same applies to the walls of a building. There is only so much pressure that walls can take from flood waters. When water is not allowed in, the pressure is only on one side of the wall, this can cause cracking or collapse. By allowing water into a prepared space in the structure, such as a basement or crawlspace, the water then exerts pressure on both sides equalizing itself. This is only effective in low flood levels that do not exceed the height of the floodproofed space. Allowing water in also reduces the hydrostatic uplift that can cause the building to float. 

Wet Floodproofing with a subgrade basement (Source: FEMA)

In order to allow water movement throughout a space, flood vents must be installed. “The total number, size, and locations of the vents is based on the square footage of the building and the anticipated performance of the vents” (Jenifer Eggleston, Jennifer Parker, and Jennifer Wellock 2019). Obviously, all mechanical and utility equipment should be relocated in order to avoid damage to those systems. Ideally, they should be elevated to the highest floor in case water exceeds the wet floodproofed space. 

The basement or crawlspace that is being adapted must also be prepared to have water in it. The walls, floors, and other surfaces need to be water resistant and made from impervious materials. For example if the walls are drywall that will need to be replaced by a water-resistant materials, such as non-paper-backed gypsum or marine wood.  In the case of sub-grade spaces, as in the image above, the grade will need to match the grade of the outside space. This will allow for water to move as it needs to and drain while also ensuring that pressure equilibrium.

National Flood Insurance Program

FEMA manages the National Flood Insurance Program (NFIP) which aims at reducing the effects of flooding on private and public structures by creating a standardized baseline for adaptation, while also providing insurance.  

The policies set forth by the NFIP for wet floodproofing are outlined in Technical Bulletin 7-93 and below briefly:

  • Wet floodproofing is allowed in specific situations in A zones 
  • If a building is in a V zone, it must meet a very specific set of requirements for anything below base flood elevation (BFE)
  • Enclosed areas below the BFE can be used for parking, building access, or limited storage. 
  • A garage that is attached must be wet floodproofed with additional entry points.

There are variances that are allowed in specific cases. Variance is a legal term which is an allowance of deviation from the stated law or policy. To receive a variance, it requires proof of hardship. The conditions for a variance are—(1) the hardship must not be self-created, (2) the facts of the hardship must be unique to the specific property in question and not common to area or neighborhood, (3) that granting the requested variance cannot alter the character of the neighborhood. A preservation example is a homeowner wants to elevate their house to BFE instead of DFE because the massing would be skewed by the extra two feet of freeboard required by the local government. They can apply for an area variance, which deals with dimensional deviation requests, on the grounds of harming the integrity of the historic resource thus altering the character. Alternatively, there is also a use variance, which allows a deviation from the use standard of the city thought the means of financial hardship. 

So if we are looking at Nantucket and its historic buildings, FEMA has built in a variance. “Variances may be issued for the repair or rehabilitation of historic structures upon a determination that the proposed repair or rehabilitation will not preclude the structure’s continued designation as a historic structure and the variance is the minimum necessary to preserve the historic character and design of the structure.” (Code of Federal Regulations, CFR 44 §60.6 (a)). With that being said, this is a great segue into what wet floodproofing is for historic structures. 

Wet Floodproofing Historic Structures

Turning to the National Park Service, the Secretary of the Interior’s Standards for the Treatment of Historic Properties is a publication that lays out the guidelines for Preservationists to use. Resilience projects, such as wet floodproofing, are a part of Rehabilitation, due to the nature of physical alterations that are attempting to adapt the building to changing climates and more frequent flooding. An addendum of sorts is the new Guidelines on Flood Adaptation for Rehabilitating Historic BuildingsThis document was published in November 2019 as a guide into what flood adaptation looks like under the rehabilitation standard. Instead of organizing by material or architectural features, this document is organized by adaptation method. Similar to the Standards, there are “Recommended” and “Not Recommended” sub-sections in every adaptation method which are more technical support. Unlike the Standards, there is a sub-section in every adaptation chapter that lays out “Technical Limitations.” These state that an adaptation method may or may not be ideal for a particular structure. Obviously, if a building has a high level of interior integrity, then flooding that interior on purpose is not recommended. All building components must be able to withstand hydrostatic forces. There needs to be adequate ventilation to dry out what did flood. Understanding what is required and thinking through how water will move is important because what the water is doing on the exterior is what it will do inside the space. 

The “Recommended” and “Not Recommended” section is as it sounds. For wet floodproofing, the NPS addresses structural needs, utilities, site drainage and venting, interior alterations, and property clean up post flooding event. Browse through the gallery below to see the recommendations for each category. 


Being proactive and pragmatic is important to protecting structures, and particularly historic structures, from water and the future of climate change. The earlier adaptation methods are pursued, the more time to adjust and innovate there is. Maybe someone in Nantucket comes up with a new way to wet floodproof that works better than the methods described above. Looking at these issues in a constructive and inventive manner is always encouraged. 

So Let’s Take ACKtion!

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