Home Builder’s Guide to Coastal Construction
FEMA 499/June 2005
Technical Fact Sheet No. 15
Purpose: To discuss the use of foundation walls in coastal buildings.
--Foundation walls include stemwalls, cripple walls, and other solid walls.
--Foundation walls are prohibited by the National Flood Insurance Program (NFIP)
in V zones. (read note below)
--Use of foundation walls in A zones in coastal areas should be limited to
locations where only shallow flooding occurs, and where the potential for
erosion and breaking waves is low.
--Where foundation walls are used, flood-resistant design of foundation walls
must consider embedment, height, materials and workmanship, lateral support at
the top of the wall, flood openings and ventilation openings, and interior grade
Note: Note that the use of shearwalls below the design flood elevation (DFE) may
be permitted in limited circumstances (e.g., lateral wind/seismic loads cannot
be resisted with a braced, open foundation. In such cases, minimize the length
of shearwalls and the degree of obstruction to floodwaters and waves, orient
shearwalls parallel to the direction of flow/waves, do not form enclosures).
Consult the authority having jurisdiction for guidance concerning shearwalls
below the DFE.
Foundation Walls – When Are They Appropriate?
Use of foundation walls – such as those in crawlspace and other solid-wall
foundations – is potentially troublesome in coastal areas for two reasons: (1)
they present an obstruction to breaking waves and fast-moving flood waters, and
(2) they are typically constructed on shallow footings, which are vulnerable to
erosion. For these reasons, their use in coastal areas should be limited to
sites subject to shallow flooding, where erosion potential is low and where
breaking waves do not occur during the Base Flood. The NFIP prohibits the use of
foundation walls in V zones (read note above). This Home Builder’s Guide to
Coastal Construction recommends against their use in many A zones in coastal
areas. Deeply embedded pile or column foundations are recommended because they
present less of an obstruction to floodwaters and are less vulnerable to
Graphic: Foundation walls: flood-resistant design considerations
Design Considerations for Foundation Walls
The design of foundation walls is covered by building codes and standards (e.g.,
Standard for Hurricane Resistant Residential Construction, SSTD 10, by the
Southern Building Code Congress International). For flood design purposes, there
are six additional design considerations: (1) embedment, (2) height, (3)
materials and workmanship, (4) lateral support at the top of the wall, (5) flood
openings and ventilation openings, and (6) interior grade elevation.
Embedment – The top of the footing should be no higher than the anticipated
depth of erosion and scour (this basic requirement is the same as that for
piers; see figure at right and Fact Sheet No. 14). If the required embedment
cannot be achieved without extensive excavation, consider a pile foundation
Height – The wall should be high enough to elevate the bottom of the floor
system to or above the DFE (see Fact Sheet No. 4).
Materials and Workmanship– Foundation walls can be constructed from many
materials, but masonry, concrete, and wood are the most common. Each material
can be specified and used in a manner to resist damage due to moisture and
inundation (see Fact Sheet No. 8). Workmanship for flood-resistant foundations
is crucial. Wood should be preservative-treated for foundation or marine use
(aboveground or ground-contact treatment will not be sufficient). Cuts and holes
should be field-treated. Masonry should be reinforced and fully grouted (see
Fact Sheet No. 16 for masonry details). Concrete should be reinforced and
composed of a high-strength, low water-to-cement ratio mix.
Graphic: Floor slab atop backfilled stemwall foundation. Floor joist system and
Lateral Support at the Top of the Wall – Foundation walls must be designed and
constructed to withstand all flood, wind, and seismic forces, as well as any
unbalanced soil/hydrostatic loads. The walls will typically require lateral
support from the floor system and diaphragm, and connections to the top of the
walls must be detailed properly. Cripple walls, where used, should be firmly
attached and braced.
Flood Openings and Ventilation Openings – Any area below the DFE enclosed by
foundation walls must be equipped with openings capable of automatically
equalizing the water levels inside and outside the enclosure. Specific flood
opening requirements are included in Fact Sheet No. 27. Flood openings are not
required for backfilled stemwall foundations supporting a slab. Air ventilation
openings required by building codes do not generally satisfy the flood opening
requirement; the air vents are typically installed near the top of the wall, the
flood vents must be installed near the bottom, and opening areas for air flow
may be insufficient for flood flow.
Interior Grade Elevation – Conventional practice for crawlspace construction
calls for excavation of the crawlspace and use of the excavated soil to promote
drainage away from the structure (see left-hand figure on page 3). This approach
may be acceptable for non-floodplain areas, but in floodplains, this practice
can result in increased lateral loads (e.g., from saturated soil) against the
foundation walls and ponding in the crawlspace area. If the interior grade of
the crawlspace is below the DFE, NFIP requirements can be met by ensuring that
the interior grade is at or above the lowest exterior grade adjacent to the
building (see right-hand figure on page 3). When floodwaters recede, the flood
openings in the foundation walls allow floodwaters to automatically exit the
crawlspace. FEMA may accept a crawlspace elevation up to 2 feet below the lowest
adjacent exterior grade; however, the community must adopt specific requirements
in order for this type of crawlspace to be constructed in a floodplain.
If a stemwall and floor slab system is used, the interior space beneath the slab
should be backfilled with compacted gravel (or such materials as required by the
building code). As long as the system can act monolithically, it will resist
most flood forces. However, if the backfill settles or washes out, the slab will
collapse and the wall will lose lateral support.
Graphic: crawlspace construction: interior grade elevation for A zones not
subject to breaking waves and erosion.