Construction in Flood Zones

Notice:   This information is only intended to highlight certain common everyday aspects of floodplain related construction.  It's not intended nor designed to be a complete resource for all codes and requirements.  Contractors and owner-builders are still expected to educate themselves regarding all the necessary requirements.

Construction standards vary depending upon the site's flood zone rating. 

In Key West, most buildings/lots are located within type "A" flood zones.  These are zones more inland from the coastline, where a static rise in water levels can be expected. 

Land close to the shoreline is usually located in type "V" flood zones.  The "V" stands for "velocity," or breaking waves with a force that's considerably more damaging. As a consequence, building standards are much higher within "V" zones.

Type "V" and "A" zones appear on flood maps as "AE-#" and "VE-#.  The "E" denotes this flood zone has an elevation level assigned to it, and is expressed as "AE-6" or "VE-9," with the trailing number indicating the Base Flood Elevation (BFE) for that zone.  That BFE number notes the height above sea level flood waters can be expected to rise... at a minimum. 

The BFE is a flood level reference height for building elevations and flood insurance.  A building below BFE is a building below the flood level for that site. Building codes usually require one extra foot of elevation, known as the Design Flood Elevation (DFE).

Building codes require most new and Substantially Improved buildings be at a minimum height of BFE plus one-foot.

Thus, a new or elevated building in an "AE-6" flood zone would need to be elevated elevated to a minimum of seven (7) feet above mean-sea-level.  An Elevation Certificate is usually needed to determine the height of the land above sea level.  If, for example, the land is determined to be four (4) feet above sea level, then the building would need to be three (3) feet above the ground.

Some historic buildings are exempt from building elevation requirements when being renovated. However, these buildings are often significantly below flood levels, and will result in drastically higher premiums for flood insurance if not elevated.  (For additional information on this historic building exemption, see Historic Building Exemptions.

There are two "X" zones within the city, mostly encircling the highest areas of the downtown or the Old Town section of the city. and on Sunset Key. Within these higher elevations, construction is mostly unregulated for floodplain purposes.  

A-Zone Construction

Important Note: Each building needs its own Building Permit Application, even if they're on the same lot.  Only one set of plans is needed, not a set for each application.

New construction and substantial renovations are required to have the First Finished Floor at a minimum height of Base Flood Elevation (BFE) Plus one-foot. New construction allowed under Rate of Growth Ordinances (ROG) or Building Permit Allocation System (BPAS) have a minimum elevation of BFE plus eighteen inches.

If there's useable space between the ground and bottom of the house, it's considered non-habitable space, used solely for minimal storage, parking, and access to higher levels.

If enclosed, this non-habitable area must:

  • Have appropriately sized, placed and number of approved flood vents,
  • Constructed using flood resistant materials,
  • No interior finishes (e.g. drywall, paneling, flooring etc.),
  • Usage is strictly limited to limited storage, parking & access to higher levels of the building
    • Limited storage means storage of lawn mowers, rakes, wheelbarrows, and similar pieces of equipment that are normally stored outside. Limited storage does not apply to household furnishings, workbenches, freezers, refrigerators, appliances or other items subject to damage if exposed to flood waters.
    • If you are purchasing a home with a finished enclosure below the base flood elevation, it may not have been permitted construction and may be subject to enforcement proceedings. Check before you buy.
  • Elevation Certificates are required before, during and upon final construction

Anchoring of the building and all its components is required.  The building to it's columns/piers, the piers to the ground, stairways and porches to their foundations (using corrosion resistant hardware).

Swimming pools are required to be anchored to the ground.  Otherwise, flood waters could cause empty or partly filled pools to become buoyant and pop out of the ground.

V-Zone Construction

New construction and renovations equating to a Substantial Improvement, are required to have the lowest horizontal structural cross-member at a minimum height of Base Flood Elevation (BFE) Plus 1-foot. (ROGO/BPAS =BFE +1.5').

If there's space between the ground and bottom of the house, it's considered non-habitable space, used solely for minimal storage, parking, and access to higher levels.  Appliances and electrical items are prohibited in these areas.

If enclosed, this non-habitable area must:

  • Have appropriately sized, placed and number of approved flood vents
  • Breakaway walls
    • Nothing - especially electrical wiring or plumbing pipes - either attached to these breakaway walls (attach them to the columns) that would hamper these walls from breaking-away as they're designed to do.  
      • This includes bulky items stored inside against these walls that'll obstruct the walls from breaking-free.
  • Constructed using flood resistant materials
  • No interior finishes (e.g. no drywall, paneling, flooring, very limited electrical wiring/fixtures etc.)
  • Usage is strictly limited to limited storage, parking & access to higher levels of the building
    • Limited storage means storage of lawn mowers, rakes, wheelbarrows, and similar pieces of equipment that are normally stored outside. Limited storage does not apply to household furnishings, workbenches, freezers, refrigerators, appliances or other items subject to damage if exposed to flood waters.
    • If you are purchasing a home with a finished enclosure below the base flood elevation, it may not have been permitted construction and may be subject to enforcement proceedings. Check before you buy.
  • Elevation Certificates are required before, during and upon final construction

Building Location:

New or Substantially Improved buildings in V zones must be located landward of the reach of the mean high tide.  They cannot be built over water.

