Flood-Resistant Raised-Slab Foundations

    Scope Images
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    A concrete slab floor can be elevated above local grade as a strategy to prevent flood damage.
    Scope

    If the home is located in a potential flood zone, design a raised-slab foundation to be flood-resistant.

    • Ensure the foundation footing is deep enough to prevent undermining due to scour.
    • Ensure exterior grade slopes away from the house.
    • Elevate the floor assembly.
    • Protect and elevate electrical, HVAC, and other service equipment.
    • Build with materials that are moisture, decay, and corrosion resistant.
    • Design assemblies to easily dry when they get wet.

    See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

    Description

    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.

     

    Stem wall foundation design, including additional reinforcement to resist moving floodwaters and short (1.5-ft) breaking waves
    Figure 1. 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.  (Source: Adapted from FEMA P-550, Case F). 

     

    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.

     

    A raised slab foundation elevates the level of the floor above grade
    Figure 2. 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

     

    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 at least 6-milimeters thick that is laid over a bed of sand or gravel to minimize water travel up from the compacted soil into the slab.

     

    To control surface water, the land should slope away from the building on all sides
    Figure 3.  To control surface water, the land should slope away from the building on all sides (Source: Building Science Corporation 2022). 

     

    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.

     

    Raised-slab CMU foundation including flood-resistant features: sloped grade, damp proofed stem wall, capillary break under the slab (gravel or sand), vapor barrier under the slab and capillary break at the top of the foundation wall (polyethylene sheet)
    Figure 4.  This raised-slab CMU foundation includes flood-resistant features such as a sloped grade, damp proofed stem wall, capillary break under the slab (gravel or sand), vapor barrier under the slab (polyethylene sheet), and capillary break at the top of the foundation wall (polyethylene sheet) (Source: Adapted from Building Science Corporation 2022). 

     

    This raised-slab CMU and brick foundation includes flood-resistant features such as a sloped grade, capillary break under the slab (gravel or sand), vapor barrier under the slab (polyethylene sheet), and capillary break at the top of the foundation wall
    Figure 5.  This raised-slab CMU and brick foundation includes flood-resistant features such as a sloped grade, capillary break under the slab (gravel or sand), vapor barrier under the slab (polyethylene sheet), and capillary break at the top of the foundation wall (Source: Adapted from Building Science Corporation 2022). 

     

    This raised-slab poured concrete foundation includes flood-resistant features such as a sloped grade, capillary break under the slab (gravel or sand), vapor retarder under the slab (rigid insulation), and capillary break at the top of the foundation wall
    Figure 6.  This raised-slab poured concrete foundation includes flood-resistant features such as a sloped grade, capillary break under the slab (gravel or sand), vapor retarder under the slab (rigid insulation), and capillary break at the top of the foundation wall (Source: Adapted from Building Science Corporation 2022). 

     

    Ensuring Success

    A vital step when designing or retrofitting a raised-slab foundation to be flood-resistant is to understand the local flood risk. See the Climate tab for information on determining the flood risk for a specific location. For homes within an SFHA, flood-resistance should be incorporated into any design or retrofit. If a home is not located in an SFHA, there may still be a chance of flooding. It is up to the discretion of the homeowner as to whether flood-resistance measures are worth the investment.

    Obtaining an accurate BFE and understanding how high it is above actual grade are key to successful implementation of flood-resistance measures. Unless the lowest non-flood-resistant construction material in the house is at least 12 inches above the BFE, it is reasonable to expect that flood damage will occur at some point. See the Climate tab for information on determining your flood risk using the FEMA Flood Insurance Rate Maps.

    Climate

    The flood-resistance approaches shown in this guide work in all climates. Understanding the flood risk at a particular location, however, is an important first step in designing and retrofitting for flood-resistance. Flood hazard risk areas are identified by FEMA as Special Flood Hazard Areas (SFHA) and can be found on Flood Insurance Rate Maps (FIRMs).

