Modified Fire Resistive Construction

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.

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

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.