Fire Resistive Construction

Construction Class 5

Overview

This Construction Brief will give you an overview of Verisk 's Construction Class 5 – modified fire-resistive construction. When you've completed this course, you will know what a Verisk 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.

Definition

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.

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

Examples of metal lath and plaster applied to steel:

Examples of sprayed-on materials applied to steel:

Example of gypsum board applied to steel columns and sprayed-on materials applied to the steel beams:

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? Verisk 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. Verisk individually evaluates each approved design.

The drop ceiling is the fire protection for the column

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