Flood opening

A flood opening or flood vent (also styled floodvent) is an orifice in an enclosed structure intended to allow the free passage of water between the interior and exterior.

In the United States, flood openings are used to provide for the automatic equalization of hydrostatic pressure on either side of a wall. Building codes usually require the installation of flood openings in the walls of structures located in A-type flood zones recognized by the Federal Emergency Management Agency. Various agencies in the United States define necessary characteristics for flood openings.

Most regulatory authorities in the United States that offer requirements for flood openings define two major classes of opening: engineered, and non-engineered. The requirements for non-engineered openings are typically stricter, defining necessary characteristics for aspects ranging from overall size of each opening, to allowable screening or other coverage options, to number and placement of openings. Engineered openings ignore many of these requirements, depending on the particular regulatory authority. To qualify as an engineered opening, testing and/or certification by a qualified agency (varying from regulator to regulator, and indicated below where appropriate) is required.

The American Society of Civil Engineers (ASCE) requirements apply to any structure that is not dry flood-proofed and which is in the mapped flood zone. It calls for openings in load-bearing foundation walls located below the mapped flood elevation. Where non-engineered openings are used, each opening must be at least three inches in diameter, and have no screen or other cover that interferes with the transition of water between interior and exterior. The total net open area of all flood openings in the structure must be equal to or greater than one square inch, per square foot of footprint of the enclosed area—though no fewer than two openings, total, which must be located on different walls. Openings must be placed such that the bottom of each opening is no more than one foot above the adjacent ground level.

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