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Do You Know Your Earthquake Classifications for Buildings? | WSRB Blog

Written by Robert Lacy | April 1, 2025

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.

These buildings in Seattle likely have different earthquake classifications.

 

Related:
WSRB's Essential Guide to Commercial Property Risk Assessment

 

Earthquake classifications for buildings quick reference guide

Wood Frame

A1

Wood frame or frame stucco buildings.

Excludes structures classified as wood frame for fire but have concrete-supported floors and/or some walls of unit masonry or concrete.

Previously classified as: 1C, 1D

All-Metal
Buildings

B1 

All-metal buildings, including those with wood or
cement-asbestos roofing and/or siding.

Previously classified as: 2A, 2B

Steel Frame

C1

Buildings with a complete steel frame carrying all loads.

  • Floors and roofs: Any material.
  • Exterior walls: Any non-loadbearing materials.

Previously classified as: 3A, 3B, 3C

Reinforced Concrete and Structural Steel

D1

Structural system consists of:

  • Poured-in-place reinforced concrete frame.
  • Poured-in-place reinforced concrete interior and/or
    exterior walls.
  • Partial structural steel frame with reinforced concrete.

Floors and roofs: Poured-in-place reinforced concrete
(except that roofs of buildings over three stories may use
other materials).

Previously classified as: 4A, 4B

D2

Reinforced concrete frame or combined reinforced concrete
and structural steel frame.

  • Floors and roofs: Any material.
  • Exterior and interior walls: Any non-loadbearing materials.

Previously classified as: 4D, 5AA (not load bearing tilt-up)

D3

Structural system includes:

  • Partial or complete load-carrying system of precast concrete.
  • Reinforced concrete lift-slab floors and/or roofs.
  • Otherwise qualifying for Classes D1 and D2.

Previously classified as: 4C, 5AA (load bearing tilt-up, whether
less than 6 inches or >=6 inches)

Concrete, Brick,
or Block

E2

Buildings with load-bearing walls of:

  • Unreinforced brick.
  • Other unreinforced solid unit masonry.
  • Hollow tile or other hollow unit masonry construction.

Floors and roofs: Any material.

Previously classified as: 5B, 5C

E3

Buildings having load-bearing walls of adobe.

Also includes buildings not covered by any other classification.

Previously classified as: 5C

 

Exceptions and common mis-categorizations

  • Load-bearing tilt-up concrete construction is D3, not D2.
  • All-metal buildings with combustible sheathing and/or insulation are B1, not A1.
  • Interior finish does not affect earthquake classification.
  • All-metal buildings should be Class B1, not C1, regardless of size or type of steel supports.
  • Steel frame buildings with non-load-bearing reinforced concrete panel walls are classified C1, not D2.
  • Buildings with metal stud walls and wood truss roofs are classed as E2.
  • Basement walls are excluded unless the basement is only partially below grade and 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 earthquake risk tool: QuakeScout. Just login to try it. 

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Previous to current classifications

Previous Classification Current Classification
1C A1
1D A1
2A B1
2B B1
3A C1
3B C1
3C C1
4A D1
4B D1
4C D3
4D D2
5AA (not load bearing tilt-up) D2
5AA (load bearing tilt-up) D3
5B E2
5C E2
5C E3
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