    FEMA flood maps and related tools can be found on FEMA’s website.

    The National Flood Hazard Layer (NFHL) map is a searchable geospatial database containing current flood hazard information. Using the NFHL Viewer, accessed through FEMA’s website, you can find and print the FIRM for a specific location.

    Figure 1 is an example of the flood hazard information shown on the NFHL Viewer for a specific location.

     

    Flood hazard risk for a particular location can be assessed by viewing flood maps on the NFHL Viewer
    Figure 1.  Flood hazard risk for a particular location can be assessed by viewing flood maps on the NFHL Viewer. (Source: FEMA 2022). 

     

    Figure 2 provides a closer view of one portion of the map in Figure 1. Note the SFHAs are designated by color and pattern and the BFEs are designated by wavy black lines.

     

    A FIRM will identify specific SFHAs (colored and hatched areas) and localized BFEs (wavy black lines)
    Figure 2.  A FIRM will identify specific SFHAs (colored and hatched areas) and localized BFEs (wavy black lines) (Source: FEMA 2022). 
    Right and Wrong Images
    Image
    Right – The raised slab foundation has a 3-ft stem wall of filled concrete block, then is back-filled with compacted dirt and crushed rock, then insulated with 1” rigid foam covered with taped vapor barrier, under a floor slab.
    Right – The raised slab foundation has a 3-ft stem wall of filled concrete block, then is back-filled with compacted dirt and crushed rock, then insulated with 1” rigid foam covered with taped vapor barrier, under a floor slab.
    Image
    Right – This HVAC condensing unit is elevated on a raised-slab
    Right – This HVAC condensing unit is elevated on a raised-slab
    Image
    Right – The raised-slab, CMU block stem wall, above-grade walls, and roof of this house use flood damage-resistant materials, integrated water, vapor, and air control layers, and construction methods which promote good drainage and rapid drying
    Right – The raised-slab, CMU block stem wall, above-grade walls, and roof of this house use flood damage-resistant materials, integrated water, vapor, and air control layers, and construction methods which promote good drainage and rapid drying
    Image
    Right – The raised-slab, brick-and-block stem wall, above-grade walls, and roof of this house use flood damage-resistant materials, integrated water, vapor, and air control layers, and construction methods which promote good drainage and rapid drying
    Right – The raised-slab, brick-and-block stem wall, above-grade walls, and roof of this house use flood damage-resistant materials, integrated water, vapor, and air control layers, and construction methods which promote good drainage and rapid drying
    Image
    Right – The raised-slab, poured-concrete stem wall, above-grade walls, and roof of this house use flood damage-resistant materials, integrated water, vapor, and air control layers, and construction methods which promote good drainage and rapid drying
    Right – The raised-slab, poured-concrete stem wall, above-grade walls, and roof of this house use flood damage-resistant materials, integrated water, vapor, and air control layers, and construction methods which promote good drainage and rapid drying
    Image
    Wrong – The footers of this house were not deeper than the depth of scour during a flood, causing undermining of the foundation
    Wrong – The footers of this house were not deeper than the depth of scour during a flood, causing undermining of the foundation

    Compliance

    The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

    2009201220152018, and 2021 International Residential Code (IRC)

    Section R322 Flood-Resistant Construction. Discusses the determination of the design flood elevation; protection of electrical, HVAC, and plumbing equipment; flood-resistant materials; and flood mitigation requirements.

    R322.3.3 Foundations. This section indicates that raised-slab foundations may not be used in coastal high-hazard areas other than Coastal A Zones. 2009 and 2012 IRC do not include Coastal A Zone in this section.

    2009-2021 IECC and IRC Insulation Requirements Table

    The minimum insulation requirements for ceilings, walls, floors, and foundations in new homes, as listed in the 2009, 2012, 2015, 2018, and 2021 IECC and IRC, can be found in this table

    Retrofit: 2009, 2012, 2012 and 2018 IECC

    Section R501.1.1 in 2018 IRC. Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

    Existing Homes

    A raised-slab retrofit could be challenging and expensive, but the requirements for a retrofit are the same as those for a new design. If the existing slab is 12 inches or more above the BFE, a retrofit may simply involve ensuring the site is graded for positive drainage, elevating and protecting HVAC, electrical, and service equipment, and damp proofing the stem walls.

