Storm Shelter Standards
ICC-500 Explained
ICC-500 is the code standard that defines how storm shelters are designed and built. Here is what it requires, how it differs from FEMA P-320, and why an Oklahoma shelter should meet both.
The Short Answer
What Is ICC-500?
ICC-500 is the enforceable code standard for the design and construction of storm shelters, referenced directly by the building codes.
Its full name is the ICC/NSSA Standard for the Design and Construction of Storm Shelters, developed jointly by the International Code Council and the National Storm Shelter Association. It is an ANSI consensus standard, which means it was created through a formal, reviewed process, and it is referenced by the International Building Code, the International Residential Code, and the International Existing Building Code. Because the codes point to it, ICC-500 carries the force of an enforceable regulation.
Where FEMA P-320 reads like guidance for homeowners, ICC-500 reads like code. It uses the language of requirements, "shall" and "required," and it sets the measurable criteria that a storm shelter has to satisfy: design wind speed, debris impact resistance, the floor area required per occupant, ventilation, door performance, and anchoring.
Side by Side
ICC-500 vs FEMA P-320
The two are closely related and often confused. The simplest way to keep them straight is that ICC-500 is the code standard and FEMA P-320 is the federal guidance that applies it to homes.
| Factor | ICC-500 | FEMA P-320 |
|---|---|---|
| Type | Enforceable code standard | Federal design guidance |
| Referenced by building codes | Yes (IBC, IRC, IEBC) | No |
| Language | "Shall" and "required" | "Should" and "recommended" |
| Protection level | Life safety | Near-absolute protection |
| Tornado design wind | 130 to 250 mph by location | 250 mph everywhere |
| Scope | All storm shelters | Residential safe rooms |
Read the companion guide: FEMA P-320 explained.
Working Together
How ICC-500 and FEMA Standards Fit Together
These standards are not competitors. FEMA P-320, and the related FEMA P-361 guidance for larger and community safe rooms, build directly on ICC-500. FEMA P-320 applies the ICC-500 requirements to residential safe rooms and adds a few things on top, including a 250 mph design wind for homes everywhere and extra best practices for siting and construction.
There is a useful way to remember the relationship: all safe rooms are storm shelters, but not all storm shelters are safe rooms. A FEMA safe room meets ICC-500 and then goes further toward near-absolute protection. For an Oklahoma homeowner, the takeaway is simple. Look for a unit documented to meet both ICC-500 and FEMA P-320, which is the combination that gives you code-recognized construction and federal-grade protection in one shelter.
Inside the Standard
What ICC-500 Requires
ICC-500 sets the tornado design wind speed by location, and all of Oklahoma falls in the highest zone, which requires a 250 mph design wind, the same extreme threshold FEMA uses. It also defines the windborne debris impact criteria a shelter must pass, the minimum floor area per occupant so a unit's rated capacity reflects real usable space, the ventilation needed for a sealed shelter, and the performance of the door and its anchoring.
The debris test is worth understanding, because flying debris causes most tornado injuries. For the 250 mph tornado zone that covers Oklahoma, the representative test missile is a 15-pound wooden 2x4 fired at the shelter at 100 mph, per the FEMA-published criteria that accompany the standard. Walls, doors, and every other surface of a compliant shelter have to stop that impact. When an installer says a unit is "tested," this is the test to ask about.
That last point about occupancy is worth remembering when you shop. Because ICC-500 specifies a minimum space per person, a unit's rated capacity is a real number, not a marketing figure. Ask the installer for the rated occupancy and choose a shelter that comfortably fits everyone you need to protect.
Edition History
ICC-500 Editions: 2008, 2014, 2020, and 2023
ICC-500 was first published in 2008. The 2014 edition brought a substantial revision, with updated design and testing requirements and a stronger emphasis on occupant life safety: means of egress, lighting, sanitation, ventilation, fire safety, and maximum occupant density. The 2020 edition refined those requirements further, and the current edition, ANSI/ICC 500-2023, moved the standard onto continuous maintenance, so it now evolves on an ongoing basis instead of waiting for a full new edition.
For a homeowner paying cash, the documentation matters more than the edition number. For a homeowner using rebate money, the edition itself is the deciding factor, and the date to know is January 1, 2024. That is when FEMA's grace period allowing FEMA-funded prefabricated residential safe rooms to comply with FEMA P-361 (2015) and ICC 500-2014 expired. Applications for a FEMA-funded prefabricated residential safe room must now demonstrate compliance with FEMA P-361 (2021) and ICC 500-2020 (FEMA, Safe Room Funding, page updated April 8, 2026).
