Decks, Pools, and Accessory Structures
Home Builders Guide to Coastal Construction
FEMA 499/June 2005
Technical Fact Sheet No. 28
Purpose: To summarize National Flood Insurance Program (NFIP) requirements and
general guidelines for the construction and installation of decks, access stairs
and elevators, swimming pools, and accessory buildings under or near coastal
buildings.
Key Issues
--Any deck, accessory building, or other construction element that is
structurally dependent on or attached to a V-zone building is considered part of
the building and must meet the NFIP regulatory requirements for construction in
the V zone (see NFIP Technical Bulletin 5-93 and Fact Sheet Nos. 2, 4, 5, 8, 11,
27, and 30). Attached construction elements that do not meet these requirements
are prohibited.
--If prohibited elements are attached to a building that is otherwise compliant
with NFIP requirements, a higher flood insurance premium may be assessed against
the entire building.
--Swimming pools, accessory buildings, and other construction elements outside
the perimeter (footprint) of, and not attached to, a coastal building may alter
the characteristics of flooding significantly or increase wave or debris impact
forces affecting the building and nearby buildings. If such elements are to be
constructed, a design professional should consider their potential effects on
the building and nearby buildings.
--This Home Builders Guide to Coastal Construction strongly recommends that all
decks, pools, accessory structures, and other construction elements in A zones
in coastal areas be designed and constructed to meet the NFIP V-zone
requirements.
--Post-storm investigations frequently reveal envelope and structural damage (to
elevated buildings) initiated by failure of a deck due to flood and/or wind
forces. Decks should be given the same level of design and construction
attention as the main building, and failure to do so could lead to severe
building damage.
Graphic: Damage from Hurricane Opal in Florida. This deck was designed to meet
State of Florida Coastal Construction Control Line (CCCL) requirements. The
house predated the CCCL and was not.
Decks
Requirements
--If a deck is structurally attached to a V-zone building, the bottom of the
lowest horizontal member of the deck must be elevated to or above the elevation
of the bottom of the buildings lowest horizontal member.
--A deck built below the Design Flood Elevation (DFE) must be structurally
independent of the main building and must not cause an obstruction.
--If an at-grade, structurally independent deck is to be constructed, a design
professional must evaluate the proposed deck to determine whether it will
adversely affect the building and nearby buildings (e.g., by diverting flood
flows or creating damaging debris).
Recommendations
--Decks should be built on the same type of foundation as the primary building.
Decks should be structurally independent of the primary structure and designed
to resist the expected wind and water forces.
--Alternatively, decks can be cantilevered from the primary structure; this
technique can minimize the need for additional foundation members.
--A breakaway deck design is discouraged because of the large debris that can
result.
--A breakaway deck on the seaward side poses a damage hazard to the primary
structure.
--Decks should be constructed of flood-resistant materials, and all fasteners
should be made of corrosion-resistant materials.
Access Stairs and Elevators
Requirements
--Open stairs and elevators attached to or beneath an elevated building in a V
zone are excluded from the NFIP breakaway wall requirements (see NFIP Technical
Bulletin9-99 and Fact Sheet No. 27), but must meet the NFIP requirement for the
use of flood-resistant materials (see NFIP Technical Bulletin 2-93 and Fact
Sheet No. 8). Large solid staircases that block flow under a building are a
violation of NFIP free-of-obstruction requirements (see NIFIP Technical Bulletin
5-93)
--Although they need not be designed to break away under flood forces, access
stairs and elevators are obstructions; therefore, the loads they may transfer to
the main building must be considered by the design professional.
Recommendations
--Open stair handrails and risers should be used because they allow wind and
water to pass through rather than act as a barrier to flow.
--The bottom of the stair, like the foundation of the primary structure, should
be designed and constructed to remain in place during a windstorm or a flood.
--Stairways not considered the primary means of egress can be constructed with
hinged connections that allow them to be raised in the event of an impending
storm or flood (check code requirements before employing this technique).
--Elevators should be installed in accordance with the guidance in NFIP
Technical Bulletin 4-93 and the building code.
