Openings in Foundation 

Walls and Walls of 

Enclosures

Below Elevated Buildings in Special Flood Hazard Areas

in accordance with the National Flood Insurance Program

Technical Bulletin 1 / August 2008
Comments on the Technical Bulletins should be directed to:

Department of Homeland Security 

FEMA Mitigation Directorate

500 C Street, SW.

Washington, D.C. 20472

Technical Bulletin 1-08 replaces Technical Bulletin 1-93, Openings in Foundation Walls.

Photograph Credits:

Figure 3. Bill Bryant, Anne Arundel County, Maryland

Figure 4. Smart Vent, Inc.

Figure 17. North Carolina Emergency Management/T. Riddle

Table of Contents

Introduction ..................................................................................................................................1

NFIP Regulations ..........................................................................................................................4

How Openings Affect Flood Insurance Rates..............................................................................4

Documenting Elevations and Information About Openings .....................................................5

Enclosed Areas Below Elevated Buildings....................................................................................5

Enclosures That Require Openings..................................................................................6

Situations That Do Not Require Openings....................................................................12

Requirements and Guidance for Installation of Openings.......................................................13

Minimum Number of Openings.....................................................................................13

Height of Openings Above Grade ..................................................................................14

Installation Examples ......................................................................................................15

Non-Engineered Openings and Engineered Openings............................................................18

Unacceptable Measures...................................................................................................19

Non-Engineered Openings .............................................................................................20

Engineered Openings .....................................................................................................24

The NFIP ......................................................................................................................................28

NFIP Technical Bulletins ............................................................................................................28

Ordering Technical Bulletins .....................................................................................................28

Further Information ...................................................................................................................29

Glossary ........................................................................................................................................30
Technical Bulletin 1 – August 2008 1

Introduction 

Protecting buildings that are constructed in special flood hazard areas (SFHAs) from damage caused by flood forces is an important objective of the National Flood Insurance Program 

(NFIP). In support of this objective, the NFIP regulations include minimum building design criteria that apply to new construction, repair of substantially damaged buildings, and 

substantial improvement of existing buildings in SFHAs. The base flood is used to delineate 

SFHAs on Flood Insurance Rate Maps (FIRMs) prepared by the NFIP. The base flood is the 

flood that has a 1-percent chance of being equaled or exceeded in any given year (commonly called the “100-year” flood). 

Certain terms used in this Technical Bulletin are defined in 

the Glossary.

The NFIP regulations require that residential buildings 

constructed in A zones have the lowest floor (including basement) elevated to or above the base flood elevation (BFE). In 

this Technical Bulletin, the term “A zones” includes all zones 

shown on FIRMs as Zones A, AE, A1-A30, AR, AO, and AH. 

Enclosed areas (enclosures) are permitted under elevated 

buildings provided the enclosed areas meet certain use restrictions and construction requirements related to flood 

resistance, including use of flood damage-resistant materials 

and installation of openings to allow for automatic entry and 

exit of floodwaters. Enclosures under buildings in V zones 

(includes all Zones V, VE, and V1-V30) must meet the same enclosure requirements except that openings are not required 

and walls must be non-supporting breakaway walls, open lattice-work, or insect screening (see Technical Bulletin 9, Design 

and Construction Guidance for Breakaway Walls Below Elevated 

Coastal Buildings).

The NFIP regulations for new construction and substantial 

improvements of existing buildings require that enclosed areas under elevated non-residential buildings meet the same 

requirements as those for enclosures under elevated residential buildings. New non-residential buildings constructed in A 

zones, and substantial improvements of existing non-residential buildings, must either have their lowest floors elevated to 

or above the BFE or be floodproofed (made watertight) to or 

above the BFE. 

Many types of foundations are used to elevate buildings. While 

the main portions of elevated buildings are above the BFE, 

the foundation and any enclosed areas below the BFE will 

be exposed to flood forces. Enclosed areas below the BFE 

Under the NFIP, the “lowest floor” is the floor of the 

lowest enclosed area of 

a building. An unfinished 

or flood-resistant enclosure that is used solely for 

parking of vehicles, building access, or storage is 

not the lowest floor, provided the enclosure is built in 

compliance with applicable 

requirements.

As used by the NFIP, an 

“enclosure” is an area that 

is enclosed on all sides by 

walls. 

The NFIP defines a “basement” as any area that is 

below-grade on all sides. 

The regulations do not allow 

basements to extend below 

the BFE. 

Owners of existing elevated 

buildings with enclosures 

below the BFE may wish 

to retrofit the enclosures. 

Lower NFIP flood insurance rates may apply if the 

retrofit enclosures have 

openings that meet the requirements in this Technical 

Bulletin and also meet other 

requirements for enclosures 

(limited use, flood damage-resistant materials, and 

elevated utilities). 
2 Technical Bulletin 1 – August 2008

(including crawlspaces) are permitted if used only for parking of vehicles, building access, 

and storage. Figure 1 illustrates a typical crawlspace foundation wall and a typical framed wall 

surrounding an enclosed area. 

If enclosure walls are not designed with openings to relieve the 

pressure of standing or slow-moving water against them (called 

hydrostatic loads), the walls can be damaged or fail during a 

flood. If the walls are “load-bearing” walls that support the elevated building, failure of the walls may result in damage to, 

or collapse of, the building. To address this concern, the NFIP 

regulations require that enclosure walls contain openings that 

will allow for the automatic entry and exit of floodwaters. These 

openings allow floodwaters to reach equal levels on both sides 

of the walls, thereby lessening the potential for damage caused 

by a difference in hydrostatic loads on opposite sides of the 

walls. In A zones, the requirement for flood openings applies 

to all enclosed areas below new elevated buildings and below 

substantially improved buildings. 

This Technical Bulletin explains the NFIP requirements for flood openings and provides 

guidance for prescriptive (non-engineered) openings and engineered openings. Non-engineered openings are used to meet the NFIP’s prescriptive requirement of 1 square inch of net 

open area for every square foot of enclosed area. As an alternative, engineered openings that 

have characteristics that differ from non-engineered openings may be used provided they are 

designed and certified by a registered design professional as meeting certain performance 

characteristics described in this Technical Bulletin. 

Areas of shallow flooding may be shown as AO 

zones on FIRMs. Rather 

than BFEs, AO zones have 

“flood depths” that range 

from 1 to 3 feet. In these 

zones, all NFIP requirements related to BFEs 

apply, including elevation of 

the lowest floor to or above 

the designated flood depth 

and requirements for enclosures with flood openings 

that are located so that 

floodwaters will flow in and 

out. 

Figure 1. Typical enclosures with flood openings 
Technical Bulletin 1 – August 2008 3

This Technical Bulletin also discusses how openings could affect flood insurance premiums, 

provides examples of enclosures that require openings and situations where openings are not 

required, and outlines the requirements for, and provides guidance on, the following:

n Installation of openings, including the minimum number of openings and height of openings above grade,

n Non-engineered openings, and

n Engineered openings. 