That reaches Oklahoma homeowners directly, because the state says it "is providing this safe room rebate program through FEMA Hazard Mitigation grants" (Oklahoma Emergency Management rules and regulations, read 2026-08-14). So if you intend to use SoonerSafe rebate money on a prefabricated unit, ask the installer for testing and labeling documentation to ICC 500-2020 or newer, not 2014. If a quote references only the 2008 or 2014 edition, that is the question to settle before you sign anything.
Buyer Checklist
Confirm Both Standards Before You Buy
Ask any installer for documentation that the unit meets ICC-500 and FEMA P-320, and confirm its rated occupancy and tested anchoring. This is also what the SoonerSafe rebate generally requires, so the same paperwork protects your family and your rebate.
Common Questions
ICC-500 Questions
What is ICC-500?
ICC-500 is the ICC/NSSA Standard for the Design and Construction of Storm Shelters, written by the International Code Council and the National Storm Shelter Association. It is an ANSI consensus standard referenced by the building codes, so it is enforceable, and it defines design wind speed, debris impact, occupant space, ventilation, doors, and anchoring for storm shelters.
How is ICC-500 different from FEMA P-320?
ICC-500 is the enforceable code standard that uses language like shall and required and provides life-safety protection, with a tornado design wind that ranges from 130 to 250 mph depending on location. FEMA P-320 is federal guidance for residential safe rooms that uses a 250 mph wind everywhere and aims for near-absolute protection. FEMA P-320 actually applies ICC-500 to homes.
Does ICC-500 apply to my home shelter?
Yes. ICC-500 covers storm shelters generally, including residential ones. A quality home shelter in Oklahoma is typically built to meet both ICC-500 and FEMA P-320, since the two are closely aligned and FEMA P-320 builds on ICC-500.
What tornado wind speed does ICC-500 require in Oklahoma?
ICC-500 sets the tornado design wind speed by location, and all of Oklahoma falls in the highest zone, which calls for a 250 mph design wind. That matches the 250 mph used by FEMA P-320, so a compliant Oklahoma shelter is built for the same extreme winds either way.
Is ICC-500 or FEMA P-320 better?
Neither is better; they work together. ICC-500 is the code standard that makes shelters enforceable in building codes, and FEMA P-320 applies and adds to it for residential safe rooms aiming at near-absolute protection. The strongest position for a homeowner is a unit documented to meet both.
How do I confirm a shelter meets ICC-500?
Ask the installer for the engineering or manufacturer documentation showing the unit was designed and tested to ICC-500, and confirm its rated occupancy, since ICC-500 sets a minimum floor area per person. The documentation, not the sales pitch, is what proves compliance.
What are the editions of ICC-500, and which is current?
ICC-500 was first published in 2008, with revised editions in 2014, 2020, and 2023. The 2023 edition (ANSI/ICC 500-2023) is the most recent, and the standard is now on continuous maintenance, meaning updates are developed on an ongoing basis. The edition matters most when federal money is involved. FEMA's grace period allowing FEMA-funded prefabricated residential safe rooms to comply with FEMA P-361 (2015) and ICC 500-2014 expired on January 1, 2024, so applications for a FEMA-funded prefabricated unit must now show compliance with FEMA P-361 (2021) and ICC 500-2020. SoonerSafe is provided through FEMA Hazard Mitigation grants, so if you plan to use rebate money on a prefabricated shelter, ask the installer for documentation to ICC 500-2020 or newer rather than 2014.
What is the difference between FEMA P-361 and ICC-500?
FEMA P-361 is the federal design guidance for safe rooms, including community and larger residential safe rooms, while ICC-500 is the enforceable code standard for storm shelters. They are closely aligned on structural criteria, and P-361 points to ICC-500, but P-361 targets near-absolute protection and is the document FEMA grant programs typically reference. For a home shelter in Oklahoma, look for documentation naming ICC-500 plus FEMA P-320 (and P-361 where the rebate paperwork asks for it).
Want a Code-Compliant Storm Shelter?
Get a free consultation from a licensed local installer who builds to ICC-500 and FEMA P-320.
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