Construction types

Modified Fire Resistive (ISO Class 5, IBC Type IB)

Modified Fire Resistive construction is ISO Class 5. ISO Class 5 encompasses IBC Type IB.

Modified Fire Resistive Buildings are buildings where 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.

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

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

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. 

Advantages of the modified 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


expensive to construct and repair
provides a false sense of security


Fire Resistive (ISO Class 6, IBC Type IA)