Examples are provided to illustrate types of buildings and enclosures that require openings, 

and to address several commonly encountered situations. Other situations may require the advice of a registered design professional. Questions should be directed to the appropriate local 

official, NFIP State Coordinating Office, or FEMA Regional Office. 

Solid perimeter foundation walls and walls surrounding enclosed areas below the BFE may be damaged by forces related 

to moving floodwaters and wave impacts (called hydrodynamic 

loads), and debris impacts. The requirement for openings is 

intended to reduce only flood damage associated with hydrostatic – not hydrodynamic – loads. 

Hydrodynamic loads and debris impacts may be significant in 

some flood hazard areas shown as A zones on FIRMs, including riverine areas where high flow velocities are likely (e.g., 

faster than 5 feet per second) and areas where wave heights of 

1.5 feet or more are possible. In these areas, it is recommended that a registered design professional evaluate foundation 

designs. Open foundations without enclosed areas are less vulnerable to the type of damage that can be caused by high flow 

velocities and wave action. 

Buildings in V zones (Zones V, VE, and V1-V30) must meet certain design and construction 

requirements that are specified in the NFIP regulations at Section 60.3(e). The area below the 

lowest floors of buildings in V zones must be free of obstruction or, if enclosed, the walls of 

enclosures must be constructed with non-supporting breakaway walls, open wood lattice-work, 

or insect screening. Openings may be provided, but are not required, in breakaway walls under buildings in V zones. For information on V-zone design and construction requirements, 

refer to the NFIP regulations, the Technical Bulletin series (especially Technical Bulletin 

5, Free-of-Obstruction Requirements and Technical Bulletin 9, Design and Construction Guidance 

for Breakaway Walls Below Elevated Coastal Buildings), the Coastal Construction Manual (FEMA 

55CD), Flood Resistant Design and Construction (ASCE 24), and Home Builder’s Guide to Coastal 

Construction (FEMA 499).

This Technical Bulletin discusses openings in walls 

below the BFE. Readers 

should check with the 

community to determine 

whether a higher elevation standard is enforced. 

For example, communities may add freeboard or 

may regulate to the design 

flood elevation (DFE). In 

those cases, references to 

the BFE in this Technical 

Bulletin should be construed as references to 

the community’s elevation 

requirement.
4 Technical Bulletin 1 – August 2008

NFIP Regulations 

The NFIP regulations for enclosures are codified in Title 44 of the Code of Federal Regulations, in Section 60.3(c)(5), which states that a community shall: 

“Require, for all new construction and substantial improvements, that fully enclosed areas below the lowest floor that are usable solely for parking of vehicles, building access, 

or storage in an area other than a basement and which are subject to flooding shall be 

designed to automatically equalize hydrostatic flood forces on exterior walls by allowing 

for the entry and exit of floodwaters. Designs for meeting this requirement must either be 

certified by a registered professional engineer or architect or meet or exceed the following 

minimum criteria: A minimum of two openings having a total net area of not less than 

one square inch for every square foot of enclosed area subject to flooding shall be provided. 

The bottom of all openings shall be no higher than one foot above grade. Openings may 

be equipped with screens, louvers, valves, or other coverings or devices provided that they 

permit the automatic entry and exit of floodwaters.” 

Proposals for substantial improvement of existing buildings in SFHAs, and proposals to repair 

those that have sustained substantial damage, must comply with the requirements for new 

construction. In A zones, the applicable requirements include openings in the walls surrounding enclosed areas below the BFE. As part of issuing permits, community officials must review 

such proposals to determine whether they comply with the requirements. Further information on substantial improvement and substantial damage is found in Answers to Questions About 

Substantially Damaged Buildings (FEMA 213). 

How Openings Affect Flood Insurance Rates

Careful attention to compliance with the NFIP regulations for flood openings is important 

during design, plan review, construction, and inspection. Compliance influences both the 

vulnerability to flood damage and the cost of NFIP flood insurance. If openings are not compliant, the floor of the crawlspace or the floor of the enclosure becomes the “lowest floor.” In 

those cases, the result may be significantly higher flood insurance premiums, especially if the 

floor of the crawlspace or enclosure is more than a foot or two below the BFE. 

The NFIP Technical Bulletins provide guidance on the minimum requirements of the NFIP regulations. 

Community or State requirements that exceed those of the NFIP take precedence. Design professionals should contact the community to determine whether more restrictive provisions apply to the 

building or site in question. All other applicable requirements of the State or local building codes must 

also be met for buildings in flood hazard areas.
Technical Bulletin 1 – August 2008 5

Documenting Elevations and Information About Openings 

Communities are required to collect data from permittees to document the surveyed elevation 

of the lowest floors of new buildings and existing buildings that are substantially improved. 

Although the data may be provided in other formats, the NFIP’s Elevation Certificate (FEMA 

Form 81-31) is designed specifically for this purpose. The current version of the Elevation Certificate is online at http://www.fema.gov/business/nfip/elvinst.shtm. 

The Elevation Certificate is designed to collect information that facilitates determining compliance of new construction and to provide data necessary for the proper rating of NFIP flood 

insurance. For guidance, see the instructions that accompany the Elevation Certificate and 

the Floodplain Management Bulletin: Elevation Certificate (FEMA 467-1). 

The Elevation Certificate has blanks that are to be completed if there are enclosures under 

elevated buildings, including:

n The square footage of the enclosed area,

n The number of flood openings within 1.0 foot above adjacent grade, and

n The total net area of flood openings.

The Elevation Certificate provides space for comments. As noted above and explained in more 

detail below, the regulations provide two ways to satisfy the requirements for openings. Comments should be provided when engineered openings are used, and when there are other 

aspects of enclosures and openings that comply with the requirements but that, without close 

inspection, may appear to be non-compliant. The documentation required for engineered 

openings should be attached to the Elevation Certificate (described on page 25, Documentation of engineered openings for flood insurance). 

Enclosed Areas Below Elevated Buildings

The NFIP regulations specify that enclosed areas under elevated buildings may be allowed 

provided the enclosed areas are used solely for:

n Parking of vehicles (attached garages or parking areas below elevated buildings)

n Building access (stairwells, foyers, elevators)

n Storage (low-value items)

Although crawlspaces are not listed explicitly as an allowable 

use, buildings may be elevated using perimeter foundation 

walls that create enclosed areas, typically called crawlspaces or 

under-floor spaces. Crawlspaces provide access to under-floor 

utilities such as pipes, ductwork, and electric conduits. 

Some communities require 

permittees to execute a 

“non-conversion” agreement 

to document their understanding that the use of 

enclosures is limited, that 

conversion to other uses is 

not allowed, and that modification of enclosures may 

result in higher NFIP flood 

insurance rates.
6 Technical Bulletin 1 – August 2008

It is important to understand how an otherwise compliant enclosed area below the BFE can be rendered non-compliant by 

installing features that are not consistent with the limitations 

on uses. The following are not allowed below the BFE because 

of potential damage and their presence is inconsistent with 

the allowed uses: appliances, heating and cooling equipment, 

plumbing fixtures, more than the minimum electric service 

required to address life safety and electric code requirements 

for building access and storage areas, and materials that are 

not flood damage-resistant.

