A soft story building is a structure where one floor, usually the ground level, is significantly less rigid than the floors above it. This weakness typically comes from large openings like garage doors, storefront windows, or open parking areas that leave the ground floor without enough solid wall space to resist sideways forces during an earthquake. The term matters most in seismic zones, where soft story buildings are among the most likely to collapse.
What Makes a Story “Soft”
The word “soft” here refers to stiffness, not the building material. Every floor of a building resists lateral (side-to-side) movement through its walls, bracing, and structural connections. When one floor has far fewer walls or much larger openings than the floors above it, that floor can’t resist the same amount of force. It becomes the weak link.
The most common cause is a ground floor designed for a different purpose than the upper floors. Think of an apartment building with open parking underneath, a row of shops with housing above, or a residential building where the garage takes up most of the first level. All of these designs remove the solid wall space that would otherwise brace the structure. The upper stories, packed with interior partition walls and smaller windows, end up much stiffer than the open ground floor below them.
In engineering terms, retrofit guidelines from the International Code Council require that a retrofitted soft story provide at least 80% of the strength of the level above it, with stiffness reaching roughly 60% of what would be required in new construction. When a building falls well below those thresholds, it qualifies as a soft story structure.
How to Spot One
You don’t need an engineering degree to identify a likely soft story building. The visual clues are straightforward:
- Tuck-under parking: The ground floor is mostly open space for cars, with thin columns or posts supporting the building above. This is the single most common soft story configuration in residential buildings.
- Large storefront windows: A commercial ground floor with floor-to-ceiling glass across most of the facade, while the upper floors have standard-sized windows and solid walls.
- Few interior walls at ground level: Retail spaces, lobbies, or garages tend to be wide open, with minimal partition walls compared to the apartments or offices upstairs.
- Wood-frame construction: Older wood-frame buildings with these open ground floors are especially vulnerable because wood framing relies heavily on its wall sheathing for lateral strength.
San Francisco’s retrofit program specifically flags buildings with ground floor commercial uses and buildings located in mapped liquefaction zones as the highest priority tier for evaluation.
Why They Fail in Earthquakes
During an earthquake, the ground moves sideways beneath a building. The upper floors, heavy and rigid, resist that motion through inertia. All of that lateral force concentrates at the weakest point, which in a soft story building is the flexible ground floor. The ground level gets pushed sideways while the upper stories stay relatively intact as a block.
Research from the NEES-Soft project, a major U.S. shake table testing program, demonstrated exactly how this plays out. During simulated earthquakes, the upper stories of test buildings behaved rigidly while the ground floor deformed dramatically, confirming a clear soft story failure mechanism. In one collapse test, the building fell forward and twisted, showing that failure resulted from a combination of sideways displacement and rotational forces at the first story.
The physics get worse as the shaking continues. As the ground floor deforms and weakens, the building’s natural vibration period lengthens, which can amplify the shaking further. At that point, the collapse mechanism shifts from a strength problem to a geometry problem. The weight of the upper floors, now offset from the base, creates increasing downward-and-sideways force (engineers call this the P-delta effect) that the already damaged ground floor can’t counteract. The building pancakes, with the upper stories dropping onto the collapsed ground level.
The researchers found that large reversing ground deformations, not just peak acceleration, are what initiate collapse in wood-frame buildings. A quake that whips the ground back and forth over a wide distance is more dangerous to these structures than one with sharp but narrow jolts.
Real-World Consequences
Soft story failures have caused some of the most concentrated damage in modern U.S. earthquakes. The 1989 Loma Prieta and 1994 Northridge earthquakes both destroyed dozens of soft story apartment buildings in the San Francisco Bay Area and Los Angeles, displacing thousands of residents. Many of these were wood-frame buildings with tuck-under parking, a design extremely common in apartments built from the 1950s through the 1970s across California.
The cost of not retrofitting extends beyond the buildings themselves. FEMA estimates that mass care and sheltering for displaced residents runs about $222 per person per day, or roughly $666,000 for every 100 people per month. Add fire, rescue, and police response costs, and the economic argument for prevention becomes hard to ignore.
How Retrofitting Works
Retrofitting a soft story building means adding structural elements to the ground floor so it can match the stiffness and strength of the floors above. The two most common approaches work together or separately depending on the building.
The first is adding plywood or engineered wood panel shear walls to the ground floor’s perimeter and interior. These panels, properly nailed and anchored, turn previously open wall sections into solid bracing that resists lateral forces. The second is installing steel moment frames, which are rigid steel rectangles bolted into the structure at key points. Steel frames are especially useful where the building needs to keep large openings, like a garage entrance that can’t be walled over. The NEES-Soft research program tested a combined system using wood structural panel sheathing alongside steel special moment frames and found it effective at preventing the soft story collapse mechanism.
The work typically involves reinforcing the foundation as well, since the new lateral bracing needs a solid base to push against. From a resident’s perspective, the construction is mostly confined to the ground floor and exterior walls. You’ll see steel framing installed around garage openings, new plywood sheathing applied to walls (sometimes visible, sometimes hidden behind new drywall), and foundation bolting or cripple wall bracing in crawl spaces.
Mandatory Retrofit Programs
Several California cities now require owners of soft story buildings to retrofit them, not just recommend it. Los Angeles launched one of the largest programs in the country, covering thousands of buildings. Property owners receive an order to comply and then face a structured timeline: two years to submit retrofit plans, three and a half years to obtain a construction permit, and seven years to complete the work. The city prioritized larger buildings first, starting with structures of 16 or more units in 2016 and phasing in smaller buildings and condos through 2017.
San Francisco’s mandatory program similarly requires buildings with soft or weak first stories to be retrofitted. San José received a $4.6 million FEMA grant in 2018 to help property owners cover costs. Other California cities with active or developing programs include Berkeley, Oakland, Pasadena, Santa Monica, and West Hollywood.
For property owners looking for financial help, FEMA offers Hazard Mitigation Assistance grants that fund measures reducing long-term disaster risk, and the National Earthquake Hazards Reduction Program provides state-level assistance grants specifically for earthquake risk reduction. Each program has its own eligibility requirements, but both are designed to offset the cost of exactly this type of work.
What Renters Should Know
If you live in a multi-unit building in an earthquake-prone area and your ground floor is mostly parking or retail space, your building may be a soft story structure. In cities with mandatory programs, your landlord is legally required to retrofit. You can check your city’s building department website, as many publish searchable lists of affected properties and their compliance status. In Los Angeles, for example, the Department of Building and Safety maintains a public database of buildings under the soft story ordinance.
If your city doesn’t have a mandatory program, the building’s vulnerability doesn’t change, just the legal requirement to fix it. Knowing what to look for, and understanding that the ground floor of your building is the most likely point of failure, can inform decisions about renter’s insurance, emergency planning, and where you choose to live.