Vantage Point: Open Up and Let the Waters In

The concept of letting water into a building in order to protect it may seem a little perverse. But the idea is to keep the pressure of floodwaters building up on the outside of a structure from overcoming the strength of the structure’s walls to withstand it. This is called “wet floodproofing”, and aside from elevating or moving the building, it is the only acceptable method of residential building protection under the National Flood Insurance Program (NFIP).

In addition to wet floodproofing, nonresidential structures can also utilize “dry floodproofing,” an example being the application of sealant to the outside of a building. Even so, the limit of sealant above the ground is three feet, to prevent too much hydrostatic buildup, even though flood insurance requires sealant to be applied to a height of a foot above the Base Flood Elevation for it to be considered in rating flood insurance policies. At approximately 62 pounds per cubic foot for freshwater and 64 pounds per cubic foot for saltwater, even one foot of floodwater along a 20-foot long wall exerts considerable pressure. That kind of stress should make it clear why wet floodproofing is important for residences, which don’t usually have walls as thick as fortresses. Too much pressure can deform and implode walls and foundations, or push a home off its foundation.

There are several approaches to wet floodproofing. It can be as simple as leaving out a few bricks or cinderblocks from the foundation, provided that the bottom of the opening (where water actually flows in) is within 1.0 foot of the lowest adjacent grade just below the opening. It can be a little more involved by installing a screened cover to keep out raccoons and snakes. Or it can be a device designed to open on its own, strictly through the pressure of floodwaters and not requiring any human intervention. This last is called an “engineered opening,” and is the primary focus of today’s column.

Local code officials, floodplain managers, and flood insurance agents are concerned about engineered openings, because they need to know the “rated” opening area to know if a building complies with NFIP regulations. For “non-engineered” flood openings, the Elevation Certificate asks for the area in square inches that will allow water to flow into and out of the structure. This might require measuring the area obstructed by bars, scrollwork, or other coverings, then subtracting it from the overall size of the flood opening. But the calculation is pretty straight forward for regulatory and insurance aspects: one square inch of opening for each square foot of enclosed area.

“Engineered openings” come in two flavors: uniquely designed flood vents for a particular structure, and mass-produced manufactured flood vents. Either flavor may be designed to accommodate more than the nominal square foot of enclosure per square inch of flood opening. And this is where confusion breeds. Because of their design, the total area of engineered flood vent openings may be less than if the openings were nonengineered. For this reason, if engineered openings are incorrectly identified, a building may be considered to have insufficient flood openings, and therefore be denied a Certificate of Occupancy or be slapped with higher flood insurance premiums.

Each engineered opening is supposed to have a certification indicating the area it is designed to accommodate. For uniquely designed openings, the engineer should have provided a certification to the property owner (hopefully a copy has also been provided to the local code official and floodplain manager). For mass-produced engineered flood openings, if the property owner hasn’t supplied them, try looking for evaluation reports on (International Code Council Evaluation Service). This is an independent non-profit evaluation group, so reports issued are not self-certifications by manufacturers. Look up the name of the manufacturer on this site, and the PDF reports list the rated opening for the different models evaluated. If a manufacturer or model is not listed, then it hasn’t been evaluated and no certification report is available through this site. Make sure you are searching for “FLOOD vents” or you will be sifting through listings for air vents and steam vents to get to what you want.

What if no certification can be found for an engineered opening? In such cases, the opening will be treated as a non-engineered opening for regulatory and insurance purposes. Since you may not know right away if you will find a certification, the best approach is to document everything you find in the field, including the actual area of each flood vent (as well as any identification as to manufacturer and model).

It is possible that a building will have various models of engineered flood openings by different manufacturers. While complicating note keeping, keeping track of how many of each will help in separating out “rated” opening area from actual opening area–but only after certifications have been secured.

About the Author

Wendy Lathrop, PS, CFM, CFS

Wendy Lathrop is licensed as a Professional Land Surveyor in NJ, PA, DE, and MD, and has been involved since 1974 in surveying projects ranging from construction to boundary to environmental land use disputes. She is a Professional Planner in NJ, and a Certified Floodplain Manager through ASFPM.