But many suburban homes built in so-called Tornado Alley don't have basements due to rocky soil conditions and high water tables, which often make building basements impossible. An interior room or closet—often recommended for those without basements—may not be effective when extreme winds or debris strike.

On Wednesday, the mayor of Moore, Oklahoma (map)—the hardest-hit town in this week's storm—announced that he was planning to propose an ordinance that would modify building codes and require all new homes built in the town to have either a reinforced storm shelter or safe room installed. The Federal Emergency Management Agency (FEMA) also encourages homeowners in tornado-prone regions to build safe rooms.

A storm shelter is a structure designed to protect people from