A house on a basement foundation is usually built on a foundation constructed of poured concrete or concrete masonry. Most concrete foundations are reinforced with steel bars or straps, but many concrete masonry foundation walls have no steel reinforcement. The framing for the floor above the basement is supported by the exterior foundation walls and sometimes by a center beam.

In a new or existing house with a basement, the shelter should be built in the basement. You can build the shelter as an entirely separate structure with its own walls, or you can use one or more of the basement walls as walls of the shelter. If you use the basement walls, they will have to be specially reinforced. Typical reinforcement techniques used in residential basement walls will not provide sufficient protection from missiles. In new construction, B&L Homes can reinforce the walls near the shelter during the construction or your house. Reinforcing the basement walls of an existing house is not practical.

The likelihood of missiles entering the basement is lower than for above ground areas; however, there is a significant chance that missiles or falling debris will enter the basement through an opening left when a window, a door, or the first floor above has been torn off by extreme wind. Therefore, your basement shelter must have its own reinforced ceiling; the basement ceiling cannot be used as the ceiling of the shelter.