The NFIP regulations require that enclosed areas surrounded 

by solid walls that extend below the BFE have flood openings. The requirement applies whether the walls are load-bearing walls or non-load-bearing 

walls. Therefore, openings are required in solid perimeter foundation walls that surround 

crawlspaces and openings are required in the walls of fully enclosed areas that meet the use 

limitations (parking of vehicles, building access, or storage). The requirement applies to new 

construction and to buildings that are undergoing substantial improvement, including repair 

of substantial damage. 

Enclosures That Require Openings

Several examples of enclosures that require openings are described below:

n Solid perimeter foundation walls (crawlspaces or under-floor spaces)

n Solid perimeter foundation walls (below-grade crawlspaces)

n Solid perimeter foundation walls (with full-height under-floor spaces) 

n Garages attached to elevated buildings

n Enclosed areas under buildings elevated on open foundations in A zones

n Enclosed areas with breakaway walls under buildings elevated on open foundations in A 

zones

n Solid perimeter foundation walls on which manufactured homes are installed

n Accessory structures (detached garages and storage sheds)

Solid perimeter foundation walls (crawlspaces or under-floor 

spaces)

The crawlspace or under-floor space that is created when a 

building is elevated on a solid perimeter foundation wall is an 

enclosed area below the BFE that must meet all of the requirements for enclosed areas (refer to Figure 1). If a brick veneer, 

siding, or other material covers the wall, then the openings 

must completely penetrate into the enclosed area. A crawlspace 

access with a door does not qualify as a flood opening unless 

In many parts of the country, a common practice is 

to build “conditioned crawlspaces” that are sealed and 

have mechanical ventilation. 

In SFHAs, all crawlspaces 

must have flood openings 

that meet the requirements 

of the NFIP and the building 

codes.

The only exception to the 

openings requirement is 

for non-residential buildings that are engineered to 

be floodproofed by meeting 

stringent requirements to 

be watertight. For information on floodproofing, refer 

to Technical Bulletin 3, NonResidential Floodproofing 

– Requirements and 

Certification.
Technical Bulletin 1 – August 2008 7

the door has an opening installed in it or otherwise meets the performance requirement that 

it will allow automatic entry and exit of floodwaters. 

As explained on page 14 (Height of Openings Above Grade), the bottom of each opening is 

to be located no higher than 1 foot above the higher of the final interior or exterior grades 

under the opening. Therefore, placement of the openings in the foundation wall requires 

knowledge of the expected finished exterior grade and the final interior grade of the crawlspace.

Building code requirements may call for ventilation of certain under-floor spaces. Ventilation 

openings typically are positioned near the top of the foundation wall to facilitate air flow. In 

most cases, ventilation openings will be too high above grade to satisfy the requirements for 

flood openings.

Solid perimeter foundation walls (below-grade crawlspaces)

The NFIP regulations do not allow buildings to be constructed 

with areas that are below grade on all sides (basements), except for certain engineered non-residential buildings that are 

designed and certified to be floodproofed. Therefore, crawlspaces that are below-grade on all sides are not allowed because 

they are basements. An exception is available only in shallow 

floodplains, and then only if certain other requirements and 

limitations are met. Those requirements and limitations are 

detailed in Technical Bulletin 11, Crawlspace Construction for Buildings Located in Special Flood 

Hazard Areas: National Flood Insurance Program Interim Guidance. 

According to this guidance, below-grade crawlspaces may be 

allowed provided the wall height 

is less than 4 feet when measured 

from bottom of the floor joist/

truss to the top of footing, which 

must be no more than 2 feet below-grade (see Figure 2). Flood 

openings are required in the 

foundation walls surrounding 

these crawlspaces and, as noted 

above, air ventilation may be required.

Although crawlspaces that satisfy 

the limitations in TB 11 are not 

considered basements for floodplain management purposes, it 

is important to note that they 

are basements for NFIP flood 

Communities are required 

to adopt specific provisions in their ordinances 

to be consistent with the 

limitations in TB 11 in order to permit below-grade 

crawlspaces.

Figure 2. Limitations on below-grade crawlspaces in shallow 

flood hazard areas (TB 11) 
8 Technical Bulletin 1 – August 2008

insurance purposes. Therefore, NFIP flood insurance will be more expensive if the grade inside 

the crawlspace is below the exterior grade on all sides. In addition, below-grade crawlspaces 

may contribute to increased humidity and mold growth. TB 11 requires that an adequate 

drainage system be provided in order to minimize floodwater contact with crawlspace materials and related moisture damage.

Solid perimeter foundation walls (with full-height under-floor spaces)

In SFHAs where the BFE is more than 4 or 5 feet above grade, or where owners want enough 

head room to allow for parking of vehicles and storage, solid perimeter foundation walls may 

be used to create full-height under-floor spaces (see Figure 3). The walls surrounding the under-floor space must meet all of the opening requirements. 

It is important that full-height under-floor spaces also meet all other NFIP requirements to 

minimize the likelihood of future conversion to uses other than the allowed uses (parking 

of vehicles, building access, or storage). As noted in the discussion of limitations on uses of 

enclosures, the following are not allowed below the BFE in full-height enclosures because of 

potential damage and their presence is inconsistent with the allowed uses: appliances, heating 

and cooling equipment, plumbing fixtures, more than the minimum electric service required 

to address life safety and electric code requirements for building access and storage areas, and 

materials that are not flood damage-resistant.

Garages attached to elevated buildings

Many buildings, especially homes, are designed with attached garages. An attached garage 

may have its floor below the BFE provided the garage meets all of the requirements for an enclosed area below the BFE. The use of the garage space must be limited to parking of vehicles, 

building access, and storage. 

Figure 3. Full-height 

solid perimeter walls 

surrounding garage and 

storage area (only two 

openings visible)

Flood openings

(only 2 shown)
Technical Bulletin 1 – August 2008 9

Openings are required in the exterior walls of the garage, and openings may be installed in 

exit doors and garage doors (see Figure 4). It is important to note that garage doors themselves do not meet the requirements for openings. Human intervention would be necessary 

to open garage doors when flooding is expected, which is inconsistent with the requirement 

that openings allow for the automatic entry and exit of floodwaters. Similarly, gaps that may 

be present between the garage door and the door jamb or walls do not guarantee automatic 

entry and exit of floodwaters and do not count towards the net open area requirement. 

If an attached garage is built with its floor below the BFE and it does not have compliant 

openings, the garage floor becomes the lowest floor. Flood insurance premiums may be significantly higher than if the garage complies with the requirements for openings and other 

requirements, such as flood damage-resistant materials and elevated utilities.

Enclosed areas under buildings elevated on open foundations in A zones 

A building that is elevated on an open foundation (e.g., piers, posts, columns, or pilings) in an 

A zone may have enclosed areas below the elevated floor (see Figure 5). Sometimes only part 

of the footprint is enclosed, such as for a stairwell or storage room. All of the requirements for 

enclosed areas apply, including openings, elevated utilities, flood damage-resistant materials, 

and limitations on use (parking of vehicles, building access, 

and storage). 

