Foundations in Coastal Areas
Home Builder’s Guide to Coastal Construction
FEMA 499/June 2005
Technical Fact Sheet No. 11
Purpose: To describe foundation types suitable for coastal environments.
--Foundations in coastal areas must elevate buildings above the Base Flood
Elevation (BFE), while withstanding flood forces, high winds, scour and erosion,
and floating debris.
--Foundations used for inland construction are generally not suitable for
--Deeply embedded pile or column foundations are required for many coastal
areas; in other coastal areas, they are recommended – instead of solid wall,
crawlspace, slab, or other shallow foundations that can be undermined easily.
("Deeply embedded" means sufficient penetration into the ground to accommodate
storm-induced scour and erosion and to resist all design vertical and lateral
loads without structural damage.)
--Areas below elevated buildings in V zones must be “free of obstructions” that
can transfer flood loads to the foundation and building (see Fact Sheet No. 27).
Graphic: Storm surge and waves overtopping a barrier island during Hurricane
Foundation Design Criteria
All foundations for buildings in flood hazard areas must be constructed with
flood-damage-resistant materials (see Fact Sheet No. 8) and must do two things
in addition to meeting the requirements for conventional construction: (1)
elevate the building above the BFE, and (2) prevent flotation, collapse, and
lateral movement of the building, resulting from loads and conditions during the
design flood event (in coastal areas, these loads and conditions include
inundation by fast-moving water, breaking waves, floating debris, erosion, and
Because the most hazardous coastal areas are subject to erosion and extreme
flood loads, the only practical way to perform these two functions is to elevate
a building on a deeply embedded and “open” (i.e., pile or column) foundation.
This approach resists storm-induced erosion and scour, and it minimizes the
foundation surface area subject to lateral flood loads – it is required by the
National Flood Insurance Program (NFIP) in V zones (even when the ground
elevation lies above the BFE) and is recommended for coastal A zones. However,
even a deeply embedded open pile foundation will not prevent eventual
undermining and loss due to long-term erosion (see Fact Sheet No. 7).
Performance of Various Foundation Types in Coastal Areas
There are many ways to elevate buildings above the BFE: fill, slab-on-grade,
crawlspace, stemwall, solid wall, pier (column), and pile. Not all of these are
suitable for coastal areas. In fact, several of them are prohibited in V zones
and are not recommended by the Home Builder’s Guide to Coastal Construction for
A zones in coastal areas.
Fill – Because fill is susceptible to erosion, it is prohibited as a means of
providing structural support to buildings in V zones and must not be used as a
means of elevating buildings in any other coastal area subject to erosion,
waves, or fast-moving water.
Slab-on-Grade – Slab-on-grade foundations are also susceptible to erosion and
are therefore prohibited in V zones. They also are not recommended for A zones
in coastal areas. (Note that parking slabs are often permitted below elevated
buildings, but are themselves susceptible to undermining and collapse.)
Graphic: Building failure caused by undermining of slab-on-grade foundation
during Hurricane Fran.
Crawlspace – Crawlspace foundations are prohibited in V zones and are not
recommended for A zones in coastal areas. They are susceptible to erosion when
the footing depth is inadequate to prevent undermining. Crawlspace walls are
also vulnerable to wave attack. Where used, crawlspace foundations must be
equipped with flood openings; grade elevations should be such that water is not
trapped in the crawlspace (see Fact Sheet Nos. 15 and 27).
Graphic: Failure of crawlspace foundation undermined by scour.
Stemwall – Stemwall foundations are similar to crawlspace foundations in
construction, but the interior space that would otherwise form the crawlspace is
often backfilled with gravel that supports a floor slab. Stemwall foundations
have been observed to perform better during storms than many crawlspace and pier
foundations. However, the building code may limit stemwall height to just a few
feet. Flood openings are not required in a backfilled stemwall foundation.
Stemwall foundations are prohibited in V zones but are recommended in A zones
subject to limited wave action, as long as embedment of the wall is sufficient
to resist erosion and scour.
Solid Foundation Walls – Solid foundation walls are prohibited by the NFIP in V
zones and are not recommended for A zones subject to breaking waves or other
large flood forces – the walls act as an obstruction to flood flow. Like
crawlspace walls, they are susceptible to erosion when the footing depth is
inadequate to prevent undermining. Solid walls have been used in some regions to
elevate buildings one story in height. Where used, the walls must allow
floodwaters to pass between or through the walls (using flood openings). See
Fact Sheet Nos. 15 and 27.
Pier (column) – Pier foundations are recommended for A zones where erosion
potential and flood forces are small. This open foundation is commonly
constructed with reinforced and grouted masonry units atop a concrete footing.
Shallow pier foundations are extremely vulnerable to erosion and overturning if
the footing depth and size are inadequate. They are also vulnerable to breakage
if materials and workmanship are not first rate. Fact Sheet No. 14 provides
guidance on how to determine whether pier foundations are appropriate, and how
to design and construct them.
Graphic: Pier (column) failures: footings undermined and columns separated from
Pile – Pile foundations are recommended for V zones and many A zones in coastal
areas. These open foundations are constructed with square or round, wood,
concrete, or steel piles, driven or jetted into the ground, or set into augered
holes. Critical aspects of a pile foundation include the pile size, installation
method and embedment depth, bracing, and the connections to the elevated
structure (see Fact Sheet Nos. 12 and 13). Pile foundations with inadequate
embedment will lead to building collapse. Inadequately sized piles are
vulnerable to breakage by waves and debris.
Graphic: Pile failures led to collapse of floor of elevated building.
Foundations for High-Elevation Coastal Areas
Foundation design is problematic in bluff areas that are vulnerable to coastal
erosion but outside mapped flood hazard areas. Although NFIP requirements may
not apply, the threat of undermining is not diminished.
Moreover, both shallow and deep foundations will fail in such situations. Long-
term solutions to the problem may involve better siting (see Fact Sheet No. 7),
moving the building when it is threatened, or (where permitted and economically
feasible) controlling erosion through slope stabilization and structural
Graphic: Insufficient pile embedment and failure of connections at tops of piles
allowed elevated building to be floated off its foundation.
Foundations in V Zones With Ground Elevations Above the BFE
In some instances, coastal areas will be mapped on an NFIP Flood Insurance Rate
Map (FIRM) as V zones, but will have dunes or bluffs with ground elevations
above the BFE shown on the FIRM. Deeply embedded pile or column foundations are
still required in these areas, and solid or shallow foundations are still
prohibited. The presence of a V-zone designation in these instances indicates
that the dune or bluff is expected to erode during the base flood event and that
V-zone wave conditions are expected after the erosion occurs. The presence of
ground elevations above the BFE in a V zone should not be taken to mean that the
area is free from Base Flood and erosion effects.
Graphic: House undermined by bluff erosion. Photograph by Lesley Ewing. Courtesy
of California Coastal Commission.