Flood vents in a foundation wall

A house raised above the ground on solid foundation walls has a space underneath, a crawl space, a garage or a storage area. In a flood zone, the walls around that space have flood openings, often called flood vents. They let floodwater flow in and out on its own, so the water level inside the walls rises and falls with the water outside. Without them, standing or slow-moving water pushes on one side of the wall only, and the force can damage the foundation or collapse the building. The openings don't protect against fast-moving water, waves or floating debris.

What they look like

Flood vents sit low on the foundation wall, close to the ground, on more than one side of the house. Many look like ordinary foundation air vents, a metal frame with fixed blades, a grille or an insect screen. Others are concrete blocks turned on their side with screening or engineered vents with moving parts.

The ventilation vents some crawl spaces have near the top of the wall are usually too high to work as flood openings. A crawl space access door doesn't count unless it has an opening built into it.

What stops them working

A flood opening has to let water through with nobody there to open it.

• Closed vents. A standard air vent that can be shut by hand only works as a flood opening if it's permanently fixed open. Closing it blocks the water.
• Covers. A solid cover put over a vent, even one meant to come off, stops the water as long as it's on.
• Clogged screens. Where floodwater carries grass clippings and leaves, insect screens clog.
• Buried vents. Backfill, topsoil or landscaping built up against the wall can raise the ground around the openings.
• Windows and a solid garage door. Neither works as a flood opening. Floodwater can't be expected to break a window, and a garage door without openings has to be opened by someone.

What the space is for

The space under an elevated house is meant for parking, getting into the house and storage, and the storage is best kept to low-value things. Floodwater will reach anything stored or installed below the flood level, from appliances and heating and cooling equipment to ductwork and plumbing fixtures.

An annotated example

What to do next

Walk around the outside of the house and find every flood vent. Clear leaves and clippings off the screens, take off any cover, and don't close any that can be shut. Keep mulch, soil and planters from building up around them. Keep only low-value things stored in the space underneath. What your flood zone means covers how to find out whether the house is in a flood zone.