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How to Size Your Commercial Storm Shelter to Accommodate Your Team

At RemainSafe, we know purchasing a storm shelter/safe room for your business or community space can be complicated. There are many questions that arise when planning to install storm shelters. How large are storm shelters? Where should it be placed on your site? Most importantly, what size storm shelter/safe room or how many shelters will you need to accommodate your facility? Let’s begin by determining what size you will need. In this blog, we will focus on the sizing guidelines for tornado shelters based on criteria provided by FEMA and International Code Council (ICC) 500.

No Two Emergencies are the Same for Your Team

According to FEMA guidelines for Safe Rooms for Tornadoes and Hurricanes, “The minimum area per occupant criteria for safe rooms in ICC 500 are based on the use of the space during a storm event and are not intended to be space recommendations for a safe room that might be used for recovery proposes.”

This means that the occupancy codes FEMA guidelines reference are written only for storm shelters/safe rooms specifically built for protection against severe weather events, and not long-term protection from other threats or other applications.

Due to the fast and destructive nature of tornados, the threat tends to be severe, but shorter-lived than the threat of a hurricane. This means that the space needed for standing occupants is less when a storm shelter is intended to protect against tornadoes than it is for hurricanes, which present a lengthier threat. The below guidelines focus only on the specific criteria for storm shelters/safe rooms made for tornadoes. Be on the lookout for our next blog focusing on hurricane shelter occupancy guidelines.

How Many People Does Your Shelter Need to Accommodate?

The size of your storm shelter will be determined by the number of team members or business patrons that you will need to protect in the event of a storm. When determining the maximum FEMA-certified capacity for your tornado shelter are as follows, according to official FEMA guidelines:

First, you must assume that the storm shelter/safe room will be used at the time of day when the maximum number of occupants is expected.

For example, if you are purchasing for a school or college campus, your needed capacity will be based on the time when most students and staff are present at the facility.

Second, the planning process of the storm shelter/safe room should also consider the potential for an increase in the number of occupants over time.

During a severe weather event, especially one as sudden as a tornado, it is possible that others outside of your institution may seek shelter in your facilities. It is essential to allow space for a few additional occupants after you have calculated the maximum required population of your business or community space needs.

How to Calculate Occupant Density for Tornado Shelters

Once you have calculated the number of occupants your storm shelter needs to protect, you are ready to determine what size storm shelter you will need.

FEMA guidance requires a Minimum Usable Floor Area for each standing person to be as follows:

  1. The required space for each standing occupant is 5 square feet
  2. The required space for each occupant using a wheelchair is 10 square feet
  3. The required space for any occupants needing a bed or stretcher is 30 square feet

How to Calculate Shelter Size

Below are two methods published by FEMA in their Safe Rooms for Tornadoes and Hurricanes guidebook (p153) to calculate the usable storm shelter/safe room floor area in an existing or new storm shelter/safe room.  

Method One

  1. Reduce the gross floor area of safe rooms with concentrated furnishings or fixed seating by at least 50%
  2. Reduce the gross floor area of safe rooms with unconcentrated furnishings and without fixed seating by at least 35%
  3. Reduce the gross floor area of safe rooms with open plan furnishings and without fixed seating by at least 15%

Method Two

Reduce the gross area of the safe room by excluding spaces associated with partitions and walls, columns, fixed or movable equipment or any other features that cannot be moved. The remaining area is considered the usable storm shelter/safe room area.

Not sure where to begin? No problem, our team will work with you to determine which storm shelter size and features is best for your business or application. The RemainSafe team will visit your site and determine the appropriate size and storm shelter quantity options customized for your applications. We work with EHS and Safety Coordinators to understand your business and incorporate the use of storm shelters/safe rooms into your campus, facility site and safety preparedness plan. Safety is the top priority at RemainSafe. Contact us to schedule a site visit and learn more about our storm shelter/safe room options.

To learn more, visit our website, or read our blog on the benefits of installing a storm shelter in your business.

To read further details of FEMA occupancy compliance, read pages 150-154 of the FEMA Safe Rooms for Tornadoes and Hurricanes Guidebook.