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Do You Know Your Earthquake Classifications for Buildings?

Robert Lacy
March 24, 2020

One day — when is anybody's guess — a major earthquake will hit Washington state. When it does, what kind of property damage can you expect?

We can't predict the severity of the quake itself, but we can help you better understand how individual properties will hold up during an earthquake. That information is captured in a building's earthquake classification, a numerical value assigned to a building based on construction features affecting its ability to withstand an earthquake.

Unlike building codes, earthquake classifications are based on potential property loss, not life safety. From a property insurance perspective, they predict how specific building materials should perform in terms of loss.

When evaluating a risk for earthquake coverage, there are many factors to consider, including distance to fault, soil type, liquefaction, and, of course, construction materials. Here’s a quick guide to the construction component, along with some common exceptions and incorrect categorizations.

Seattle buildings earthquake classificationsThese buildings in Seattle likely have different earthquake classifications.


WSRB's Essential Guide to Commercial Property Risk Assessment


Earthquake classifications for buildings quick reference guide

Wood frame

1C -

Habitational: Dwellings, 100% apartment and condominium buildings, not exceeding two stories. No area limit.

Non-habitational: Three stories or less and 3,000 square feet or less in ground floor area.

1D -

Area and height limits not qualifying for 1C. 

Metal frame

2A -

One story and 20,000 square feet or less ground floor area.

2B -

Area and height limits not qualifying for 2A. 

Steel frame

3A -

Floors and roofs: Poured-in-place reinforced concrete or concrete fill on metal deck. Open web steel joists excluded.

Exterior walls: Non-load-bearing and poured-in-place reinforced concrete or reinforced unit masonry.

*Column-free areas greater than 2,500 square feet do not qualify.

3B -

Floor and roof: Poured-in-place reinforced concrete, metal, or any combination, except buildings over three stories, may have roofs of any material.

Exterior walls: Any non-load-bearing material.

3C -

Floor and roof: Any material.

Exterior walls: Any non-load-bearing material. 

Reinforced concrete frame

4A -

Exterior walls: Poured-in-place reinforced concrete or reinforced unit masonry.

a. Poured-in-place reinforced concrete frame;

b. Poured-in-place reinforced bearing walls;

c. Partial structural steel frame with a. and/or b.

*Column-free areas greater than 2,500 square feet do not qualify.

4B -

Exterior walls: Any non-load-bearing material.

Structural system as in 4A.

4C -

d. Pre-cast load carrying system and/or;

e. Reinforced concrete lift-floor slabs and/or roof; and

f. Otherwise qualifying for 4A or 4B.

4D -

Structural system as above but with:

Exterior walls: Any non-load-bearing material.

Floors and roofs: Any material. 


5A -

This earthquake classification is not used in Washington state.

5AA -

Floors and roofs: Wood or metal.

Exterior walls: Load-bearing:

a. Poured-in-place reinforced concrete; and/or

b. Precast reinforced concrete; and/or

c. Reinforced brick masonry; and/or

d. Reinforced hollow concrete block.

5B -

Floors and roofs: Any material.

Exterior walls: Load-bearing of unreinforced brick or other unreinforced solid masonry units, excluding adobe.

5C -

Floors and roofs: Any material.

Walls: Load-bearing of hollow tile or other hollow unit masonry construction and/or adobe.

Also included are buildings not covered by any other class.


Exceptions and common mis-categorizations

  • Load-bearing tilt-up concrete construction is 5AA, not 4C.
  • All-metal buildings with combustible sheathing and/or insulation are 2A or 2B, not 1C or 1D.
  • Interior finish does not affect earthquake classification.
  • All-metal buildings should be Class 2, not Class 3, regardless of size or type of steel supports.
  • Steel frame buildings with non-load-bearing reinforced concrete panel walls are classified 3B or 3C, not 5AA.
  • Buildings with metal stud walls and wood truss roofs are classed as 5C. Structures that are classified as wood frame but have concrete-supported floors and/or some walls of unit masonry or concrete are excluded.
  • Basement walls are generally excluded unless the basement is only partially below grade so is considered a story.
  • Brick and stone veneers do not affect a building’s earthquake classification; however, there may be an additional premium if the veneer is covered under the policy.

You'll find the earthquake classification for buildings WSRB has inspected in our commercial property reports. When you look at the classifications, remember that they're just one part of the equation. Multiple factors affect a property's potential for earthquake damage. Learn more about them in our blog post, and get data on these factors in our new Earthquake Risk tool. Just login to try it. 

Stay informed with future content by signing up for notifications.

More earthquake contentRISK ASSESSMENT GUIDE

Robert Lacy, WSRB's Vice President, Inspection Services & Professional Development, oversees our team of commercial property analysts as they produce advisory loss costs, commercial property reports, and automatic fire sprinkler system evaluations. He is involved in the annual evaluation of our loss cost levels, ensuring we are current on coding and rating issues for commercial property. Additionally, Robert works to encourage professional development throughout the organization.



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