Frequently Asked Questions About Building Science

Use the filters below to find answers to frequently asked questions about Building Science guidance regarding:

  • General Building Science questions
  • Substantial Damage Estimator (SDE) tool
  • Safe rooms
  • Floods
  • High winds
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If you need additional information, please email the FEMA Building Science Helpline or call at 866-927-2104.

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Please visit the Building Science Resource Library to reference all hazard-specific publications and guidance documents.

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In which Special Flood Hazard Area zones can fill be used to elevate a home?

The only Special Flood Hazard Area (SFHA) zone in which fill can be used to elevate a structure above the base flood elevation (BFE) is Zone A. Fill cannot be used in Zone V. It is recommended that Coastal A Zones be treated as Zone V, and hence it is strongly recommended that fill not be used to elevate structures in Coastal A Zones.

Furthermore, not all Zone A areas freely allow the use of fill to elevate a home. The placement of fill in the SFHA can increase the BFE by reducing the soil’s ability to convey and store floodwaters. An increase in the BFE can result in increased flood damage to both upstream and downstream properties. To prevent these possible results, some communities prohibit the use of fill, require compensatory storage for filled areas, and/or identify a more restrictive floodway to preclude the use of fill in certain areas. For more information regarding using fill to elevate a structure in an SFHA, see Technical Bulletin 10, Ensuring That Structures Built on Fill In or Near Special Flood Hazard Areas Are Reasonably Safe from Flooding (FEMA, 2001). Check with the local jurisdiction on additional requirements on placing fill in the SFHA.

References: FEMA. 2001. Ensuring That Structures Built on Fill In or Near Special Flood Hazard Areas Are Reasonably Safe from Flooding. NFIP Technical Bulletin 10. Washington, DC, May 2001.

Why do I need to determine Substantial Damage or Substantial Improvement for structures located in the Special Flood Hazard Area (SFHA)?

Communities that participate in the National Flood Insurance Program (NFIP) are required to determine whether damage of any origin meets the criteria for Substantial Damage when the damage is to a structure located in a mapped 100 year floodplain. Similarly, improvements to structures located in a mapped 100-year floodplain must be evaluated to determine whether the improvements meet the criteria for Substantial Improvement. Improvements may include rehabilitation, remodeling, and lateral or vertical additions. Check with the local jurisdiction on additional requirements for substantial damage and substantial improvement requirements.

What Is the Difference Between a Residential Safe Room and a Community Safe Room?

A residential safe room serves occupants of dwelling units and has a design occupant capacity of 16 persons or fewer. A community safe room is any safe room not defined as a residential safe room. This includes safe rooms intended for use by the general public, by building occupants or a combination of both.

What Is the Minimum Square Footage per Person for a Tornado Community Safe Room?

 For a tornado community safe room, the number of spaces for standing or seated, wheelchair-bound, or bedridden people should be determined based on the expected occupancy of the safe room as calculated by the designer and the applicable authority having jurisdiction. As a minimum, each community safe room should be sized to accommodate at least one wheelchair space for every 200 occupants.

It is also important to note that community safe rooms should provide equal access for all persons in accordance with all federal, state, and local Americans with Disabilities Act (ADA) requirements and ordinances. Table 1 shows the minimum safe room floor area per occupant for tornado community safe rooms.

Tornado Community Safe Room OccupantMinimum Usable Floor Area* per Safe Room Occupant
Standing or Seated5 square feet
Using a Wheelchair10 square feet
Relocated to a Bed or a Stretcher30 square feet
Table 1. Occupant Density for Tornado Community Safe Rooms

*See FEMA P-361 for more information on usable safe room floor area.

Are There Any Flood Hazard Restrictions for the Location of a Community Safe Room?

Yes. FEMA provides guidance on the location of community safe rooms in relation to flood hazards. Per FEMA P-361, flood hazards should be considered when designing a community safe room. Flood loads acting on a structure containing a safe room are strongly influenced by the structure’s location relative to the flood source. Community safe rooms should be located outside of the following high-risk flood hazard areas:

  • Flood hazard areas subject to high-velocity wave action (Zone V areas) and Coastal A Zones (where mapped)
  • Floodways

Exceptions can be made to these restrictions in some instances; for more information and additional elevation requirements, see the Flood Hazard Siting and Floor Elevation Criteria for Community Safe Rooms (fact sheet), and FEMA P-361.

What Should I Do If I Am Unsure a Safe Room or Storm Shelter Product Meets Criteria?

As noted in Chapter 4 (Consumer Guide) of FEMA P-320, your State Hazard Mitigation Officer (SHMO) may be able to recommend a qualified safe room inspector. A list of SHMOs can be found at https://www.fema.gov/grants/mitigation/ state-contacts.

The age of the safe room or storm shelter is a key factor in determining which criteria were in effect when the product was designed and constructed or installed. FEMA P-361 was introduced in 2000 and updated in 2008, 2015, and 2021. ICC 500, which is referenced in the model building codes and includes the minimum requirements for residential and community storm shelters, was first published in 2008 and updated in 2014 and 2020.

Lastly, an independent RDP licensed in your state and familiar with safe room design and construction may also be consulted to verify that the safe room meets or exceeds the criteria found in FEMA P-361 or ICC 500.

Emergency Management Institute

The Emergency Management Institute (EMI) serves as the national focal point for the development and delivery of emergency management training to enhance the capabilities of federal, state, local, tribal and territorial government officials, volunteer organizations and the public and private sectors to minimize the impact of disasters.

Explore Independent Study Courses or resident and field courses.

When does a flood-damaged home need to be elevated?

