Walls

• In order for walls to be ISO Code 6, or fire-resistant, they must meet several criteria. Solid masonry must include reinforced concrete at least 4 inches thick. Hollow masonry must be 12 inches thick or 8 inches thick with a fire-resistance rating of two or more hours. Load-bearing exterior walls must be made of noncombustible material.
  
  

Roof and Floors

• Both floors and the roof of a fire-resistant building must contain reinforced concrete with a thickness of at least 4 inches. All assemblies must have a two-hour or longer fire-resistance rating. In addition, all load-bearing supports in the building must have at least a two-hour, fire-resistance rating.
  
  

Concrete

• All concrete units must have steel cables to provide them with tensile strength. During construction, these cables are manipulated by a contractor to stress and strengthen the concrete properly. The manipulation of the cables is how concrete is classified as "pre-" or "post-" stressed.