This Construction Brief will give you an overview of ISO's Construction Class 5 – modified fire resistive construction. When you've completed this course, you'll know what an ISO report means when it says a building is modified fire resistive construction. And you'll understand why a particular structure is—or isn't—modified fire resistive.


  • Buildings with exterior walls, floors, and roofs of masonry materials described in the definition of fire resistive (Construction Class 6)—less thick than required for fire-resistive structures but not less than four inches thick, or
  • Fire-resistive materials with a fire-resistance rating less than two hours but not less than one hour

Wall, floor, and roof assembly with noncombustible panels, with one-hour fire-resistance rating

Poured gypsum roof with one-hour fire-resistance rating


Exterior walls

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.

Here's an example of concrete applied to steel

Structural steel protection

Modified fire-resistive buildings also include structural steel protection techniques—fire-protection material applied to steel.

Materials include:

  • Concrete
  • Plaster
  • Clay tile
  • Brick or other masonry units
  • Gypsum block
  • Gypsum wallboard
  • Mastic coatings
  • Mineral and fiberboard
  • Mineral wool

Metal lath and plaster

Here are examples of metal lath and plaster applied to steel

Metal lath and plaster protects the beam.


Here are examples of sprayed-on materials applied to steel:


Here is an example of gypsum board applied to steel:


Ceilings protecting steel beams or joists

What happens when there is no fire-protection material applied to steel beams or joists that support floors or roofs? ISO still considers a building modified fire resistive if it has a suitable ceiling.

Ceilings can be plaster or gypsum wallboard or suspended mineral tile.

The entire floor-ceiling (a fire-resistive ceiling protecting a floor) or roof-ceiling (a fire-resistive ceiling protecting roof supports) should conform to construction details in a UL-listed or Factory Mutual (FM)-approved design. ISO individually evaluates each approved design.

The drop ceiling is the fire protection for the beam

One-hour UL-listed assembly concrete on steel deck

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 modified fire-resistive construction

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