Work on flood-damaged homes located in communities that participate in the National Flood Insurance Program (NFIP) is subject to specific requirements. If the flood-damaged home is in a floodplain and is substantially damaged (see definition below), the NFIP requires that reconstruction and repairs, termed substantial improvements (see definition below), be performed to bring the home into compliance with local floodplain management regulations. These regulations include elevating the home so that its lowest floor is at or above the base flood elevation (BFE). The local jurisdiction may require elevating the home higher than the BFE, to an elevation called the design flood elevation (DFE). Other requirements include using the proper type of foundation (depending on the flood zone in which the house is located), properly anchoring the home to the foundation, using flood-resistant materials, complying with limitations on the use of enclosed areas below the elevated home, and protecting utilities and equipment. Check with the local jurisdiction for additional requirements that must be met.

Reconstruction of a home that is destroyed or that has been so severely damaged that it must be rebuilt is considered construction of a new home, and new homes must comply with the local floodplain management regulations.

Definition: Substantial damage means damage of any origin for which the cost of restoring the structure to its before-damaged condition would equal or exceed 50 percent of the market value of the structure before the damage occurred (FEMA, 2010).

Definition: Substantial improvement means any reconstruction, rehabilitation, addition, or other improvement of a structure for which the cost of the work equals or exceeds 50 percent of the market value of the structure before the start of construction of the improvement. This term includes structures that have incurred substantial damage regardless of the actual repair work performed (FEMA, 2010).

References: FEMA. 2010. Substantial Improvement/Substantial Damage Desk Reference. FEMA P-758. Washington, DC. May 2010.

How is the net open area of a flood opening ("flood vent") measured?

FEMA Technical Bulletin (TB) 1, Openings in Foundation Walls and Walls of Enclosures (2008), states that the term “net open area” refers to the permanently open area of a non-engineered opening. The National Flood Insurance Program (NFIP) regulation permits flood openings that are equipped with coverings or dividers as long as they allow the automatic exit and entry of floodwaters. When calculating the net open area of a flood opening with a cover, coverings that have solid obstructions such as grilles, fixed louvers, or faceplates must be considered. Figure 14 of TB 1 shows a standard air vent faceplate that provides 42 square inches of net open area. This number is derived by multiplying the 0.5-inch width of the openings times the 6.5-inch height of the openings times the 13 total openings. Such estimates may be used when no other data are available.

According to FEMA TB 1, manufacturers of devices intended for use as standard air vents typically indicate the number of square inches that each device provides for airflow (either stamped onto the metal frame or noted on the packaging). This number should be used for the net open area when these devices are installed as non-engineered flood openings. To qualify as flood openings that permit automatic entry and exit of floodwaters, openings must not have solid covers installed. Similarly, typical air vent devices used as flood openings designed to be opened and closed manually must be disabled permanently in the open position.

Insect screens that do not impede the entry and exit of floodwaters are allowed and do not affect the determination of net open area. If a community has adopted the International Building Code (IBC) or International Residential Code (IRC), a screen is required to cover ventilation openings to keep out animals and insects. The IBC and IRC provide a list of acceptable covering materials. The commentaries that accompany the codes note that some covering materials may reduce the gross open area by up to 50 percent, in turn reducing the net open area. As a result of this reduction, in communities where floodwaters are expected to carry debris, local officials may determine that additional openings are required to compensate for the possibility that some openings may become clogged with debris.

FEMA TB 1, page 27, presents the equation that can be used to determine the area of an engineered flood opening. The equation is taken from American Society of Civil Engineers (ASCE) 24-05, Flood Resistant Design and Construction (2005).

References:

  • FEMA. 2008. Openings in Foundations Walls and Walls of Enclosures in Special Flood Hazard Areas in Accordance with the National Flood Insurance Program. NFIP Technical Bulletin 1. Washington, DC. August 2008.
  • American Society of Civil Engineers (ASCE). 2005. Flood Resistant Design and Construction. ASCE 24-05.
I have a row house. How do I elevate it?

Since the late 19th Century, row houses have been a common style in New York and New Jersey. Many of these buildings consist of a lower level or walk-out basement (not a basement per the NFIP) and 2 or 3 stories of upper floors.

If Substantially Damaged, row houses in mapped flood hazard areas have to be brought into compliance with the requirements for new homes. There may be several ways compliance can be achieved, each depending on several factors, including how high the BFE is above the ground. For example, it may be necessary to abandon the lowest level and then modify the upper floors.  If the next lowest floor is below the BFE or local elevation requirement, the floor system may need to be removed and reconstructed to appropriate elevations.  The figure below illustrates the scenario where the lowest floor would be partially filled to grade (with the remaining area on the lowest floor being wet floodproofed and used as storage), and the top two floors would each be raised.

High ceilings typical of older row homes may make it easier to meet the requirements by building a “false floor” to create a crawlspace, although doing this will reduce ceiling heights while retaining the original floor space of the upper levels. It may not be possible to modify common walls (party walls) but the exterior walls at the front and rear of the building can probably be modified to raise windows. In a more modern townhouse it may be possible to abandon the lowest floor and make adjustments/repurpose the upper floors.

Regardless of the type of construction, if a row house was Substantially Damaged the requirements for areas below the BFE must be followed in order to have the lowest floor above the BFE considered the building’s lowest floor. Modification options for row homes in areas subject to high velocity wave action (Zone V) may be limited.

Figure indicating the “before” and “after” of flood mitigation for a row house. The “after” includes fill in the lowest floor up to the grade level and the living area floors are moved to above the freeboard level.

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