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Do You Know Your Construction Classes? Construction Class 3: Non-Combustible

July 29, 2013

Construction Class 3 (CC 3) begins to enter the world of fire-resistive construction. CC 3 encompasses metal buildings where although the structure doesn’t add fuel to the fire, it doesn’t resist the fire well. The structural steel used is unprotected, thus causing it to become structurally unsound or even collapse at high temperatures.  These buildings are, however, inexpensive to design and fairly quick to construct.

 “Buildings where the exterior walls, floors, and roof are constructed of, and supported by metal, asbestos, gypsum, or other non-combustible materials.”  Commercial Lines Manual, Rule 15.B.3

Exterior Walls:

–          Structural, horizontal, and vertical load-bearing unprotected metal supports (rated as noncombustible)

–          Metal or masonry, panels or curtain walls supported by metal frame (rated as noncombustible)

Unprotected metal buildings are fairly easy to recognize, so long as the horizontal and vertical load-bearing supports are visible.

Construction Class 3 building

From the outside, a CC 3 building generally appears to be metal siding with a metal roof.

noncombustible building construction

The picture above is an excellent example of the inside.  The roof and roof supports are unprotected metal and the roofing insulation is a fiberglass batt.  WSRB used to rate metal buildings with fiberglass batt insulation As Combustible, but in recent years, building codes have changed to require exposed materials to be of a limited flame spread.

Floors and Roof:

–          Metal or masonry, floors or roof decks supported by metal frame (rated as noncombustible)

–          Floor and roof assemblies with a fire-resistive rating less than R.1 hour

non-combustible building materials

This building also shows the roof and wall supports of an unprotected metal building.  In later articles, we will show examples of types of protected metals, but for now, unprotected metal typically looks like bare steel that may or may not be painted.

A few final notes:

–          Construction Class 3 buildings are generally 1 or 1=2-story light industrial metal buildings, although occasionally they may be multiple-story retail or office buildings.

–          If any exposed insulation on the walls states that it’s to be covered, the building may be rated As Combustible.

–          If the roofing contains a rigid type of insulation, below the roof but above the decking, WSRB will consider the building As Combustible.

Next week we’ll discuss Construction Class 4, Masonry Non-Combustible.

Article by: Kristen Skinner

2 Comments leave one →
  1. November 4, 2013 9:02 pm

    This roof is showing unprotected metal and roofing insulation is a fiberglass batt.

    • Kristen Skinner permalink
      November 5, 2013 1:34 pm

      Thanks for your comment, Alina. That is fiberglass batt insulation on the roof. WSRB used to consider buildings with this insulation As Combustible unless we had documentation showing that the flamespread was <25. We now consider it incombustible unless the insulation indicates specifically that it needs to be covered.

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