How Thick Does a Concrete Wall Need to Be for a Tornado Room?

About 1,000 tornadoes hit the United State each year, according to the NOAA National Severe Storms Laboratory. Sixty people, on average, die from tornadoes annually, making concrete safe rooms with the proper thickness an important feature of a house or property, especially in you live in Tornado Alley. The structure must be thick enough to withstand winds of up 250 mph that causes flying debris to turn into deadly missiles.

Most tornado rooms consist of an enclosed space completely encased in thick concrete, including the walls, ceiling and floor. Most builders add the room to a central area of the home because the family can safely access the room without going outside in the storm. Other concrete tornado rooms are added as part of a remodel, with the structures often added to an outside wall to keep costs down. A few builders construct concrete tornado rooms in the ground outside of the home, similar to a cellar, but you must walk outside to reach the shelter.

In order for a concrete room to effectively resist high winds and flying debris, most rooms built with precast concrete walls rely on 12- to 16-inch thick blocks. To keep the house and the room from being sucked into the air during a tornado, large footings that work like anchors are added to the walls to hold the room in place.

According to Texas Tech University's Wind, Science & Engineering Research Center, rooms built with 6- to 8-inch thick reinforced concrete masonry provide resistance for F5 tornadoes, the most powerful storm possible. When concrete masonry is used, you must also add conventional reinforcing bars to give the structure more strength. In addition, cavities in the concrete block are filled with concrete to give the walls even more resistance.

A concrete tornado room is only as good as its weakest point, making the door to the room an important factor. Because concrete doors would weigh too much for people to maneuver when they need to access the room, fabricated or store-bought missile-resistant steel doors work best, according to Texas Tech University's Wind, Science & Engineering Research Center. Doors consisting of 11-gauge sheet steel include the use of two 3/4-inch plywood pieces glued together. Some builders use hollow steel doors available in 14-, 16- or 20-gauge metal skins. The 16- and 20-steel gauge doors require strengthening with a single layer of 14-gauge steel to make them work for a tornado room.