Seismic Retrofitting

Thinking of getting earthquake insurance? Give some serious thought to seismic retrofitting. Don't just insure your home against earthquakes. Retrofit so you can prevent major earthquake damage.

Seismic retrofitting strengthens your foundation so your home is better equipped to withstand the violent shaking of an earthquake. This can mean significant savings on earthquake damage repair costs. More importantly, seismic retrofitting will give you peace of mind because you'll know your home can keep your family safe and secure against the next earthquake.

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The Importance of Seismic Retrofitting: An Illustration

Play this flash presentation to see exactly how a seismic retrofit significantly strengthens your home and improves its earthquake resistance. Use the underlined navigation prompts to play the elements on a page. Use the Back and Next buttons to navigate from one page to the next.

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Why Retrofit Your Home?

Nothing should be more important to you than the safety and well-being of your family — and seismic retrofitting can give you this assurance. This alone is sufficient reason for a seismic retrofit. But there are other equally compelling reasons to retrofit your home:

Seismic retrofitting drastically improves your home's chances of surviving an earthquake.

Why risk major earthquake damage when you can avoid it? A study on earthquake retrofitting by the Association of Bay Area Governments shows that an optimally retrofitted home can be exponentially more resistant to earthquake damage than a house that has no such modifications.

It is cheaper to have seismic retrofitting done than to have your home repaired after it has been ravaged by an earthquake.

Seismic retrofitting can spell the difference between a $5,000 and a $260,000 post-earthquake repair bill.

Seismic retrofitting may actually lead to reduced insurance premiums.

Insurance underwriters assess various risk factors to calculate insurance premiums. Since the risk of earthquake damage is correlated to reduced earthquake damage risks, seismic retrofitting can actually result in reduced insurance premiums.


Architect Michael O'Hearn owned two houses on Elm Street, House #210 and House #214. The two houses were practically identical. They:

  • were around 100 years old
  • had the same design
  • were made of the same materials, and
  • were built using the same construction techniques by the same builder.

When the Loma Prieta Earthquake struck in 1989, however, House #214 came apart in 4 sections while House #210 suffered only minor damage.

What made the difference?

Seismic retrofitting made all the difference.

Prior to the earthquake, O'Hearn had had House #210's sill plate bolted to its foundation and its cripple wall framing braced with shear walls. Unfortunately, the earthquake struck before he could have the same improvements done on House #214.

The result?

After the quake, the retrofitted house, House #210, cost only about $5,000 to repair. On the other hand, the repair costs for the non-retrofitted house, House #214, came up to a whopping $260,000.

Seismic Retrofit: Usual Procedure

Seismic retrofitting usually involves foundation bolting, cripple wall bracing and attaching the floor framing to the cripple wall.

Foundation Bolting

Foundation bolting ties the wood framing members of a building to its concrete foundation. If the mudsill is not connected to the foundation, the entire house could slide off its foundation during an earthquake.

This procedure usually involves bolting the mudsill to the stem wall, usually by using 5/8" diameter anchor bolts embedded at a minimum of 7". If there is no access to bolt the mudsill vertically, alternative methods can be used such as the placement of Universal Foundation Plates.

Foundation bolting also reinforces the strength of existing connections. From the countless foundation inspections we have conducted, we know that insufficient connections between the home's wood framing and its concrete foundation are a common problem. Sometimes, existing bolts are few and far between. Other times, the bolts are so severely rusted they are practically useless as connectors.

foundation bolt
Foundation bolt drilled vertically for earthquake proofing

universal plate
Universal plates used in lieu of vertical foundation bolts

Cripple Wall Bracing

Most older houses have a short wood-framed wall in the crawl space. This is the cripple wall. It may be anywhere from a few inches to several feet in height, extending from the top of the concrete foundation stem wall to the bottom of the floor joists.

The cripple wall carries an enormous amount of load and, in an earthquake, it bears the brunt of the earth's tremors. If you live in an old house, neglecting to reinforce the cripple wall puts yourself and your family at risk. In older houses, the cripple wall is usually the weakest part of the structure, and cripple wall collapse was actually one of the main causes of building failure in the 1989 Loma Prieta earthquake.

Cripple wall bracing is the application of structural-grade plywood to the cripple wall. This strengthens and reinforces the cripple wall so that it will not collapse easily during an earthquake.

In cripple wall bracing, city-engineered 1/2" structural-grade 5-ply APA rated plywood sheathing is custom cut with ventilation holes, covered with galvanized wire mesh screens and securely nailed to the cripple wall framing according to structural nailing schedules. For the bracing of cripple walls which are greater than 4 feet in height, the services of a structural engineer are required by the City of Los Angeles Department of Building and Safety.

cripple wall plywood
Structural-grade plywood for cripple wall bracing

cripple wall wire mesh screen
Ventilation hole covered with wire mesh screen

cripple wall installation
Installation of cripple wall reinforcement

Securing the Floor Framing to the Cripple Wall

framing anchor
Floor attached to cripple wall through framing anchors

Properly connecting the floor framing to the cripple wall further strengthens the house's capacity to withstand quakes. The failure of the connection between the floor framing and the cripple wall is another usual cause of earthquake damage.

Simpson A-35 Framing Anchors are usually used to connect the floor framing to the cripple wall.

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"Sinai recently performed major earthquake retrofit and foundation reconstruction on my older home. I choose Sinai because they impressed me as a well-staffed and professional company and because of their technical expertise and explanation of their solution. They finished the job on time, with no major problems. They were extremely responsive to my inquiries and met all of my expectations and requests. They even performed some tasks that were beyond the contract specs at no cost to me. I would use Sinai again and recommend them to anyone. Their technical solution was far superior to two other companies that provided bids and I am confident I got the best job."

Bill Wong, Los Angeles

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