Foundations in Flood Hazard Areas

Shandy Heil, Skinner, Patricia

Slab on Fill within Stemwalls Showing Termite Protection.

In locations that fall in FIRM A zones, the building elevation is measured at the top of the finished floor. In V zones (at risk for storm surge and waves), the elevation is measured at the bottom of the lowest horizontal structural member. The Coastal Construction Manual of the Federal Emergency Management Agency (FEMA) advises applying V-zone standards to construction in A zones that are near open water or V zones where buildings could be exposed to wave action.

Foundations for homes in A zones (except Coastal A):

Slab on compacted fill: Fill dirt is often used to elevate building sites. Some communities prohibit placing fill in the floodplain. Others allow fill but require that an equal volume of flood storage capacity be generated in the same vicinity.

Limiting fill to 2 feet, and only under the footprint of the home, minimizes the loss of flood storage capacity. The fill should slope to natural grade within 2 feet of the foundation. This limited-fill technique has the added advantage of reducing the impact of fill on trees. Foundation fill must be compacted to have adequate bearing capacity and resist erosion; this may be referred to as engineered fill.

When additional elevation is needed, one of the other elevated foundation methods can be used instead or on top of the minimal fill.

Slab cap on filled stemwall: To build on a slab when you need to elevate more than 2 feet above existing grade level, foundation stemwalls can be constructed to the needed height and the interior filled with compacted soil. The stemwall can consist of reinforced concrete block walls on footings or cast-in-place concrete foundation walls engineered for the soil conditions. The top of the stemwall can be shaped to provide a form for the slab; L-shaped blocks are made for this purpose. Even though the slab cap is elevated, a durable moisture barrier of plastic sheeting is still needed over the soil and should extend over the stemwall before the slab is poured.

For appearance purposes, the exterior cladding or brick veneer can extend down over the stemwall but should end above finished grade. This type of elevated foundation is more expensive than other options but may provide architectural and market appeal since it can make the home look larger or more majestic. It also avoids potential crawl space problems (moisture, animals, pipe freeze, etc.) and makes it easier to add a safe room to the interior.

Unfinished, floodable crawl space or level: A similar architectural look can be achieved with a framed floor system over a crawl space without the expense of adding and compacting fill within the stemwall foundation. (See Moisture Control and Termite sections for additional guidelines.)

When a raised house is built with an enclosed crawl space (or any enclosure beneath the BFE), it must be designed to allow free entry and exit of floodwater. The interior cannot be finished, and all materials used below base flood elevation must be flood-resistant. The space below BFE can be used only for parking, access and limited storage.

For any enclosure below BFE, the NFIP requires openings (flood vents) on at least two sides totaling 1 square inch of opening for each square foot of enclosed area. The bottom of these openings must be no more than a foot above the outside grade. Flood vents are not required if an engineer or architect certifies that the design will allow for the automatic equalization of flood water forces on exterior walls. This usually involves areas designed to open or break away with water pressure.

Openings may be fitted with closing vents if those vents conform to the building code for automatic flood venting requirements. A window, door or garage door is not considered a flood opening, but flood openings can be located in them.

Pile, post, column or pier foundation: In areas of slow-moving floodwater, any of these raised foundation types is acceptable. The traditional pier-and-beam foundation with a frame floor system is generally the least-expensive elevated foundation option. Traditional architectural styles in most southern states often include pier-and-beam foundations left open, so this look can offer cultural appeal and community value.

Foundations for homes in V zones and coastal A zones:

Continuous pile, open foundation: The use of fill or stemwall foundations for structural support of buildings is prohibited in V zones. The code requires homes to be elevated on a pile or column foundation, and an engineer or architect must certify that the foundation and structure are anchored to resist floatation, collapse and lateral movement. A pile/column joint may weaken the foundation and require special attention in design and construction. Using a continuous pile - one that extends seamlessly from below grade all the way to the floor (or roof) - avoids that potential weak point. See FEMA 55 and FEMA 550 (under Related Information, below) for more on foundation design.

Free of obstruction or minimal obstruction: For buildings in V zones, the space below BFE must be free of obstruction, meaning left open. If the local ordinance allows, this space can be enclosed with non-supporting breakaway walls, open latticework or insect screening designed to collapse under wind and water loads without causing damage to structural supports or the structure.

Minimum code requirements do not take into account the damage that can be caused when breakaway walls do just that and slam into neighboring buildings. For this reason, it’s best to avoid the use of breakaway components, even if allowed.

Foundations for homes in minimal risk zones (C and X) near A- or V- zone property

Lowest floor is at BFE for the nearest higher risk zone. The flood zone lines shown on a FIRM are adequate for rating insurance, since the zone-boundary errors tend to cancel each other. However, they are not a reliable system for protecting your home.

If your property is near an A or V zone, check the elevation of your property against the BFE for the adjacent zones. If your community does not require you to elevate and your mortgage lender does not require flood insurance, it’s wise to choose to protect yourself by elevating to or above the nearby BFE. It is not unusual for home sites outside of a FIRM flood zone to be lower than the flood zone. Also, flood patterns often change over time, reducing the accuracy of FIRMs, and floods can and do exceed the level of the BFE.

Related information:


1/6/2007 1:40:43 AM
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