Graphic: The rails on these stairs were enclosed with siding, presenting a
greater obstacle to the flow of floodwater and contributing to the flood damage
shown here.
Graphic: Large solid stairs such as these block flow under a building and are a
violation of NFIP free-of-obstruction requirements.
Swimming Pools
Requirements
--An at-grade or elevated pool adjacent to a coastal building is allowed only if
the pool will not act as an obstruction that will result in damage to the
building or nearby buildings.
--When a pool is constructed near a building in a V zone, the design
professional must assure community officials that the pool will not increase the
potential for damage to the foundation or elevated portion of the building or
any nearby buildings. Pools can be designed to break up (frangible pools)
during a flood event, thereby reducing the potential for adverse impacts on
nearby buildings.
--Any pool constructed adjacent to a coastal building must be structurally
independent of the building and its foundation.
--A swimming pool may be placed beneath a coastal building only if the top of
the pool and the accompanying pool deck or walkway are flush with the existing
grade and only if the lower area (below the lowest floor) remains unenclosed.
Under the NFIP, lower-area enclosures around pools constitute a recreational use
and are not allowed, even if constructed to breakaway standards.
Recommendations
--Pools should be oriented with their narrowest dimension perpendicular to the
direction of flood flow.
--Concrete decks or walkways around pools should be frangible (i.e., they will
break apart under flood forces).
--Molded fiberglass pools should be installed and elevated on a pile-supported
structural frame.
--No aboveground pools should be constructed in a V-zone site unless they are
above the DFE and have an open, wind- and flood-resistant foundation.
--Pool equipment should be located above the DFE whenever practical.
--Check with community officials before constructing pools in V zones.
Graphic: siting and design recommendations for swimming pools in coastal areas.
Accessory Buildings
Requirements
--Unless properly elevated (to or above the DFE) on piles or columns, an
accessory building in a V zone is likely to be destroyed during a coastal storm;
therefore, these buildings must be limited to small, low-value structures (e.g.,
small wood or metal sheds) that are disposable. See NFIP Technical Bulletin 5-
93.
--If a community wishes to allow unelevated accessory buildings, it must define
"small" and "low cost." NFIP Technical Bulletin 5-93 defines "small" as less
than 100 square feet and "low cost" as less than $500. Unelevated accessory
buildings must be unfinished inside, constructed with flood-resistant materials,
and used only for storage.
--When an accessory building is placed in a V zone, the design professional must
determine the effect that debris from the accessory building will have on nearby
buildings. If the accessory building is large enough that its failure could
create damaging debris or divert flood flows, it must be elevated above the DFE.
Recommendations
--Whenever practical, accessory buildings should not be constructed. Instead,
the functions of an accessory building should be incorporated into the primary
building.
--All accessory buildings should be located above the DFE whenever practical.
--All accessory buildings should be designed and constructed to resist the
locally expected wind and water forces whenever practical.
--The roof, wall, and foundation connections in accessory buildings should meet
the requirements for connections in primary buildings.
--Accessory buildings below the DFE should be anchored to resist being blown
away by high winds or carried away by floodwaters.
--Accessory buildings (including their foundations) must not be attached to the
primary building; otherwise, failure of the accessory building could damage the
primary building.
--Orienting the narrowest dimension of an accessory building perpendicular to
the expected flow of water will create less of an obstruction to flowing water
or wave action, and may result in less damage.
Graphic: Small accessory building anchored to resist wind forces.
Additional Resources
FEMA. NFIP Technical Bulletin 2-93, Flood Resistant Materials Requirements for
Buildings Located in Special Flood Hazard Areas.
(http://www.fema.gov/fima/techbul.shtm)
FEMA. NFIP Technical Bulletin 4-93, Elevator Installation for Buildings Located
in Special Flood Hazard Areas. (http://www.fema.gov/fima/techbul.shtm)
FEMA. NFIP Technical Bulletin 5-93, Free-of-Obstructions Requirements for
Buildings Located in Coastal High Hazard Areas.
(http://www.fema.gov/fima/techbul.shtm)
[End of Fact Sheet No. 28]