This Construction Brief will give you an overview of ISO's Construction Class 2 – joisted masonry construction. When you've completed this course, you'll know what an ISO report means when it says a building is joisted masonry construction. And you'll understand why a particular structure is—or isn't—joisted masonry.


Buildings with exterior walls of masonry or fire-resistive construction rated for not less than one hour and with combustible floors and roofs.

Types of joisted masonry

There are several types of masonry used in the exterior bearing walls of joisted masonry buildings:

  • Brick
  • Concrete—either reinforced or nonreinforced
  • Hollow concrete masonry units
  • Tile
  • Stone

Exterior bearing walls may also be any noncombustible materials with fire-resistance ratings of not less than one hour.

Glass block:




Concrete - either reinforced or nonreinforced:

Hollow concrete blocks:

Hollow tile:

Variation of joisted masonry construction

There's one variation on joisted masonry construction that doesn't change the construction class—heavy timber or mill construction.

Heavy timber construction uses wood members much larger than those found in frame (Construction Class 1) or other joisted masonry construction.

If the building uses steel columns or beams for walls, the beams must be protected so they have a fire-resistance rating of not less than one hour.

Heavy timber construction

ISO classifies the building as heavy timber construction if it meets these requirements:

  • Walls of masonry construction
  • Floors of three inch wood plank or four inch laminated plank, both surfaced with one inch flooring
  • Roof of two inch wood plank, three inch laminated plank, or 1-1/8 inch tongue-and-groove plywood deck
  • Wood column supports not less than eight inch x eight inch, wood beams or girders not less than six inch x six inch, or protected metal

Heavy timber advantages

Heavy timber construction offers these advantages:

  • Harder to ignite
  • Consumed more slowly by fire
  • More structural stability
  • Greater salvage value
  • Lack of concealed spaces

Joisted masonry disadvantages

Joisted masonry construction has these disadvantages:

  • Floors and roofs of combustible materials subject to damage by fire
  • Presence of concealed spaces

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