By Fionna M. D. Samuels, Ph.D., Optimum Seismic Fellow (@Fairy__Hedgehog)
Citation: Samuels, Fionna M. D., 2023, Thinking of retrofitting your home? Here’s what you should know, Temblor, http://doi.org/10.32858/temblor.324
Chris Gordon, The Geodisclosure Team
In areas with high seismic risk — like many parts of California — earthquake preparedness is a necessity. There are many ways to prepare for the Big One (and even moderate ones!). “With an earthquake, you want to prepare beforehand in a way where you feel empowered rather than frightened,” says Janiele Maffei, the executive director of the California Residential Mitigation Program, or CRMP. The key, she explains, is taking baby steps.
Start small. People living in single-family, wood-framed homes in earthquake country should secure the items susceptible to falling throughout their dwellings. They should make a family plan for what to do during — and after — a quake and have a disaster kit made in advance.
Then, folks might consider taking the leap of retrofitting their home to better withstand earthquakes. The process of retrofitting starts easy, Maffei says: look for clues that your home might be vulnerable in a quake. And then, get cracking.
Two homes after the magnitude-6.0 earthquake in Napa, Calif., in 2014. The one that is not retrofitted was too structurally damaged to be immediately habitable (left) while the retrofitted home next door was structurally sound (right). Credit: California Earthquake Authority
What homes are most at risk? Age matters
“First and foremost, we start with age, because age correlates to building code,” says Maffei. Age is a key factor in deciding to retrofit because California building codes are tweaked every few years. “The most critical year for single family dwellings was 1979,” she says. “That was because of the 1971 San Fernando Valley earthquake.”
The epicenter of the destructive magnitude-6.6 earthquake was just north of Los Angeles, near Magic Mountain. Sixty-five people were killed, thousands were injured and the quake resulted in more than half a billion dollars of property damages. In addition to severely damaging hospitals and other infrastructure, many single-family homes were completely destroyed or significantly damaged, which led to significant changes to the building code by 1979. The age of a home can be found on myriad legal documents, including tax forms.
Is my older home at risk? Visual cues can help
“Once you know the [home’s] age, there are a couple of vulnerabilities that will stand out,” Maffei says. The first she calls the “crawlspace vulnerability,” which arises from how crawlspaces are reinforced. A wood-framed house can sit either directly on its foundation or on short crawlspace walls (often called “cripple walls”). To make these homes safe, the first floor of the house needs to be securely, directly bolted to either the foundation or to the crawlspace walls. Builders also sheath the short crawlspace walls in plywood or something similar for extra support.
Before (left) and after (right) images of a crawlspace in the process of being retrofitted. A foundation plate is used to securely attach the first floor to the foundation rather than directly screwing them together. Credit: California Earthquake Authority
Without these added supports, “these houses don’t have proper anchorage to the foundation or proper bracing of the crawlspace walls,” says Maffei. In an earthquake, such homes can slide and potentially topple off their foundations.
“What I tell people is if you have a pre-1980 house, you may have this vulnerability,” says Maffei, “and if you have a pre-1940 house, you do have this vulnerability — unless it’s been mitigated — just because of the age of construction.”
Luckily, the bolts and braced crawlspace walls are fairly easy to see. Maffei suggests peeking under your house, into the crawlspace, to look for bolts and plywood. If there are obvious signs of a retrofit, like shiny new bolts, you might be alright, Maffei says. Otherwise it may be time to consider a retrofit.
Before (left) and after (right) images of a crawlspace in the process of being retrofitted. Plywood is bolted in place to brace the short walls in the crawlspace. Credit: California Earthquake Authority
For wood-framed houses with a so-called soft-story — a large empty space like the garage beneath part of the living quarters — look to the garage door for an easy-to-see vulnerability. Although soft-story construction is usually seen in multifamily homes or apartments, many single-family homes also have this vulnerability.
Like crawlspace walls in pre-1980 homes, these kinds of garages in older homes are often not reinforced with plywood. The garage door also presents a particularly weak spot in the structure during an earthquake. “Now you’re talking about a collapse of a full story, and that can be extremely dangerous,” Maffei says.
There are also homes with less common but easily observable vulnerabilities, like brick-and-mortar chimneys. Chimneys can be so large and heavy that they damage the house. Worse, they can topple onto people and cars around the structure. The bricks can even become projectiles during a quake. These days, Maffei says that it’s often recommended to simply take a chimney down if it’s ever damaged, and replace it with a safer, equally aesthetic alternative.
