The United States has the most tornadoes of any country. Though we experience approximately 1,200 each year, a busy year could see more than 1,500 tornadoes. We also have the strongest and most violent tornadoes of any country because of our geography and size.
Assessing Your Risk
Our building codes contain design data that provide guidance for weather, seismic and other events. The weather data provides information like precipitation (snow loads) and wind loads. None of the design guidelines for wind loads come close to the force exerted by extreme weather events like tornadoes.
The basic wind speed information in the 2012 International Residential Code (IRC) shows a wind speed of 90 miles per hour for most of the United States [Figure R301.2(4)A]. Coastal areas receive higher wind speed ratings, up to 140 miles per hour, because of hurricanes. Even moderate tornadoes like an F1 measured on the Enhanced Fujita Scale can exceed the wind load used to design our houses in most of the country.
NOAA’s National Climactic Data Center publishes information on extreme weather events like tornadoes. You might be surprised by the risk in certain states. For example, would you have guessed that Florida experienced more tornadoes (by far) on average than any other southeastern state from 1991 to 2010?
You can also use records from NOAA’s Storm Prediction Center to assess the risk for your location. The data from the SPC is also startling – there were 758 tornadoes in the United States during April 2011. In addition to information on tornadoes, you can find all sorts of weather and sesmic events recorded on our government websites to help you assess your risk.
Safe Room & Storm Shelter Standards
The Federal Emergency Management Agency (FEMA) publishes a series of construction standards and other related documents for buildings in areas known for weather-related hazards like hurricanes and tornadoes. FEMA has a safe room standard for these extreme weather events. FEMA describes safe rooms as, “a hardened structure specifically designed to meet the Federal Emergency Management Agency (FEMA) criteria and provide ‘near-absolute protection’ in extreme weather events, including tornadoes and hurricanes.”
Safe rooms are typically above-ground rooms in your home. This is in contrast to an underground storm shelter. You can find FEMA’s guidance for safe rooms in its P-320 “Taking Shelter from the Storm” document. Safe rooms and most storm shelters are designed for a small number of occupants that you’d expect in a home or small business. The ICC 500 standard from the International Code Council provides guidance for larger shelters that you’d expect for schools and commercial buildings.
Installing a safe room in an existing home can be a significant challenge because of the potential amount of demolition and structural work required. The room needs to be adequately connected to the structure and foundation of the house to resist the wind and other loads delivered in a weather event like a tornado or hurricane. Safe rooms seem to be a much better solution for new construction where they can be included in the initial design.
If you’d like to add a shelter to your existing home, you should consider a prefabricated storm shelter like the one shown in the following image.
In the image above, the existing garage floor was cut and the dirt was excavated to make room for the storm shelter to be dropped in and back filled with concrete. By setting the shelter in the ground, there’s no wind load because there are no exposed walls that have to resist the force of the storm.
You can also find prefabricated units designed to be buried in your yard so there’s no disruption to your house. These might be helpful solutions for mobile homes and houses with challenging foundations – steeply-sloping lots, houses on piers, etc.
The image above is another view of the storm shelter installation in the second image. You can see the storm shelter has been installed in the garage floor. We prefer storm shelter doors to open to the exterior of the house. What happens if a weather event knocks the garage down on top of the storm shelter in the garage floor while you’re occupying it?
To address this issue, many municipalities have created a storm shelter registry so they know to check each storm shelter to be sure people aren’t trapped inside. Oklahoma City’s storm shelter registry is an example you can review. If your municipality doesn’t have a storm shelter registry, you should give more thought to where you locate your storm shelter access.
You also need to be able to operate your storm shelter access. In the second image, you can see the metal door to the storm shelter. Before you choose a storm shelter, please make sure the anticipated occupants are strong enough to open and close the shelter by themselves. You may choose to spend more money on a safe room that’s easier to access, open and close if you anticipate young, elderly or disabled occupants.
After watching news coverage of recent storms, we’ve revised this post to include this section – Please don’t forget to size your storm shelter to include your pets. It’s amazing to see how many people lose track of their pets when they’re separated during weather events. It’s also critical to have your pets microchipped so they can be identified and returned to you if you do become separated.
Know When to Go
If you’ve assessed your risk and think there’s any chance that you might experience an extreme weather event, please consider investing in a weather radio. Many people rely on outdoor warning sirens to alert them though these are typically designed only to alert people who are outside – away from their weather radios. What’s the point of having a storm shelter if you don’t know when to use it?
You can also find active alerts on the National Weather Service website. It’s a helpful resource that lets you check alerts by state so you can see weather event concerns even when you’re traveling.
Since you’ve assessed your risk for weather events, please take a moment to coordinate your insurance. You may find that you can get credit toward your premiums for code-plus construction that helps your home resists weather events.
We also suggest that you take some time to research flood insurance, even if you think you don’t need it. Weather events often include rain that can create flash flood events that aren’t covered under many home owner’s insurance policies. We don’t want you to be in a situation where your home owner’s policy provider argues that damage was caused by water intrusion and is thus excluded from your coverage.
You can find lots of information on the National Flood Insurance Policy website. Please beware that there’s typically a 30-day waiting period for new policies – you can’t wait to call when bad weather is approaching!
Many severe storms are “fast moving”. There’s no time to pack up and escape so you need a better option than trying to ride out a tornado in your bathtub. Very few buildings are “storm proof”. We can design and construct buildings that are impervious to all but the most violent events like a nuclear strike but who wants to live in a bunker all the time?
If you’re thinking about protecting your family from weather events, please consider starting with a narrow focus: a first aid kit, a weather radio and a storm shelter. Once you’ve gotten that basic level of protection, you can consider survival supplies and hardening the rest of your property to resist storms.
If you need some help deciding between a storm shelter and a safe room or you’re considering options to help you property resist the force of a storm, do some research to find a local advocate focused on your type of construction. We’re partial to architects and engineers as advocates because we want you to have an impartial advocate – be wary of people with a product to sell while they claim to be impartial. We think an advocate can be a great investment in your safety and help you make some sense of your options… especially when comparing costs!
Since this is a post on safety, we respectfully ask that you share it with anyone who might benefit from this information. Do you know anyone subject to extreme weather events like tornadoes and hurricanes? Thanks!