#Block stem wall foundation how to

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.

#Block stem wall foundation how to

This guide provides key information on how to design a raised-slab foundation to reduce the risk of damage during a flood event.