Open foundations are recommended in riverine flood hazard areas where flow velocities are expected to exceed 5 feet 

per second because of the anticipated hydrodynamic loads 

and potential for debris impact and scour. These loads may 

be sufficient to damage typical solid perimeter foundation 

walls, even though flood openings are provided. 

Figure 4. Attached 

garage, with engineered 

openings installed in the 

garage door 

Flood openings

ASCE 24 and several of 

the fact sheets included in 

the Home Builder’s Guide 

to Coastal Construction 

(FEMA 499) are excellent 

resources for flood-resistant 

building methods in coastal 

A hazard areas.
10 Technical Bulletin 1 – August 2008

If a waterway was studied using detailed methods and a floodway is shown on a FIRM, then 

the Floodway Data Table in the Flood Insurance Study should be reviewed for data that can 

be used to estimate velocities. For each cross section, the table provides the mean velocity that 

can be used to approximate velocities in the floodplain outside of the floodway. For other waterways in areas known to have fast-moving water, standard methods can be used to compute 

an approximate velocity. Examples of other sources of information that should be reviewed 

include local observations and studies prepared by State and local agencies.

Enclosed areas with breakaway walls under buildings elevated on open foundations in A zones

Open foundations are also recommended in A zones in coastal areas where breaking wave 

heights can be between 1.5 and 3.0 feet (called Coastal A Zones). In these areas, it is recommended that walls surrounding enclosed areas be designed as breakaway walls. Flood openings 

are required in breakaway walls in A zones in order to comply with the NFIP requirements. 

ASCE 24 includes specific provisions for openings in breakaway walls.

Solid perimeter foundation walls on which manufactured homes are installed 

Manufactured homes may be installed on solid perimeter foundation walls that enclose space below the homes (see Figure 6). 

Even if it is not part of the load-bearing foundation, a solid 

perimeter wall is required to have openings, otherwise hydrostatic loads may damage the perimeter wall, which could, in 

turn, damage the home’s supporting foundation and anchor 

system.

Figure 7 shows an example of a framed enclosure below an elevated manufactured home. In this case, the full-height enclosed 

Openings are required in 

rigid skirting that is attached 

to frames or foundations of 

manufactured homes to relieve hydrostatic loads and 

minimize transferring loads 

that can damage homes 

and their supporting foundation systems.

Figure 5. Enclosure with flood 

openings, under house elevated on 

pilings
Technical Bulletin 1 – August 2008 11

area is used for parking and storage. Openings are required because the walls surrounding 

the enclosed area are solid walls. As indicated by the driveway on the left, the interior slab is 

higher than the exterior grade along the side of the building. The openings shall be located 

within 1 foot of the interior grade. 

Figure 6. Manufactured home 

supported on piers; masonry perimeter 

wall with flood openings (ground 

anchors not shown)

Figure 7. Manufactured 

home installed above 

a full-height framed 

garage (note elevation 

of driveway slab on left; 

the openings are within 

1 foot of interior grade of 

the slab)
12 Technical Bulletin 1 – August 2008

Accessory structures: detached garages and storage sheds

Detached garages and detached storage buildings in A zones may be permitted without requiring them to be elevated if they comply with all of the requirements for enclosures. Garages 

and other accessory buildings must be used only for parking of vehicles and storage, utilities 

must be elevated, flood damage-resistant materials must be used below the BFE, the requirements for flood openings must be satisfied, and they must be anchored to resist flotation, 

collapse, or lateral movement under flood conditions.

Communities are required to regulate all development in SFHAs, including the placement of 

small storage sheds. Storage sheds in A zones are not required to be elevated if they comply 

with all of the requirements for enclosures. They must be used only for storage, utilities must 

be elevated, flood damage-resistant materials must be used below the BFE, and the requirements for flood openings must be satisfied. In addition, sheds are to be anchored to prevent 

flotation, collapse, or lateral movement under flood conditions.

Situations That Do Not Require Openings

Two situations that do not require openings are described below:

n Manufactured home with skirting 

n Back-filled stem wall foundation 

Manufactured home with flexible skirting

Skirting used to enclose the area under manufactured homes 

typically is made of weather-resistant material and extends 

from the bottom of the home down to grade. Flexible skirting and rigid skirting that are not attached to the frame or 

foundation of a manufactured home are not required to 

have openings. However, where floodwaters are expected to 

rise rapidly, there may be concerns about the skirting being 

pushed against foundation systems. In these areas, open lattice may be more appropriate to minimize the potential for 

flood damage.

Filled stem wall foundation 

A filled stem wall foundation (also called a chain wall) can 

look like a solid perimeter foundation wall from the outside, 

but this type of foundation is backfilled with compacted structural fill that supports the floor slab (see Figure 8). Because of the fill, unbalanced lateral 

loads against the walls will be minimized as floodwaters, and thus openings are not required.

It is important that the final Elevation Certificate, or other documentation of elevations, include an explanation when stem wall foundations are used to avoid the assumption that it 

is a crawlspace that lacks the required openings. The Elevation Certificate diagrams do not 

illustrate filled stem wall foundations. A note in the comment section should describe the 

foundation so that insurance agents are alerted as to why there are no openings.

The National Fire Protection 

Association’s standard, 

Model Manufactured Home 

Installation Standard (NFPA 

225), specifies that installation of skirting does not 

trigger the requirement for 

flood openings provided 

the skirting does not provide structural support and 

will collapse under wind and 

water loads that are less 

than those expected during 

the base flood event without 

causing structural damage 

to the elevated home or the 

foundation. 
Technical Bulletin 1 – August 2008 13

Requirements and Guidance for Installation of Openings

The NFIP regulations specify certain installation requirements that must be met by all flood 

openings, whether non-engineered openings or engineered openings, which are described 

starting on page 18. The installation requirements address the minimum number of openings 

and the maximum height of openings above grade. Additional guidance and explanations for 

various situations are described below.

Minimum Number of Openings

Each enclosed area is required to have a minimum of two openings on exterior walls to allow 

floodwaters to enter directly. In order to meet the requirement, the openings must be located 

so that the portion of the opening intended to allow for inflow and outflow is below the BFE. 

Openings that are entirely above the BFE (or any portion of an opening that is above the BFE) 

will not serve the intended purpose during base flood conditions and thus are not counted 

towards the compliance with the flood opening requirements. 

The openings should be installed on at least two sides of each 

enclosed area to decrease the chances that all openings could 

be blocked with floating debris and to allow for more even 

filling by floodwater and draining of the enclosed area. It is 

recommended that openings be reasonably distributed around 

the perimeter of the enclosed area unless there is clear justification for putting all openings on just one or two sides (such 

as in townhouses or buildings set into sloping sites).

