Flood Vents

collapsed foundation wallsWithout properly placed flood vents, the pressure from floodwaters can collapse walls of enclosures under buildings either when the water is rising or receding. Flood vents allow the water to pass easily through the walls and equalize the water pressure on both sides of the walls preventing collapse. When floodwaters rise to 1.5-feet, the pressure exerted upon walls becomes strong enough to collapse those walls.

In type "A" flood zones, enclosed areas below the flood level may only be utilized for minimal storage, parking, and access to upper levels.  Such enclosed areas must have installed an appropriate number of and properly placed flood vents, depending upon the size of the enclosed area.

In type "V" flood zones, both flood vents and breakaway walls are required. (See Online Flood Maps)

vent open block smallFlood vents can be as simple as holes cut in walls or as elaborate as specially engineered vents that remain closed until floodwaters rise then automatically open to allow for the passage of floodwaters in and out of the enclosed space.  

Floodwaters often recede faster than they rose. Standing water trapped inside an enclosure 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.
  • Flood vents may be as simple as openings or gaps in walls, created by skipping a block as shown in the photo to the left.
  • Some builders opt to turn a few blocks on their side so the openings are parallel to the ground. 
  • So long as these openings are the correct size, placement, and number.
  • Commercial buildings have the option of floodproofing rather than installing flood vents.

Quick Tips:

  • Bottom of flood vents can't be higher than 12" above the outside adjacent grade.
    • Except, when the interior grade (concrete slab) is higher than the outside grade; then it's no higher than 12" above the interior slab.
  • 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
      • 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.
  • Air vents don't qualify as flood vents.
  • Engineered flood vents must be certified by a State of Florida licensed engineer.

Additional Flood Vent / FEMA Technical Bulletin publications

Engineered Flood Vents:

There are manufactured flood vents designed to pExample engineered flood vent dispelling waterass a greater volume of water than a standard vent opening. Thus often 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 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.

Important 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 often 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.