Elevating Homes with Substructures

If you are elevating a house that has been substantially damaged or is being substantially improved, your community’s floodplain management ordinance or law will not allow you to have a basement, as defined under the NFIP.

The NFIP regulations define a basement as “any area of the building having its floor subgrade on all sides.” If your house has such a basement, you will be required to fill it in as part of any elevation project. Note that the National Flood Insurance Program (NFIP) definition of basement does not include what is typically referred to as a “walkout-on-grade” basement, whose floor would be at or above grade on at least one side.

If your house has been substantially damaged or is being substantially improved and is in a Coastal High Hazard Area (Zone V, VE, or V1-V30 on the Flood Insurance Rate Map (FIRM) for your community), your community’s floodplain management ordinance or law will require that the bottom of the lowest horizontal structural member (rather than the lowest floor) be elevated to or above the BFE. In many houses, the lowest horizontal structural member is a beam that supports the framing of the lowest floor.

With the exception of Elevating on an Open Foundation, described at the end of this chapter, the elevation techniques presented in this guide are not appropriate for houses in Coastal High Hazard Areas. If you have any doubt about the type of flood hazards that may affect your house, check with your local officials. Existing Foundation In general, the most economical approach to elevating a house is to use as much of the existing foundation as possible.

Although some elevation methods do not allow this approach, most do. If you choose one of the latter, a design professional must evaluate the ability of your existing foundation to support the loads that will be imposed by the elevated house and, as discussed in the next section, the loads expected to result from flooding and other hazards at the site. If changes must be made to the foundation to increase its strength and stability, they can be made as part of your retrofitting project, but they can increase both the cost of the project and the time required to complete it.

The type of foundation on which your house was originally built (basement, crawlspace, slab-on-grade, piers, posts, pilings) also can affect the elevation process. This issue is discussed later in this chapter, in the section The Elevation Techniques. Hazards Because so many elevation techniques are available, elevation is practical for almost any flood situation, but the flooding conditions and other hazards at the house site must be examined so that the most suitable technique can be determined.

Regardless of the elevation technique used, the foundation of the elevated house must be able to withstand, at a minimum, the expected loads from hydrostatic pressure, hydrodynamic pressure, and debris impact. It must also be able to resist undermining by any expected erosion and scour. If you are elevating a house in an area subject to high winds, earthquakes, or other hazards, a design professional should determine whether the elevated house, including its foundation, will be able to withstand all of the horizontal and vertical forces expected to act on it. In making this determination, the design professional must consider a number of factors, including the structure and condition of the house, the soil conditions at the site, the proposed elevation technique, and the hazards at the site. The conclusion may be that additional modifications must be made during the retrofitting project.

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