Figure 8. Back-filled stem wall 

foundation (openings not required)

The International 

Residential Code® and the

International Building Code®

(by reference to ASCE 24) 

both require a “minimum 

of two openings on different sides of each enclosed 

area.” 
14 Technical Bulletin 1 – August 2008

Figure 9 shows a sketch illustrating where openings could be located when an elevated building has multiple enclosed areas. [Note: the number of openings shown in Figure 9 is for 

illustration only; the total number of openings and the adequacy of the net open area of those 

openings depend on the type of opening, covering, and whether vent devices or engineered 

openings are installed.]

Height of Openings Above Grade

The bottom of each opening is to be located no higher than 1 foot above the grade that is 

immediately under each opening. The purpose of this requirement is to satisfy the performance expectation that the difference in water levels between the interior and exterior will 

not exceed 1 foot as water begins to rise and as floodwaters recede from the site. Note that the 

openings (or those portions that count towards the required net open area) must be located 

below the BFE. In areas with shallow flood depths, this may require positioning the openings 

closer to grade than the maximum 1 foot allowed. 

Given the requirement that the bottom of openings shall not be higher than 1 foot above 

grade, a question arises if the interior and exterior grades are different: which grade should 

be used to determine placement of flood openings? The higher of the final interior grade 

and the finished exterior grade that is immediately under each opening is used to make this 

determination: 

n Finished exterior grade. Care should be taken when placing backfill, topsoil, and landscaping materials around the outside of enclosures, especially solid perimeter foundation 

Figure 9. Sketch of foundation plan of home with multiple enclosed areas, each with flood openings 

(number of openings for illustration purposes only)
Technical Bulletin 1 – August 2008 15

walls. If the finished exterior grade is higher than the interior grade on all sides of the 

building, then the enclosed area becomes a basement as defined by the NFIP. 

n Final interior grade. The trench that is excavated to construct footings and foundation 

walls must be backfilled completely, otherwise a basement is created. If the interior grade 

is higher than the exterior grade, the openings are to be no higher than 1-foot above the 

interior grade.

Installation Examples 

Interior grade higher than exterior grade

Consider a crawlspace enclosure that has its interior grade higher than the exterior grade. As 

water rises against the outside of the foundation, the ground or fill on the interior balances 

the hydrostatic load (see Figure 10). It is only when the water rises above the interior grade 

that the lateral load becomes unbalanced and therefore must be equalized by openings.

When viewed from the outside, a solid perimeter foundation wall or wall surrounding an enclosed area with the interior grade higher than the exterior grade will appear to not meet the 

installation requirements for openings. The openings will appear to be too high above the exterior grade (illustrated in Figure 7). Therefore, it is important that the final documentation 

of as-built elevations note the difference in interior and exterior grades. For example, if the 

NFIP Elevation Certificate is used, comments should explain that the interior grade is higher 

than the exterior grade and it should be noted whether the openings are (or are not) within 

1 foot of the higher of the two grades. 

Figure 10. Illustration of flood 

openings installed within 1 foot of the 

higher of interior or exterior grade
16 Technical Bulletin 1 – August 2008

Sloping sites

Buildings on solid perimeter foundation walls that are set into a sloping site present another 

special situation with respect to installation of openings. Careful attention must be paid to the 

following:

n The interior floor along the lower side of a building that is set into a sloping site must be 

at or above the exterior grade across the entire length of that side of the building, otherwise the enclosure becomes a basement. 

n The bottom of each opening shall be located no higher than 1 foot above the exterior or 

interior grade immediately below the opening, whichever is higher (see Figure 11).

n For openings to perform their intended function, sufficient open area must be below the 

BFE.

Townhouses with limited exterior walls

Townhouses are single-family dwelling units constructed in a group of three or more attached 

units in which each unit extends from foundation to roof and with exterior walls on at least 

two sides. Openings are required if townhouses in SFHAs are constructed with solid perimeter 

foundation walls or with solid walls surrounding enclosed areas under the elevated portion of 

the building. 

Because the interior townhouse units have less linear exterior wall length than the end units, 

it can be a challenge to meet all of the requirements, especially the requirement for adequate 

net open area and the requirement that each enclosed area have openings. If openings cannot be provided in at least two walls, the NFIP allows all openings to be installed in one wall. 

Figure 11. Openings in enclosure 

walls, sloping site
Technical Bulletin 1 – August 2008 17

Design of interior townhouse units can satisfy the guidance that openings should be on different sides if the walls inside the enclosed area have openings to connect enclosed spaces from 

front to back. Figure 12 shows suggested locations for openings. [Note: the number of openings shown in Figure 12 is for illustration only; the total number of openings and the adequacy 

of the net open area depend on the type of opening, covering, and whether a vent device is 

installed in the openings.] 

It may be even more challenging to provide adequate openings in enclosures under interior 

townhouse units if the multi-unit building is set into a sloping site, in which case it may be appropriate to consider using a filled stem wall foundation or an open foundation. Use of fill 

across one side of elevated townhouses may create a similar complication.

Openings that extend above the BFE

Only those portions of openings that are below the BFE can be counted towards the required 

net open area. Stacked vent devices may be installed or large-dimension openings may be provided (Figure 13). In both cases, if the BFE does not reach the top of the opening, only the 

portion that is below the BFE will count as contributing to the required net open area. Similarly, if the floor of a mechanical room is below the BFE (with elevated equipment inside) and 

a louvered door provides ventilation, only the open portion of the louvered door that is below 

the BFE will count towards the required net area of flood openings. 

Figure 12. Illustration of suggested flood openings in enclosures under elevated townhouses (number of 

openings for illustration purposes only)
18 Technical Bulletin 1 – August 2008

Depth of water 1 foot or less

Some FIRMs show mapped SFHAs where the depth of water will be 1-foot deep or shallower. 

Although the difference in water depth between the outside and inside of the enclosure under a building in these areas will not exceed 1 foot during the base flood, the NFIP regulations 

require openings. 

There are at least two solutions to this situation. The first is to elevate the floor of the enclosure the necessary height so that it is at or above the BFE and there is no need for openings. 

The second solution is to install openings, taking care to ensure that all of the necessary open 

area is below the BFE (otherwise the openings will not function as intended). This can be accomplished by positioning the bottom of the openings at or very close to grade, rather than 

the maximum of 1 foot above grade. In addition to complying with the regulations, the walls 

will not experience excessive differential hydrostatic pressure when floodwaters rise higher 

than the BFE.

Non-Engineered Openings and Engineered Openings

The NFIP regulations identify alternatives to provide sufficient size and number of openings 

to allow for the automatic entry and exit of floodwaters. This section describes how this level 

of performance can be satisfied by use of:

n Non-engineered openings (or covers and devices) that meet the prescriptive requirement to provide 1 square inch of net open area for each square foot of enclosed area (as 

Figure 13. Stacked vents inserted in 

large openings must be below the BFE

BFE
Technical Bulletin 1 – August 2008 19

described below, a variety of options and devices can serve 

as non-engineered openings). 

n Engineered openings (or covers and devices) that are 

specifically designed and certified by a registered design 

professional as meeting the required performance and 

design requirements outlined below (and, if applicable, 

the community’s building code).

n Engineered openings (or covers and devices) for which 

an Evaluation Report has been issued by the International Code Council (ICC) Evaluation Service, Inc. (ICC-ES), 

a subsidiary of the International Code Council, Inc. 

