The City of Key West, Florida

The Southernmost City in the Continental United States


Flood Vents

In all but type "X" flood zones, newly enclosed areas below flood levels may only be utilized for minimal storage, parking and access to upper levels.  Such enclosed areas must have flood vents installed.

In type "V" flood zones and "Coastal A Zones," flood vents must be accompanied by breakaway walls. (See Online Flood Maps)

Flood vents are necessary to keep the high-water pressure from collapsing walls.  Allowing water to flow freely under such buildings equalizes the water pressure on both side of the walls.

These vents can be as simple as holes cut in walls or as elaborate as specially engineered vents that remain closed until flood waters rise, then automatically rotate open horizontally to allow for the passage of flood waters in and out of the enclosed space.  

When flood waters rise to 1.5-feet or higher, the pressure exerted upon walls becomes strong enough to collapse those walls. 

Flood waters often recede faster than they rose. Standing water trapped inside a foundation when the flood recedes can cause walls to explode outward.  Flood vents allow water pressure to equalize on both sides of a wall, preventing collapse.

collapased foundation wall

Flood vents may be as simple as openings or gaps in walls, created by skipping a block as shown in the photo to the right.

Some builders opt to turn a few blocks on their side so the openings are parallel to the ground. 

So long as these opening are the correct size, placement and number.

vent open block

Commercial buildings have the option of floodproofing rather than installing flood vents. (See Floodproofing)

Quick Tips:

  • Bottom of flood vents can't be higher than 12" above either the exterior grade or interior floor. 
    • So... if the floor is higher than 12" above the grade - but still below the flood level - then this 12" measurement is taken from the interior floor. 
  • Vent size is determined by the size of the area being protected. 
    • Formula: One Square Inch of vent opening : One Square Foot of floor space [1-inch / 1-foot]
      • Example: 100sf of floor space would require 100si of open vent space
    • At least two (2) flood vents are required for each enclosed area. 
      • The total number of required square inches of vent opening, is divided in half.  Together, the two vents meet the total number of square inches of vent space.
  • Number of Vents - minimum of two - must be placed on at least two different walls.
  • No vent can be smaller than three inches in any direction on the plane of the wall (radius, so that's 3" from a center point in any direction, thus a minimum 6" diameter opening).
  • Lovers in vents detract from the area of open vent space.  Only the open area - free from obstructions such as louvers - can be counted toward the total number of inches required.
  • Manual closures aren't permitted.  If a vent comes with manual closures, this feature must be permanently disabled with vents in the open position.
    • Some flood vents come with plugs or covers that someone has to remove before a flood.  Such vents aren't permitted in Florida.
  • Insect screens that do not impede the entry and exit of floodwaters are allowed and do not affect the determination of net open area of vents.
  • Air vents don't qualify as flood vents.
  • Engineered flood vents must be certified by a State of Florida licensed engineer or ICC certification.

Engineered Flood Vents:

eng vents

There are manufactured flood vents designed to pass a greater volume of water than a standard vent opening.  Use of such vents often means fewer and smaller vents are necessary.

Some of these engineered vents have covers that remain closed and automatically open as flood waters rise. Absent a flood, the rotating center remains vertical and locked in placed.  The locking pins are attached to internal flotation blocks that rise with the flood waters. At some point, the pins disengage, and the center panel turns sideways, causing the water to flow and pressure to equalize on both sides of a wall, preventing collapse.

Tip: Final inspection on these vents often fail because the retaining clips weren't installed. These clips come in a small package inside the box and are frequently discarded because installers don't understand their purpose. Absent these retaining pins, when the vent is turned sideways the center section is released and can be removed. These small pins prevent the vent from floating away after being opened by rising flood waters.

These engineered automatic flood vents were installed at the proper height, but still failed inspection.

They were both placed on the same wall.  Flood vents must be on at least two different walls.

side-by-side smart vents

 Helpful Flood Vent Publications:

FEMA: Technical Bulletin 1, Openings in Foundation Walls and Walls of Enclosures (2008)

FEMA: Openings in Foundation Walls and Walls of Enclosures





Contact Us

Scott Fraser,
FEMA Coordinator
Building Department
1300 White St
Key West, FL 33040
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