Alternatives for Enclosures with Openings
Jack Anderson, FEMA
Historically, older buildings in floodplains have sustained serious damage when flooding occurred because they were not elevated high enough to avoid flood waters. Now, both Federal regulations and internationally recognized building codes contain elevation requirements for the design and construction of buildings in Special Flood Hazard Areas (SFHAs). These regulations require, for construction in the SFHA designated as Zone A, the lowest floor must be at or above the Base Flood Elevation (BFE) shown on the current Flood Insurance Rate Map (FIRM) for the community. If the building is constructed in an SFHA subject to coastal high-hazard flooding (Zone V), it must be elevated on an open foundation such as pilings or other supports, which raises its lowest horizontal structural member to or above the BFE to protect it from damage in a flood or coastal storm surge.
The space beneath elevated buildings may be used only for parking, limited storage, or building access. Enclosures below elevated buildings in Zone V must be designed and constructed to be "breakaway walls" to prevent an obstruction that could transfer flood loads to the building. In Zone A, however, property owners may enclose the area below an elevated building as a crawl space.
Keeping Enclosures Safe
It is important to design enclosures below elevated buildings so they will not be subject to collapse and will protect the structures above them in the event of a flood. In Zone A this means the installation of openings in enclosure and foundation walls to equalize the forces of floodwaters. An enclosure without openings is less likely to survive the hydrostatic forces of flood waters than one with openings, which permit water to pass through unimpeded. For this reason the NFIP created specific requirements regarding the installation of openings or vents for allowing water to flow through the enclosed space beneath an elevated building during a flood. These requirements are codified in Title 44 of the Code of Federal Regulations (44 CFR). Older elevated builings with enclosures can be retrofitted to include openings or vents.
The presence of adequate openings or vents in the enclosure beneath an elevated structure is an important factor in rating an NFIP policy and can, therefore, have a significant impact on the cost of flood insurance coverage for the building.
The Lowest Floor Guide section of the NFIP Flood Insurance Manual describes how proper openings in an enclosure can alter the rating of a flood insurance policy. Compliant enclosures which meet the NFIP prescriptive requirement of 1 square inch of net open area for every square foot of enclosure and are in place on two or more walls will shift the lowest floor designation from the enclosure floor to the first elevated floor. In most cases, the flood insurance premium will be reduced considerably.
Property owners and builders of structures which have, or will require, openings that do not meet the 1 square inch for every square foot requirement can still seek to verify these openings will provide adequate protection. For example, automatic flood vents may provide adequate protection but are smaller than those specified by the Lowest Floor Guide. In these instances, FEMA will recognize one of three alternatives for changing the rate determination.
Certification by an Engineer or Architect
A registered professional engineer or architect can certify the flood openings are designed to automatically equalize hydrostatic flood forces on exterior walls by allowing for the entry and exit of floodwaters. This certification is required to assure community officials the openings are designed in accordance with accepted standards of practice. For guidance concerning acceptable certifications, refer to FEMA Technical Bulletin 1, August 2008, Openings in Foundation Walls for Buildings Located in Special Flood Hazard Areas.
Documentation by a Community Building Official
The community building official can submit a letter or other written evidence explaining the flood openings have been accepted by the community as an alternative to the openings requirement in the International Building Code or the local ordinance based on the issuance of an Evaluation Report on openings by the International Code Council Evalution Service (ICC-ES), Inc.
ICC-ES Evaluation Report
The third acceptable alternative to the 1 inch for every square foot opening requirement is an Evaluation Report issued by the ICC-ES which states the automatic flood vents meet the code requirement. This report provides the specification on the number of flood vents required for a defined square footage of enclosed area below the BFE.
In the event one of these alternative measures is used to meet the NFIP openings in enclosures requirement, the required documentation must be submitted to the Write Your Own (WYO) Company or NFIP Servicing Agent who wrote the policy.
Jack Anderson, CFM, was an insurance adjuster for 4 years--including catastrophe response during the 2004 hurricanes--before joining the Building Science Branch of the FEMA Mitigation Directorate as a Program Specialist in 2007.