(http://www.iccsafe.org).

The following requirements for installation apply regardless of whether engineered openings 

or non-engineered openings are used to satisfy the NFIP requirements (also see page 13, Requirements and Guidance for Installation of Openings):

n Each enclosed area must have a minimum of two openings; if there are multiple enclosed 

areas, each area must have openings in its exterior walls,

n The bottom of each opening must be no more than 1 foot above the higher of the interior 

or exterior grade immediately under the opening, and

n Any screens, grates, grilles, fixed louvers, or other covers or devices must not block or impede the automatic flow of floodwaters into and out of the enclosed area.

Unacceptable Measures

It is important to note that FEMA has determined that certain measures are not acceptable as 

flood openings, including:

n Standard foundation air ventilation devices that can be closed manually, because they do 

not allow for the automatic entry and exit of floodwaters unless they are permanently disabled in the open position.

n Standard foundation air ventilation devices that have detachable solid covers that are intended to be manually installed over the opening in cold weather, because they do not 

allow for the automatic entry and exit of floodwaters when the cover is in place.

n Standard foundation air ventilation devices that are designed to open and close based on 

temperature (unless they also are designed to allow for the automatic entry and exit of 

floodwaters).

n Windows below the BFE, because the automatic entry and exit of floodwaters cannot be 

satisfied by the expectation that windows will break under rising floodwaters.

n Garage doors without openings installed in them, because human intervention is required 

to open the doors when flooding is expected. Gaps between the garage door and the door 

jamb or walls do not count towards the net open area requirement.

n Standard exterior doors without openings installed in them. 

The International 

Residential Code® includes 

both the prescriptive (nonengineered) alternative and 

the engineered openings 

alternative. 

The International Building 

Code® also includes both 

alternatives by reference to 

ASCE 24.
20 Technical Bulletin 1 – August 2008

Non-Engineered Openings

Non-engineered openings are openings that are used to satisfy the prescriptive requirement 

that calls for 1 square inch of net open area for each square foot of enclosed area. A wide variety of options is available to satisfy the prescriptive requirements. 

The term “net open area” refers to the permanently open area of a non-engineered opening. 

The NFIP regulations indicate that flood openings may be equipped with coverings or devices provided that they permit the automatic entry and exit of floodwaters. The measurement 

of the net open area must take into consideration any coverings that have solid obstructions, 

such as grilles, fixed louvers, or faceplates. Figure 14 shows a typical standard air vent faceplate 

and measurements of the net open area. 

Manufacturers of devices intended for use as standard air vents typically indicate the number 

of square inches that each device provides for air flow (either stamped into the metal frame or 

noted on the packaging). The same number should be used for the net open area calculation 

when these devices are installed as non-engineered openings. However, in order to qualify as 

flood openings that permit automatic entry and exit of floodwaters, openings must not have 

solid covers that are installed during cold weather. Similarly, typical air vent devices that are 

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 the net open area. Communities that administer the International 

Building Code® (IBC®) or the International Residential Code® (IRC®) should note the requirement to cover ventilation openings to keep animals and insects from entering. These codes 

provide a list of acceptable covering materials. The commentaries that accompany those codes 

note that some covering materials may reduce the gross open area of the vent by as much as 

50 percent. In areas where floodwaters are expected to carry debris such as grass clippings and 

leaves, it is notable that screens tend to clog (Figure 15). Local officials may determine that 

additional openings are required to increase the likelihood that openings will perform as expected, even if some become clogged with debris.

Figure 14. Typical standard air vent 

faceplate (this example provides 42 

square inches of net open area)
Technical Bulletin 1 – August 2008 21

Examples of several commonly used non-engineered openings are described below and shown 

in Figures 16 through 21: 

n Figures 16 and 17 show typical standard air ventilation devices that are intended for installation in crawlspace foundation walls. If installed as flood openings, they must be disabled 

permanently in the open position to satisfy the requirement 

for automatic entry and exit of floodwaters (note that the 

device shown in Figure 17 is not compliant because it is not 

disabled in the open position). 

n Figure 18 shows two examples where the builder provided 

decorative treatment for open holes; in each case, only the 

net open area is counted, and the area covered by the decorative treatment is not counted. 

n Figure 19 shows a common practice for solid perimeter 

foundation walls constructed of standard 16” x 8” concrete 

masonry blocks. If a block is omitted as shown, the resulting void provides 128 square inches of net open area. 

n Figure 20 shows where standard blocks are turned sideways. 

The voids in the blocks are measured to determine the net 

open area.

n Figure 21 shows a foundation in which a hole was created 

when the concrete was poured; a wood frame covered with 

screening is inserted in the hole. The framed void is measured to determine the net open area.

Figure 15. Typical air 

vent clogged by flood 

debris

The IRC and IBC (through 

reference to ASCE 24) require that flood openings 

are to be not less than 3 

inches in any direction in 

the plane of the wall. This 

requirement applies to the 

hole in the wall, excluding any device that may be 

inserted such as typical 

foundation air vent device. 

Communities usually require screens over voids 

(open holes) that are created in walls to serve as flood 

openings, to limit the entry 

of insects and rodents provided the screens do not 

impede the inflow and outflow of floodwaters.
22 Technical Bulletin 1 – August 2008

Figure 16. Examples of typical air vents used as flood openings (net open area varies)

Figure 17. Although this standard air vent was intended as flood openings, it is not acceptable because it is not 

disabled in the open position and does not allow automatic inflow and outflow of floodwaters.
Technical Bulletin 1 – August 2008 23

Figure 18. Decorative treatments using fixed louvers and brickwork (count the "net open area" or have 

certified as engineered openings)

Figure 19. Foundation 

wall with omitted blocks 

as flood openings (insect 

screen not visible)
24 Technical Bulletin 1 – August 2008

Figure 20. Concrete block turned sideways (insect 

screening shown)

Figure 21. Wood frame with insect screen inserted in 

opening in poured concrete foundation wall 

Engineered Openings

Openings that are designed and certified by a registered design professional as meeting the 

performance required by the regulations are called “engineered openings.” This section 

describes certification and documentation requirements for engineered openings and the 

specific design requirements.

Engineered openings with individual certification

For architectural or other reasons, building designers or owners may prefer to use unique 

or individually designed openings or devices. In these cases, a registered design professional 

must submit a certification. As a general rule, States require a designer to be licensed to practice in the State in which building is located.

The original certification of the engineered openings must include the design professional’s 

name, title, address, signature, type of license, license number, the State in which the license 

was issued, and the signature and applied seal of the certifying registered design professional. 