    Retrofitting an existing slab-on-grade or raised-slab foundation which is below or within 12 inches of the BFE is much more involved, but can be achieved in a few different ways. One option is to lift the entire house, including the slab, and construct a new foundation underneath it. A second option is to lift the entire house excluding the slab, construct a new foundation underneath it, and construct a new floor (raised slab over fill, or framed floor over flood-proof crawlspace or open pile foundation). See the FEMA Homeowner’s Guide to Retrofitting (FEMA P-312 2014) for more detailed information on these options.

    If the first-floor walls are masonry, a third option is to build a new floor higher than the existing slab, by as much as is necessary to get sufficiently above the BFE. The roof would then have to be lifted or demolished, and the walls would need to be extended upwards before re-installing the roof. The height gain of the walls would need to equal the distance that the floor was raised in order to maintain ceiling height. The new floor could be framed inside the masonry walls over the new flood-proofed crawl space or else the new floor could be constructed as a raised-slab by importing fill material to support the new, higher slab. Figure 1 shows the primary steps in this method. In this case the new, raised floor is shown as a wood-framed floor over a wet-floodproofed crawlspace.

     

    This left-to-right sequence shows the method of wall extension to flood-proof a masonry house on a slab foundation. In this case the new, raised floor is a wood-framed floor over a wet-floodproofed crawlspace, but using fill to create a new raised slab is also an option.
    Figure 1.  This left-to-right sequence shows the method of wall extension to flood-proof an existing masonry house on a slab foundation (Source: FEMA P-312 2014).

     

    If the home is two or more stories, and the first-floor walls are masonry, another option is to abandon the first floor completely and retrofit it to become a flood-proofed vented crawlspace. In this scenario, the second floor would become the first occupied floor. This approach would not require building a new floor, extending walls, or lifting or demolishing the roof.

    When considering these retrofit options, it is important to factor cost, time, and energy efficiency goals into the decision. As an alternative to all of the above options, the homeowner may simply choose to demolish and rebuild the home to current flood-resistance standards and recommendations.

    More Info.

    Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

    References and Resources*
    Author(s)
    Institute for Business & Home Safety
    Organization(s)
    Institute for Business & Home Safety
    Publication Date
    Description
    Report covering reducing risk from flooding.
    Author(s)
    Coulbourne,
    Jones,
    Durham,
    Kapur,
    Koumoudis,
    Line,
    Low,
    Overcash,
    Passman,
    Reeder,
    Seitz,
    Smith,
    Tezak
    Organization(s)
    FEMA
    Publication Date
    Description
    Volume 1 of a two-volume report providing a comprehensive approach to planning, siting, and risk management for homes constructed in coastal environments.
    Author(s)
    Coulbourne,
    Jones,
    Durham,
    Kapur,
    Koumoudis,
    Line,
    Low,
    Overcash,
    Passman,
    Reeder,
    Seitz,
    Smith,
    Tezak
    Organization(s)
    FEMA
    Publication Date
    Description
    Volume II of a two-volume report providing a comprehensive approach to desing, construction, and renovation of homes located in coastal environments.
    Author(s)
    Lstiburek
    Organization(s)
    BSC
    Publication Date
    Description
    Report providing guidance on designing buildings to be flood resistant.
    Author(s)
    Federal Emergency Management Agency
    Organization(s)
    FEMA
    Publication Date
    Description
    Online geospatial map with interactive viewer showing flood risk areas and base flood elevations
    *For non-dated media, such as websites, the date listed is the date accessed.
    Contributors to this Guide

    The following authors and organizations contributed to the content in this Guide.

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