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Do You Know Your Construction Classes? Mixed Construction

September 3, 2013

The end of the construction class road is learning how to handle mixed construction. It’s common to see buildings of varied construction types all over Washington. Some buildings are built using different construction methods, while others are added on to over time. As someone trying to learn construction classes it’s important to understand the ground rules before diving into mixed construction classes. And if you need a reminder, you can find our articles on Construction Classes 1, 2, 3, 4, 5, and 6 by following those links.

As always, disregard basements. Basement walls and floor areas, or the area on grade with a basement, are disregarded in determining construction class. Also, 33 1/3% is the key number to remember. If a building is constructed of two or more construction classes use the lowest construction class (1 being lowest, 6 being highest) over 33 1/3%.

To begin, determine the construction classes used in your building.


In the example above, two construction classes are used: the first floor is Construction Class 6, and the second, third, and fourth floors are Construction Class 1. Because 75% of the building is frame (CC 1) and only 25% is masonry (CC 6), this building will be classified as Construction Class 1, Frame.


In this example, 30% of the walls are Frame (CC 1) and 70% are Noncombustible (CC 3), so this building would be classified as Construction Class 3, Noncombustible. Remember: 33 1/3% of the walls would have to be frame before this building would be considered Construction Class 1.

A few notes:

–          Many retail and habitational buildings are multiple construction classes (like the first example); the ground floor, where the parking garage or first floor retail tenants are located, are generally of a concrete-type construction, with the upper floors constructed of wood.

–          In multiple-story buildings and occasionally in high-rises, the roof tends to be built using wood decking, despite the rest of the building being constructed of mostly noncombustible materials. This generally won’t affect your construction class, as it’s less than 33 1/3% of the area.

–          Don’t confuse the 33 1/3% for construction class with the 10% requirement on Earthquake classes! It’s common to have a Construction Class 1, Frame, building that’s an Earthquake Class 5C (hollow masonry), because more than 10%—but less than 33 1/3%—is hollow masonry.

–          Keep in mind that each rating bureau across the country determines wall portions slightly different from one another. If you’re ever in doubt or have a question, feel free to give us a call and we’ll be happy to help you out!

Learn about other Construction Classes here!

Article by: Kristen Skinner

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