The certification shall identify the building in which the engineered openings will be installed. The language of the certification shall address the following: 

n A statement certifying that the openings are designed to automatically equalize hydrostatic flood loads on exterior walls by allowing the automatic entry and exit of floodwaters in 

accordance with the Engineered openings, design requirements on page 26, 

n Description of the range of flood characteristics tested or computed for which the certification is valid, such as rates of rise and fall of floodwaters, and 

n Description of the installation requirements or limitations that, if not followed, will void 

the certification.
Technical Bulletin 1 – August 2008 25

Engineered openings with ICC-ES Evaluation Reports

Engineered openings or devices may be accepted by local 

officials as an alternative to non-engineered openings (prescriptive) provided the designs are certified. The certification 

may take the form of the individual certification described 

above, or it can be an Evaluation Report issued by the ICC-ES. 

The ICC-ES issues such reports for a variety of building products, methods, and materials. Evaluation Reports are issued 

only after the ICC-ES performs technical evaluations of documentation submitted by a manufacturer, including technical 

design reports, certifications, and testing that demonstrate 

code compliance and performance. 

Evaluation Reports are supported by certifications that include appropriate language describing performance of the 

openings and the name, title, address, type of license, license 

number, the State in which the license was issued, and the signature and seal of the certifying registered design professional. 

The specific provisions that are addressed in the certification 

must include: 

n A statement certifying that the openings are designed to automatically equalize hydrostatic flood loads on exterior walls by allowing the automatic entry and exit of floodwaters in 

accordance with the Engineered openings, design requirements below, 

n Description of the range of flood characteristics tested or computed for which the certification is valid, such as rates of rise and fall of floodwaters, and 

n Description of the installation requirements or limitations that, if not followed, will void 

the certification.

Documentation of engineered openings for compliance

An important part of the evidence necessary to document compliance is the certification of 

engineered openings or the Evaluation Report. A copy of the individual certification or the 

Evaluation Report is required to be kept in the community’s permanent permit files, along 

with inspection reports. The documentation can be submitted as part of the permit application and design drawings, or submitted separately. Owners should retain the certification or a 

copy of the Evaluation Report to submit along with applications for NFIP flood insurance.

Documentation of engineered openings for flood insurance

Insurance agents will request that property owners provide documentation as part of applications for NFIP flood insurance. The documentation should be attached to the Elevation 

Certificate. The following are acceptable forms of documentation:

n For engineered openings with individual certification, the certification described above 

that is signed and sealed by a registered design professional who is licensed in the State 

where the building in which the engineered openings are used is located; or 

ICC-ES has issued 

Acceptance Criteria for 

Automatic Foundation Flood 

Vents (AC364) for one type 

of engineered opening. The 

ICC-ES will develop acceptance criteria for other 

types, upon request. 

Local officials in communities that do not administer 

the International Code 

Series determine whether 

to accept building products that have received 

Evaluation Reports issued 

by the ICC-ES.
26 Technical Bulletin 1 – August 2008

n For engineered openings with ICC-ES Evaluation Reports, a copy of the Evaluation Report 

that documents that the engineered openings meet the performance requirements of the 

NFIP and the building code, and that specifies the number of such openings that are required for a specified square footage of enclosed area below the BFE; or

n For engineered openings with ICC-ES Evaluation Reports, a letter or other written evidence from the local official that use of engineered openings in a specific building is 

acceptable.

Engineered openings, design requirements

The American Society of Civil Engineers (ASCE) developed the standard Flood Resistant Design 

and Construction (ASCE 24). This standard applies to buildings and site developments proposed in flood hazard areas; it is referenced by the International Building Code. ASCE 24 Section 

2.6.2.2 contains installation and design criteria for engineered openings. ASCE 24 provides 

the equation below to determine the total net area of engineered openings that are installed 

in foundation walls or enclosure walls. The equation includes a coefficient that corresponds 

to a factor of safety of 5, which is consistent with design practices related to protection of life 

and property. This factor of safety also helps to account for the likelihood that insect screens 

may clog with flood-borne debris. The ASCE 24 commentary provides additional background 

on the derivation of the equation.

As with non-engineered openings, engineered openings must be designed to allow automatic 

entry and exit of floodwaters.

Three design and performance criteria for engineered openings are specified in ASCE 24 but 

are not explicitly identified in the NFIP regulations:

n Engineered openings are to perform such that difference between the exterior and interior water levels shall not exceed 1 foot during base flood conditions.

n Engineered openings are to be not less than 3 inches in any direction in the plane of the 

wall. This requirement applies to the hole in the wall, excluding any screen, grate, grille, 

louvers, or devices that may be placed in or over the opening. 

n In the absence of reliable data on the rates of rise and fall, engineered openings are to be 

designed based on the assumption that the minimum rate of rise and fall will be 5 feet per 

hour. Where data or analyses indicate more rapid rates of rise and fall, the required number of openings is to be increased to account for those different conditions. The number 

or size of the openings may be decreased if data or analyses indicate rates of rise and fall 

are less than 5 feet per hour.
Technical Bulletin 1 – August 2008 27

From ASCE 24, the equation to determine area of engineered openings:

Ao = 0.033 [1/c] R A

e

Where: Ao = total net area of openings required (in2)

0.033 = coefficient corresponding to a factor of safety of 5.0 (in2 • hr/ft3)

c = opening coefficient (non-dimensional; see ASCE 24,Table 2-2)

R = worst case rate of rise and fall (ft/hr)

Ae = total enclosed area (ft2)

[ASCE 24] Table 2-2

Flood Opening Coefficient of Discharge

Opening Shape and Condition c

circular, unobstructed during design flood 0.60

rectangular, long axis horizontal, short axis 

vertical, unobstructed during design flood 0.40a

square, unobstructed during design flood 0.35

rectangular, short axis horizontal, long axis 

vertical, unobstructed during design flood 0.25b

other shapes, unobstructed during design flood 0.30

Notes:

a. When the horizontal dimension is twice or more the vertical dimension, use 

0.4; as the dimensions approach a square, interpolate from 0.4 to 0.35.

b. When the horizontal dimension is half or less the vertical dimension, use 0.25; 

as the dimensions approach a square, interpolate from 0.25 to 0.35.

Used with permission from ASCE.
28 Technical Bulletin 1 – August 2008

The NFIP

The U.S. Congress established the NFIP with the passage of the National Flood Insurance 

Act of 1968. The NFIP is a Federal program enabling property owners in participating communities to purchase insurance as protection against flood losses, in exchange for State and 

community floodplain management regulations that reduce future flood damages. Participation in the NFIP is based on an agreement between communities and the Federal Government. 

If a community adopts and enforces adequate floodplain management regulations, FEMA will 

make flood insurance available within the community. 

Title 44 of the U.S Code of Federal Regulations contains the NFIP criteria for floodplain management, including design and construction standards for new and substantially improved 

buildings located in SFHAs identified on the NFIP’s Flood Insurance Rate Maps. FEMA encourages communities to adopt floodplain management regulations that exceed the NFIP 

criteria. As an insurance alternative to disaster assistance, the NFIP reduces the escalating 

costs of repairing damage to buildings and their contents caused by floods.

