![]() Electrical components such as switches, sockets, circuit breakers, and wiring should also be elevated at least 12 inches above the BFE in order to reduce damage to the electrical system and the chance of fire from short circuits. Per IRC 2021 Section 322, raised-slab floors constructed in flood hazard areas (including A zones and Coastal A zones) should have the slab elevated to at least 12 inches above the BFE. A raised-slab foundation allows the level of the floor to be elevated above grade or expected flood levels (Source: Courtesy of Building Science Corporation) Examples of this for various assemblies are shown on the Training tab and the Flood-Resistant Walls guide.įigure 2. Air, moisture, and insulation control layers of the foundation should be integrated with those layers of the above-grade walls and should be installed to meet or exceed code and home performance requirements.The foundation construction must meet local code structural load and fire risk requirements.Site grading slopes away from the building perimeter.The foundation footings are deep enough to mitigate risk from local erosion and scour.The raised-slab floor is elevated at least 12 inches above expected flood levels (Figure 2).The raised-slab foundations should be designed according to the following parameters: See the Ensuring Success tab and the Climate tab in this guide for more information on risk assessment and flood mapping. Raised-slab foundations have been observed to perform better than crawlspaces in flood conditions in terms of resisting flood forces, but they perform equally poorly in terms of erosion and scour ( FEMA P-550 2009).Ī flood hazard risk assessment should be done prior to design and construction of a home with a raised-slab foundation. Raised-slab and crawlspace foundations are considered appropriate for non-coastal A-zones as long as the local area does not experience breaking waves or significant erosion or scour. This broad wall area also causes more disruption of floodwaters, resulting in local scouring which can undermine footings ( FEMA P-55 Volume II 2011). Due to the broad wall area that raised-slab foundations expose to moving floodwaters, they can experience greater lateral forces than pile or column foundations. Still, open foundations using pilings or columns are far more resistant to damage from flooding. If constructed in Coastal A Zones, the raised-slab foundation must be designed to withstand forces from wave action, debris impact, erosion, and scour. Per IRC 2021 Section 322, raised-slab foundations are not allowed for new construction in coastal high-hazard areas other than Coastal A Zones. However, they are not recommended in coastal high-hazard areas including SFHA Coastal A Zones or V Zones. Raised-slab foundations are a common foundation approach in many regions that can experience flood events. (Source: Adapted from FEMA P-550, Case F). ![]() For a raised-slab foundation, the main floor slab rests on a bed of dirt or gravel that is raised above the exterior grade, leveled, and contained within a poured-concrete or concrete-block stem wall that sits on a poured concrete footing. ![]() Unlike vented crawlspaces, raised-slab foundation walls do not have air or flood vents the foundation walls are solid and continuous.įigure 1. They are similar in construction to a crawlspace, except that the floor above the foundation is made of concrete slab rather than wood and the space under the floor is completely filled with compacted soil and/or gravel rather than being left as empty air space. Raised-slab foundations consist of a continuous perimeter foundation wall that is backfilled on the interior up to the level of the above-grade slab floor (Figure 1). See the Flood Overview Guide for background information on types of floods, flood mapping, the National Flood Insurance Program (NFIP), and terms used throughout this guide such as Special Flood Hazard Area (SFHA), Zones V, A, and Coastal A, the Flood Insurance Rate Map (FIRM), Base Flood Elevation (BFE), Design Flood Elevation (DFE), wet floodproofing, dry floodproofing, and scour. This guide provides key information on how to design a raised-slab foundation to reduce the risk of damage during a flood event.
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