Fundamentals of Resilient Design: Wet Floodproofing

The Federal Emergency Management Agency (FEMA) defines wet floodproofing as “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.”

There are a number of key elements to this practice that we’ll look at in turn.

Wet floodproofing strategies, including openings that allow floodwaters to enter the basement. Graphic: FEMA

Letting floodwaters flow through a building without washing it away

A key aspect of wet floodproofing is properly anchoring a structure to its foundation, but allowing floodwaters to enter a building and equalize pressures on foundation walls or lower-level walls. By contrast, if floodwaters are kept out of a building (dry floodproofing, which I’ll address in a future article), there are very significant hydrostatic forces (pressures on those walls) that have to be contended with.

With wet floodproofing, you let the water in. A common approach in coastal areas is to build on structural piers with non-structural lower-level walls or openings that break-away to allow water to flow through and under the building.

With perimeter walls, opening with break-away windows or louvers can allow floodwaters into the building (see illustration).

FEMA and the National Flood Insurance Program require wet floodproofing for buildings, especially residential buildings in flood plains in which part of the building lies below the base flood elevation (BFE)—the elevation at which flooding will occur during a 100-year storm event. Building codes often specify a design flood elevation (DFE) as the base flood elevation plus some additional mount of elevation, such as one foot.

Elevating mechanical equipment

Part of the wet floodproofing approach is getting stuff out of floodable parts of a building that would be damaged by flooding. FEMA calls for mechanical equipment to be elevated above the BFE, but others, including RDI, recommend going well beyond just the BFE for placement of such equipment. I like to see mechanical equipment installed at the BFE plus five feet (BFE+5) whenever possible.

The increasingly common practice of placing mechanical equipment on the roof of larger, commercial buildings and multifamily apartment buildings is an example of this strategy. That gets equipment well above the BFE (unless the building is located in an extraordinarily bad place!), though it does add other complexities, such as building fully weatherproof enclosures, and it takes up space that might otherwise be used for a solar-electric (PV) system, rainwater harvesting, or rooftop greenhouse.

The therapeutic pool at Spaulding is located on the first floor. Photo: Spaulding Rehab Hospital — The therapeutic pool at the Spaulding Rehab Hospital in Boston is located in floodable space on the first floor. The building is designed to allow flooding of the entire first floor with minimal damage. Photo: Spaulding Rehab Hospital

Specifying materials and finishes that can get wet and dry out

An important aspect of wet floodproofing is specifying materials in floodable locations that can be soaked by flooding and dry out without lasting damage or mold growth. Standard paper-faced drywall is to be avoided in such locations, because once it’s flooded, it’s destroyed; soaked paper-faced drywall can’t be salvaged, it has to be replaced.

Other materials to be avoided in floodable spaces, such as basements, include wall-to-wall carpeting, wood and cellulostic materials, sheet materials such as wallpaper and resilient flooring that can trap moisture, and plaster finishes.

Materials that are appropriate for wet floodproofing include poured concrete floors (polished concrete makes an excellent finished floor that can be highly decorative, depending on the concrete aggregate that is exposed during the grinding and polishing process), ceramic tile, brick, and fiberglass-faced drywall (such as the Georgia Pacific DensGlass® product line).

Note that these strategies need not be limited to flood zones. Basements can flood for other reasons, such as plumbing leaks, broken water lines, and burst hoses on clothes washers or dishwashers. Such accidents can cause as much damage as a flooded river or coastal storm surge.

Spaces below the BFE that are protected using wet floodproofing measures should be limited to uses like vehicle parking and limited storage, because vehicles and stored goods can be relocated in advance of a predicted flood.

More on wet floodproofing is available from FEMA, including Technical Bulletin 7 – Wet Floodproofing Requirements (1993), Technical Bulletin 1 – Openings in Foundation Walls and Walls of Enclosures (2008) and Technical Bulletin 2 – Flood Damage-Resistant Materials Requirements (2008).

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Along with founding the Resilient Design Institute in 2012, Alex is founder of BuildingGreen, Inc. To keep up with his latest articles and musings, you can sign up for his Twitter feed. To receive e-mail notices of new blogs, sign up at the top of the page.

David W. Orr, Ph.D

David W. Orr is Paul Sears Distinguished Professor of Environmental Studies & Politics, Emeritus. He also served as “Counselor to the President” of Oberlin College for ten years. He is the author of eight books, including Dangerous Years: Climate Change and the Long Emergency (Yale, 2016) and Down to the Wire: Confronting Climate Collapse (Oxford, 2009) and co-editor of three others. He has authored over 220 articles, reviews, book chapters, and professional publications. In the past twenty-five years he has served as a board member or adviser to ten foundations and on the Boards of many organizations including the Rocky Mountain Institute, Bioneers, and the Aldo Leopold Foundation.

He is currently a Trustee of the Alliance for Sustainable Colorado, Children and Nature Network, and the WorldWatch Institute. He has been awarded nine honorary degrees and a dozen other awards including a Lyndhurst Prize, a National Achievement Award from the National Wildlife Federation, leadership awards from the U.S. Green Building Council (2014) and from Second Nature (2012), and a lifetime achievement award from Green Energy Ohio and the North American Association for Environmental Education. He has lectured at hundreds of colleges and universities throughout the U.S., Europe, Latin America, and Asia. He headed the effort to design, fund, and build the Adam Joseph Lewis Center, which was named by an AIA panel in 2010 as “the most important green building of the past thirty years,” as “one of thirty milestone buildings of the twentieth century” by the U.S. Department of Energy, and as one of “52 game-changing buildings of the past 170 years” by the editors of Building Design +Construction in January, 2016. He was instrumental in the design and funding of the Peter B. Lewis Gateway Center. He is the founder of the Oberlin Project and a founder of the journal Solutions. His current work is on the state of American democracy.