NFIP Technical Bulletins 

This is one of a series of Technical Bulletins that FEMA has produced to provide guidance 

concerning the building performance requirements of the NFIP. These requirements are contained in Title 44 of the U.S. Code of Federal Regulations at Section 60.3. The bulletins are 

intended for use by State and local officials responsible for interpreting and enforcing the requirements in their floodplain management regulations and building codes, and by members 

of the development community, such as design professionals and builders. New bulletins, as 

well as updates of existing bulletins, are issued periodically, as necessary. The bulletins do not 

create regulations; rather, they provide specific guidance for complying with the requirements 

of existing NFIP regulations. Users of the Technical Bulletins who need additional guidance 

should contact their NFIP State Coordinator or the appropriate FEMA regional office. The 

User’s Guide to Technical Bulletins (http://www.fema.gov/pdf/final/guide01.pdf) lists the bulletins issued to date.

Ordering Technical Bulletins 

The quickest and easiest way to acquire copies of FEMA’s Technical Bulletins is to download them from the FEMA website (http://www.fema.gov/plan/prevent/floodplain/techbul.

shtm).

Technical Bulletins also may be ordered free of charge from the FEMA Publications Warehouse by calling 1-800-480-2520, or by faxing a request to 301-362-5355, Monday through 

Friday between 8 a.m. and 5 p.m. EST. Please provide the FEMA publication number, title, 

and quantity of each publication requested, along with your name, address, zip code, and daytime telephone number. Written requests may be also be submitted by mail to the following 

address:
Technical Bulletin 1 – August 2008 29

FEMA Publications

P.O. Box 2012

Jessup, MD 20794

Further Information 

The following sources provide further information concerning openings in foundation walls 

and walls of enclosures.

American Society of Civil Engineers, Structural Engineering Institute. 2005. Flood Resistant Design and Construction, ASCE/SEI 24-05.

American Society of Civil Engineers, Structural Engineering Institute. 2005. Minimum Design 

Loads for Buildings and Other Structures, ASCE/SEI 7-05.

FEMA. 1991. Answers to Questions About Substantially Damaged Buildings, FEMA 213. 

FEMA. 2000. Coastal Construction Manual, FEMA 55CD (3rd edition).

FEMA. 2004. Floodplain Management Bulletin: Elevation Certificate, FEMA 467-1 (http://www.

fema.gov/pdf/fima/fema467-6-10-04.pdf).

FEMA. 2005. Home Builder’s Guide to Coastal Construction: Technical Fact Sheet Series, FEMA 499. 

FEMA. 2006. Elevation Certificate (FEMA Form 81-31, http://www.fema.gov/pdf/nfip/elvcert.

shtm).

ICC Evaluation Service, Inc. 2007. Acceptance Criteria for Automatic Foundation Flood Vents (AC364, 

http://www.icc-es.org/criteria/pdf_files/ac364.pdf). 

International Code Council, Inc. 2006. International Building Code®, IBC® 2006.

International Code Council, Inc. 2006. International Residential Code®, IRC® 2006.

National Fire Protection Association. 2005. Model Manufactured Home Installation Standard©, 

NFPA 225.

National Fire Protection Association. 2006. Building Construction and Safety Code®, NFPA 5000.
30 Technical Bulletin 1 – August 2008

Glossary 

Accessory structure − A structure that is on the same parcel of property as a principal structure, the use of which is incidental to the use of the principal structure.

Base flood − The flood having a 1-percent chance of being equaled or exceeded in any given 

year, commonly referred to as the “100-year flood.” The base flood is the national standard 

used by the NFIP and all Federal agencies for the purposes of requiring the purchase of flood 

insurance and regulating new development. 

Base flood elevation (BFE) − The height of the base (1-percent annual chance or 100-year) 

flood in relation to a specified datum, usually the National Geodetic Vertical Datum of 1929 

(NGVD), or the North American Vertical Datum of 1988 (NAVD). 

Basement − Any area of a building having its floor subgrade (below ground level) on all 

sides. 

Elevation certificate − A form developed by FEMA to collect surveyed elevations and other A form developed by FEMA to collect surveyed elevations and other 

information about a building that is necessary to obtain flood insurance.

Enclosure or enclosed area − Areas created by a crawlspace or solid walls that fully enclose Areas created by a crawlspace or solid walls that fully enclose 

areas below the BFE.

Federal Emergency Management Agency (FEMA) − The Federal agency that, in addition to 

carrying out other activities, administers the National Flood Insur­ance Program. 

Flood Insurance Rate Map (FIRM) − The official map of a community on which FEMA has 

delineated both the special flood hazard areas (SFHAs) and the risk premium zones applicable to the community.

Hydrodynamic load − The load imposed on an immersed object, such as a foundation element The load imposed on an immersed object, such as a foundation element 

or enclosure wall, by water flowing against and around it. The magnitude of the hydrodynamic load varies as a function of velocity and other factors.

Hydrostatic load − The load imposed on an immersed object such as an enclosure wall, by The load imposed on an immersed object such as an enclosure wall, by 

standing or slowly moving water. The magnitude of the hydrostatic load increases linearly with 

water depth.

Lowest floor − The lowest floor of the lowest enclosed area of a building, including a basement. The lowest floor of the lowest enclosed area of a building, including a base­ment. 

Any NFIP-compliant unfinished or flood-resistant enclosure usable used solely for parking of 

vehicles, building access, or storage (in an area other than a basement) is not considered a 

building’s lowest floor, provided the enclosure does not render the structure in violation of 

the applicable design requirements of the NFIP. 

Mitigation Directorate − The component of FEMA directly responsible for administering the 

flood hazard identification and floodplain management aspects of the NFIP. 
Technical Bulletin 1 – August 2008 31

Net open area − The permanently open area of a non-engineered opening intended to provide automatic entry and exit of floodwaters.

Opening, engineered − An engineered opening is an opening that is designed and certified An engineered opening is an opening that is designed and certified 

by a registered design professional as meeting certain performance characteristics related to 

providing automatic entry and exit of floodwaters; the certification requirement may be satisfied by an individual certification or issuance of an Evaluation Report by the ICC Evaluation 

Service, Inc.

Opening, non-engineered − A non-engineered opening is an opening that is used to meet A non-engineered opening is an opening that is used to meet 

the NFIP’s prescriptive requirement of 1 square inch of net open area for every square foot of 

enclosed area. 

Registered Design Professional − An individual who is registered or licensed to practice their 

respective design profession as defined by the statutory requirements of the professional registration laws of the State or jurisdiction in which the project is to be constructed.

Special Flood Hazard Area (SFHA) − An area delineated on a FIRM as being subject to inundation by the base flood and designated as Zone A, AE, A1-A30, AR, AO, AH, A99, V, VE, 

or V1-V30. 

Substantial damage − Damage of any origin sustained by a structure whereby the cost of re- Damage of any origin sustained by a structure whereby 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. Structures that are determined 

to be substantially damaged are considered to be substantial improvements, regardless of the 

actual repair work performed. 

Substantial improvement − Any reconstruction, rehabilitation, addition, or other improvement of a structure, the cost of which equals or exceeds 50 percent of the market value of 

the structure (or smaller percentage if established by the community) before the “start of 

construction” of the improvement. This term includes structures that have incurred “substantial damage,” regardless of the actual repair work performed.