Flood-Resistant Raised-Slab Foundations

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

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.

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). 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. Unlike vented crawlspaces, raised-slab foundation walls do not have air or flood vents; the foundation walls are solid and continuous.

Raised-slab foundations are a common foundation approach in many regions that can experience flood events. However, they are not recommended in coastal high-hazard areas including SFHA Coastal A Zones or V Zones. Per IRC 2021 Section 322, raised-slab foundations are not allowed for new construction in coastal high-hazard areas other than Coastal A Zones. If constructed in Coastal A Zones, the raised-slab foundation must be designed to withstand forces from wave action, debris impact, erosion, and scour. Still, open foundations using pilings or columns are far more resistant to damage from flooding. 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. This broad wall area also causes more disruption of floodwaters, resulting in local scouring which can undermine footings (FEMA P-55 Volume II 2011).    

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. 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).

A flood hazard risk assessment should be done prior to design and construction of a home with a raised-slab foundation. See the Ensuring Success tab and the Climate tab in this guide for more information on risk assessment and flood mapping.

The raised-slab foundations should be designed according to the following parameters:

• The raised-slab floor is elevated at least 12 inches above expected flood levels (Figure 2).
• The foundation footings are deep enough to mitigate risk from local erosion and scour.
• Site grading slopes away from the building perimeter.
• The foundation construction must meet local code structural load and fire risk requirements.
• 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. Examples of this for various assemblies are shown on the Training tab and the Flood-Resistant Walls guide.

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. 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. Plumbing, heating, ventilation, and cooling equipment, including duct work, should also be installed 12 inches or more above the BFE to prevent damage.

Beyond elevation of the floor and equipment, several basic features should be included in all flood-resistant raised-slab foundations. First, the exterior grade of the entire perimeter should be sloped to direct water away from the building (Figure 3). This allows for faster drying potential after a flood and a drier foundation in general. The foundation wall should be damp proofed on the exterior to reduce water absorption during a flood and to allow easier cleanup after a flood. Damp proofing can be achieved by relatively simple means such as covering the stem wall with a parge coat that is then painted with latex paint. A parge coat is a thin coat of a cementitious or polymeric mortar applied to concrete or masonry for refinement of the surface. While parging is not waterproofing, it can help seal the masonry surface and prepare it for waterproofing. A capillary break should be installed at the top of the foundation wall to prevent the movement of moisture up from the foundation wall into the slab floor and the wall above. Finally, the slab should be poured over a plastic vapor barrier (minimum thickness of 6 mil) that is laid over a bed of sand or gravel to minimize water travel up from the compacted soil into the slab.

Several options exist for constructing the foundation stem walls for a raised-slab foundation. One common approach is to use concrete masonry unit (CMU) blocks as shown in Figure 4 (as well as Figure 2 above). The stem wall can also be made of a combination of CMU and brick, as shown in Figure 5, or of poured concrete, as shown in Figure 6.  Note that the slab and wall insulation approaches shown in these figures represent a variety of options which may not be appropriate for all climate zones. Actual below-slab, slab-edge, and wall insulation levels should be designed and installed to meet local code requirements. Note the use of closed-cell rigid foam insulation as a flood-resistant material. Fibrous insulation such as fiberglass or cellulose should not be used in walls or floors that may experience flooding. Consult FEMA’s Flood Damage-Resistant Materials Requirements document for more information on flood-resistant materials. Various approaches to wall construction and insulation are discussed in the Flood-Resistant Walls Guide.