Underground shelters provide the most safety during a tornado, though builders of above-ground shelters that are in compliance with the National Storm Shelter Association standards report that their shelters exceed underground shelters in safety testing. Testing usually only covers the doors of underground shelters, while testing of above-ground shelters includes the walls and corner joints. Underground shelters with entrances outside the home can be difficult to access during severe storms, while above-ground shelters provide easier access.
Underground shelters, while deemed safer, are not always feasible due to water-table issues, and in harsher climates the stresses caused by frozen soil can impact the integrity of the shelter. Site preparation can also increase the costs of installing an underground shelter, although this type of shelter does not take up space within a house or lot as required for above ground shelters. In some areas new homes include safe rooms. Prebuilt above-ground shelters are also available for installation in existing homes. Both options require safety testing to ensure resistance to flying debris, according to the Federal Emergency Management Agency. FEMA recommends safe rooms even for homes with basements. The shelter can be placed in the basement if ease of access is not an issue.
Risk of flooding impacts the location of above-ground shelters as well as those built below ground. FEMA provides guidelines on locating above-ground safe rooms in relation to flood hazards.