Elevation on Piles or Columns:

All new construction and substantial improvements to buildings in V Zones must be elevated on pilings, posts, piers or columns.

Other methods of elevating buildings - on fill, solid walls or crawlspaces - and floodproofing are prohibited because these techniques present obstructions to wave action.  The force of a breaking wave is so great that these types of foundations would be severely damaged, resulting in collapse of the building.  Waves can also ramp up on fill and reach the elevated portions of the building. 

How High:

The controlling elevation is the bottom of the lowest horizontal structural member of the lowest floor (not the Finished First Floor as in type A zones).

In an elevated building, the lowest beam, joist, or other horizontal member that supports the building is the lowest horizontal structural member. Grade beams installed to support vertical foundation members where they enter the ground are not considered lowest horizontal members.

If a reinforced slab or grade beam to support a slab were employed (which in most cases it shouldn't), the lowest of these would be considered the lowest horizontal structural member.  Such a scenario would result in exceptionally high flood insurance premiums.

Wind and Water Loads:

The supporting foundation design must account for wind loads in combination with the forces that accompany the base flood forces. Cross bracing and proper connections are key to accomplishing such.

Solid columns are required; block columns and similar foundations aren't acceptable.

Piles must be embedded well below the scour depth.

A "V-Zone Certification" form, completed by a State of Florida Engineer, is required as a design document; in addition to an Elevation Certificate.

Anchoring:  

Buildings and all their components are required to be anchored.  The building to it's columns/piers, the piers to the ground, stairways and porches to their foundations (using corrosion resistant hardware).

Flood Resistant Materials:

Areas below or at flood levels must be constructed using flood resistant materials.

Breakaway Walls:

Walls below established flood levels are required to be certified as breakaway walls.  For additional information review our special section on this topic.

Backflow Prevention:

The design must include measures preventing sewage from backing up in to the building during flooding.

Concrete Slabs:

Reinforced concrete slabs are mostly prohibited in V-zones.  When subjected to very forceful churning waters, such slabs become very dangerous and destructive.  A hydroplaning, flipping or cartwheeling slab will kill people and demolish buildings. They also transmit storm forces to buildings they're attached to, unnecessarily jeopardizing a building's structural integrity during a storm.

As a result, building codes require such slabs be of frangible concrete; meaning they are designed to break into smaller pieces when undermined or struck by violent flood waters, causing the pieces to sink rather than travel.

Therefore... there are special requirements for such slabs:

  • Reinforcement - of any kind - is prohibited
  • Slabs not thicker than four (4) inches
  • Slabs must remain structurally independent of the building

There are flood insurance benefits that come with frangible detached slabs, in that if properly constructed, they aren't considered the building's lowest floor; upon which flood insurance rates are largely based.

If they are attached, too thick, supported by a grade beam or reinforced, the building will receive a flood insurance rating as if the entire building were at ground level.  The difference in annual insurance premiums could be major.

Larger Buildings:

For most circumstances and for small, low-rise V zone structures (including residences), the above alternative of frangible slabs should be employed. This alternative is also appropriate for other uses of slabs such as pool decks, sidewalks, and patios. 

For larger mid- and high-rise structures, both alternatives could be considered, although a second option of self-supporting structural slabs will likely mean a significant increase in NFIP flood insurance premiums (since the bottom of the slab – or the bottom of the lowest hori­zontal member supporting the slab – will become the lowest floor for flood insurance rating purposes). Reinforced, self-supporting structural slabs may be appropriate for large structures that are supported on large concrete piles and columns, since these structures are typically much heavier and are less prone to damage from flood loads. If a frangible parking slab is constructed beneath such a structure, timely reoccupation after a severe coastal storm event – of an otherwise intact and usable structure – may be prevented. A self-supporting structural slab could be considered in such situations

Reinforced, self-supporting structural slabs and beams beneath large buildings should be de­signed to be only as thick as necessary to support vehicle loads and other design loads, but no thicker. The slabs and beams should be connected and integral to the foundations, and all below BFE components should be designed to act together to resist flood loads and other de­sign loads. Obstructive effects will be minimized as long as the slab systems remain intact and horizontal so that floodwaters and waves pass above and below the slabs.