Lesson 17 Overview

Welcome to Lesson 17. This lesson covers the Tornado Safe Room Module overview.

At the end of this lesson you will be able to:

  • Explain key tornado hazard concepts
  • Explain Tornado Safe Room Module basic concepts and theory
  • Describe tornado safe room mitigation types
Tornado Hazard Overview

A tornado is a narrow, violently rotating column of air that extends from the base of a thunderstorm to the ground. Because wind is invisible, it is hard to see a tornado unless it forms a condensation funnel made up of water droplets, dust and debris. Tornadoes are among the most destructive weather events.

The number of detected tornadoes has increased since 1950. This is largely due to the invention and use of radar starting in 1950. More recently, storm chasing has led to more tornadoes being reported. Along with radar and other technological advances, warnings have led to a steady decline in the number of tornado fatalities.

NOAA’s National Weather Service Storm Prediction Center provides Monthly and Annual U.S. Tornado Summaries on tornado totals and deaths, available at http://www.spc.noaa.gov/climo/online/monthly/newm.html

Table 1: Enhanced Fujita Scale
Tornadoes are assigned a classification based on estimated wind speeds and related damage. The National Weather Service implemented the “Enhanced Fujita Scale,” or E-F Scale, in 2007 to classify tornadoes more consistently and accurately. Tornadoes with higher EF classifications produce stronger winds and cause more damage. The following screens show examples of damage at different EF classifications.


3-Second Gust (mph)


65 - 85


 86 - 110


111 - 135


136 - 165


166 - 200



Tornado Classifications: EF0

EF0 tornadoes have wind speeds of 65 to 85 miles per hour.

Damage includes loss of roof-covering material (<20%), gutters, and/or awnings; loss of vinyl or metal siding; tree branches broken; and shallow-rooted trees toppled.

Tornado Classifications: EF1

EF1 tornadoes have wind speeds of 86 to 110 miles per hour.

Damage includes broken glass in doors and windows, uplift of roof deck and significant loss of roof covering (>20%), collapse of chimneys and garage doors, mobile homes pushed off foundations or overturned, and moving automobiles pushed off roads.

Tornado Classifications: EF2

EF2 tornadoes have wind speeds of 111 to 135 miles per hour.

Damage includes entire houses shifted off foundations, large sections of roof structure removed, mobile homes demolished, trains overturned, large trees snapped or uprooted, and cars lifted off ground and thrown.

Tornado Classifications: EF3

EF3 tornadoes have wind speeds of 136 to 165 miles per hour.

Damage includes collapse of most walls except small interior rooms, and most trees in forest uprooted.

Tornado Classifications: EF4

EF4 tornadoes have wind speeds of 166 to 200 miles per hour.

Damage includes well-constructed houses leveled, structures blown off weak foundations, and cars and other large objects thrown about.

Tornado Classifications: EF5

EF5 tornadoes have wind speeds of greater than 200 miles per hour.

Damage includes strong frame houses lifted off foundations, carried a considerable distance, and disintegrated; automobile-sized missiles flown through the air in excess of 100 meters; trees debarked; and slabs swept clean.

Video: Tornado Safe Room Module Basic Concepts and Theory

This video explains the basic concepts and theory of the Tornado Safe Room Module of the BCA Tool.

The Tornado Safe Room Module calculates benefits of proposed safe room mitigation projects based on reduced casualties, or reducing the likelihood that a person would be killed or injured in a tornado if he or she did not have access to a safe room. These are also called “life safety” benefits.

Residential safe rooms and community safe rooms are both eligible for funding under some of FEMA’s mitigation programs.

For residential safe rooms, there is a pre-determined cost-effectiveness methodology explained in the Job Aid called “Safe Room Project Application Using Pre-Calculated Benefits”: http://www.fema.gov/media-library-data/1424368115734-86cfbaeb456f7c1d57a05d3e8e08a4bd/FINAL_SafeRoom_JobAid_13FEB15_508complete.pdf.

The rest of this overview looks only at community safe rooms.

The first major variable in the Tornado Safe Room Module is the tornado risk, which is automatically imported into the analysis based on the project location. Projects are more likely to be found cost-effective where there is a risk of large and frequent tornadoes.

Occupancy is the next significant variable in the Tornado Safe Room module. Occupancy data is important not only for properly designing the safe room with enough space, but it also determines the number of people who will be protected from harm. For example, if a safe room is to be constructed just for a school, the occupancy would be the number of students, faculty and support staff, and average number of visitors.

Applications for safe rooms for general public use will have to demonstrate where potential occupants will come from and whether the time and distance for them to reach the safe room meets programmatic requirements.

Another major variable in the Tornado Safe Room Module is the predominant structure type(s) that the safe room occupants will evacuate from. Different structure types have different “wind performance” characteristics, which means the extent they can withstand high winds and protect their inhabitants. Examples include open space like a campground that provides no protection, mobile homes that provide limited protection, and large institutional buildings like hospitals that provide a very strong wind resistance and good protection.

You can select up to two predominant structure types in the module.

For example, a school safe room may be close enough for neighboring residences to also seek safety.

When the tool knows the tornado risk for the project location, number of occupants for the safe room, and the wind performance for the buildings they will be evacuating from, the “Value of a Statistical Life,” or VSL, converts calculated injuries and fatalities into dollar terms.

These dollar values take into consideration costs for hospitalization, lost work productivity, and the long term impact of severe injuries.

The dollar value for the prevented casualties is the Damages Before Mitigation value. Since a safe room constructed to FEMA specifications provide “near absolute protection,” these Damages After Mitigation are assumed to be zero. Therefore, the entire dollar value of casualties prevented by the safe room project are considered project benefits.

This concludes the basic concepts and theory of the Tornado Safe Room Module of the BCA Tool.

Tornado Safe Room Mitigation Types

Residential and community safe rooms can protect lives and property from tornado events. Safe room construction projects include retrofits of existing facilities and new safe room construction and apply to both single- and multi-use facilities.

The Safe Room Resources page (https://www.fema.gov/safe-room-resources) on the FEMA website provides a list of publications that provide information about safe rooms.

Residential Safe Room

A residential safe room is a small, specially designed (“hardened”) room, such as a bathroom or closet, which is intended to provide a place of refuge for the people who live in the home.

An external residential safe room is similar in function and design, but it is a separate structure installed outside the home, either above or below ground.

According to FEMA Publication 320, a residential safe room or small community safe room can have a maximum occupancy of 16.

To be considered a FEMA safe room, residential safe rooms must be designed and constructed according to the guidelines specified in FEMA Publication 320, Taking Shelter from the Storm: Building a Safe Room for Your Home or Small Business, available at: http://www.fema.gov/library/viewRecord.do?id=1536

Community Safe Room

A community safe room is designed and constructed to protect a large number of people from a natural hazard event.

The number of persons taking refuge in the safe room can be several hundred or more.

To be considered a FEMA safe room, community safe rooms must be designed and constructed according to the guidelines specified in FEMA Publication 361, Design and Construction Guidance for Community Safe Rooms, available at: http://www.fema.gov/library/viewRecord.do?id=1657.

Lesson 17 Summary

Lesson 17 covered the Tornado Safe Room Module overview, including:

  • Key tornado hazard concepts
  • Tornado Safe Room Module basic concepts and theory
  • Tornado safe room mitigation types