This Construction Brief will give you an overview of ISO's Construction Class 6 – fire resistive construction. When you've completed this course, you'll know what an ISO report means when it says a building is fire resistive construction. And you'll understand why a particular structure is—or isn't—fire resistive.


  • Solid masonry, including reinforced concrete not less than four inches thick
  • Hollow masonry not less than 12 inches thick
  • Hollow masonry less than 12 inches thick, but not less than eight inches thick with a listed fire-resistance rating of not less than two hours
  • Assemblies with not less than a two-hour fire-resistance rating

Floors and roofs:

  • Reinforced concrete not less than four inches thick
  • Assemblies with not less than a two-hour fire-resistance rating

Structural metal supports:

  • Horizontal and vertical load-bearing protected metal supports—including prestressed and posttensioned concrete units—with not less than a two-hour fire-resistance rating

The exterior bearing walls and load-bearing portions of exterior walls must be of noncombustible materials or of masonry, but exterior nonbearing walls and wall panels may be slow burning, combustible, or with no fire-resistance rating.

Solid masonry

Hollow masonry

Exterior walls of noncombustible materials

Pre- and posttensioned concrete units

Both pre- and post-tensioned concrete units have steel cables installed in the concrete to provide tensile strength. With pre-stressed concrete units, builders pull the cables tight before pouring the concrete and release them as the concrete cures. With post-tensioned concrete units, builders pull one end of the cable tight after pouring the concrete.


Posttensioned concrete units

Prestressed concrete units

Advantages of fire resistive construction 

  • Uses noncombustible materials
  • Allows greater height and area than other construction classes
  • Uses load-bearing members or assemblies that resist damage from fire

Disadvantages of fire resistive construction

  • Expensive to construct and repair
  • Provides a false sense of security