Houses on hillsides, especially down-slope homes where a significant portion of the foundation is elevated on stilts, essentially making them soft-story buildings, are also vulnerable to shaking or collapse during earthquakes. After the 1994 Northridge earthquake damaged 374 hillside homes in Los Angeles, the city passed ordinances requiring new hillside homes to be built to withstand earthquakes. However, the city did not pass a mandatory retrofitting ordinance for existing structures and left it up to individual homeowners to make the choice to strengthen their homes.
What can I expect when I retrofit my wood-framed home? Minimal disruption
Retrofitting these vulnerabilities is surprisingly straightforward. For crawlspaces, a contractor can simply bolt the home to its foundation and add plywood to any crawlspace walls that need to be braced.
“It’s not disruptive, which is fantastic,” says Temblor’s cofounder and CEO Ross Stein, who’s had two homes retrofitted through bolting. He explains that because all the work was done in the crawlspace, he and his family were able to go about their daily lives without interruption.
The retrofit for a soft-story building, specifically its garage, is similarly nondisruptive: The walls and frame of the garage door are reinforced with plywood and steel bars, respectively.
How much does it cost? It depends
The cost of a retrofit will depend on how much work needs to be done on the home. Stein says the retrofit he contracted for his home in Marin County last year cost him just over $8,000. It required anchoring the first floor to the mudsill (the wooden frame of the house below the floor), and then anchoring the mudsill to the foundation. Finally, structural plywood was installed around the crawlspace walls.
This cost lines up with what Maffei has seen through the CRMP grant programs she helped develop and now manages. These programs help offset costs associated with retrofitting wood-framed houses. “We have now retrofitted more than 21,000 houses with grants throughout California,” Maffei says. The majority of retrofits are coming in under $10,000, with the median closer to $5,400 and a few costing significantly more, she says. Ultimately, the cost of a retrofit will depend on local construction costs and the specific retrofitting elements a home needs. For example, it is less expensive to simply bolt a wooden framed house to its foundation, more to add plywood to crawlspace walls, and significantly more if the foundation must also replaced.
The CRMP runs two different grant programs for California residents. Earthquake Brace + Bolt provides qualified applicants with $3,000 to offset the cost of bolting their homes to the foundations and installing plywood to support crawlspace walls. Earthquake Soft Story is a recently introduced program that offers up to $13,000 to help homeowners retrofit their garages for more stability during an earthquake.
However, registration for these programs is not always open because the CRMP depends on federal funding for its grants. “We don’t have unlimited funding,” says Maffei. “We also don’t know what amount we’re going be able to get year after year.” But for those interested in the grants, anyone can sign up for email updates, she says.
Two homes after the magnitude-6.4 earthquake in Ferndale, Calif., in 2022. One home is not retrofitted (left) the other home is retrofitted (right). Credit: California Earthquake Authority
Is a seismic retrofit worth it? Yup
For Stein, retrofitting his home was a completely sound decision. “We [as a society] all have an awful lot of our assets tied up with homes,” says Stein. “Not only do we want to protect that asset, we want a place to live after that earthquake has occurred.”
However, a retrofit does not guarantee that there will be no property damage after an earthquake. “We’re going after one particular kind of vulnerability,” says Maffei — keeping the house on its foundation. There are examples of retrofitted homes remaining livable after recent magnitude-6.0 or 6.4 earthquakes while neighboring homes are red-tagged, meaning that they’re not safe to inhabit, she says. That’s the goal — go beyond life safety standards and ensure that homes are at least repairable after a quake.
“We’re in this beautiful place and everything that makes it beautiful has been brought to us by the faults,” Stein says. Of course, that means that earthquakes are an inevitability. “Learning to live with that is being prepared,” he says. “It’s an earthquake emergency kit, it’s a communication plan, it’s having a whistle on your keychain, and it’s retrofitting your home. That should just be part of the deal.”
Fionna M. D. Samuels is Temblor’s Optimum Seismic Fellow. She is a science writer hailing from the Front Range of Colorado where she got her Ph.D. in Chemistry from Colorado State University. Her work has appeared in Eos, Scientific American and Symmetry. Optimum Seismic is sponsoring their first Temblor science writing fellow to cover important news about seismic resilience of the built environment.
By Fionna M. D. Samuels, Ph.D., Optimum Seismic Fellow (@Fairy__Hedgehog)
Citation: Samuels, Fionna M. D., 2023, Thinking of retrofitting your home? Here’s what you should know, Temblor, http://doi.org/10.32858/temblor.324
Chris Gordon, The Geodisclosure Team
