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Seismic Retrofitting

Standard Home Construction

The photo on the below is a diagram of how typical older, wood framed homes are constructed in our area.


On the right of the diagram, you will see a full-height concrete foundation. In these conditions the house is resting directly on top of the concrete foundation, eliminating the vulnerability of racking due to the fact that there is no existing pony walls. These homes have less weaknesses than the pony wall conditions, but are still very much capable of sliding off their foundations because they lack the proper hardware needed to resist lateral movement in the event of an earthquake.
The photo on the right is a diagram of how typical older, wood framed homes are constructed in our area.The left side of the diagram illustrates how most Seattle area homes are built, with short, wood-framed walls in the basements and/or crawlspaces. These walls are called “cripple” or “pony” walls. They span the distance between the top of the concrete foundation to the first-floor framing platform of the home, and can vary in height. This type of construction is the most susceptible to failure in an earthquake event, due the fact that the entire weight of the house sits directly on top of these short, unsecured walls that have little shear value. These unreinforced walls fail by racking, or folding over, due to excessive lateral forces during an earthquake; if these walls fail, the potential of a home falling off of its foundation is extremely high.

What is a seismic retrofit?

A seismic retrofit is the technical process of transferring the lateral forces generated from an earthquake in a building structure down to the foundation. This process is typically accomplished by modifying a buildings structural system to reduce or eliminate known seismic deficiencies.
We need to address the three main weaknesses in how a structure (home) could fail in the event of an earthquake: racking, sliding and overturning.
In order to maintain a continuous load path from the house to the foundation, anchor bolts, shear panels, shear transfer ties, and sometimes hold-downs, may be required to accomplish an effective retrofit.

How to Retrofit a Home

If you have a home that was constructed prior to 1976, in most cases, your home is resting on its concrete foundation by nothing but gravity. Prior to that date, there were no building codes that required newer homes to be attached to the foundation. Evergreen Seismic address those key major weaknesses by attaching the first-floor framing platform, down to the foundation by using the ABC’s of retrofitting; Anchor, Brace, and Connect. By completing this work, it significantly improves the chances of a total loss of your home being displaced from the foundation and most importantly, life safety. Below is a detailed depiction of the ABC’s.





In most situations, homes rest directly on a piece of lumber, called a mudsill, that is sandwiched between the concrete foundation and the first-floor platform (joists). If the mudsill is not attached to the foundation, the entire house could slide off of its foundation.This problem is addressed by adding anchor bolts through the mudsill, embedded several inches into the foundation. In addition, it is very important that each anchor bolt has a bearing plate to prevent the mudsill from splitting due to the lateral force. It is always guaranteed that homes that will need a seismic upgrade will require anchor bolts.
In situations where the home has a full-height foundation, and access is limited vertically, alternative methods will be needed, such as side anchor plates. These types of anchors require five lag screws that are installed into the sides of the existing mudsill and two anchor bolts that are bolted directly into the side of the foundation. The purpose of these anchors is to resist lateral side to side movement.
In rare situations, homes may have existing anchor bolts. These usually are not up to current building codes, due to their size and spacing, and may even be rusted through because they are not galvanized.Call us today for best seismic retrofitting services.

As previously mentioned in the first diagram above:
Most Seattle area homes have short wood-framed walls in the basements and/or crawlspaces. These walls are called “cripple” or “pony” walls. They span the distance between the top of the concrete foundation to the first-floor framing platform of the home, and can vary in height.
This type of construction is the most susceptible to failure in an earthquake event, due the fact that the entire weight of the house sits directly on top of these short, unsecured walls that have little shear value. These unreinforced walls fail by racking, or folding over, due to excessive lateral forces during an earthquake; if these walls fail, the potential of a home falling off of its foundation is extremely high.
We address this vulnerability by adding shear panels, which are attached securely with very tight nail spacing, to the cripple walls to add rigidity.Note that not shown in the diagram, in conditions where work may occur in unheated areas such as a crawlspace and some basements, round ventilation holes will be required in the shear panels. This allows air circulation throughout each stud bay to prevent future rotting. Without ventilation holes, moisture buildup can occur inside the pony walls and create rot.

The third area in which a home could fail during an earthquake is the point of contact between the first-floor platform (floor joists) and the top of the cripple wall. The only positive connection in these areas are diagonal nails, called “toe-nails), that span every few feet. As is the case when a home doesn’t have anchor bolts, this type of connection can cause a house to slide off of its foundation.
We address this vulnerability by installing shear transfer ties, or framing clips, which resists lateral forces, similar to anchor bolts. Sometimes, if there is a full-height concrete foundation, this area will be from the rim joist to the mudsill.

Additional hardware, called hold-downs, may be required in areas where overturning forces are likely. Areas where there is a limited length of wall to reinforce, or areas where there are very tall cripple walls to its length, the use of hold-downs will probably be required.
Hold-downs are a piece of hardware that is installed at the end of shear wall to resist uplift or overturning forces imposed on the wall do to “in-plane” lateral load applied at the top of the wall. They are drilled, cleaned and epoxied several inches into the concrete foundation with threaded rod, and bolted into the existing framing.

We Love Our Customers!

Evergreen Seismic was amazing to work with. Matt was quick to respond to our initial query, and took the time to walk us through the process and answer questions when he came out to look at the house. His bid was very clear and thorough. He's also a great communicator and made the whole process extremely smooth once we decided to go forward. We really appreciated that nothing felt rushed or unexpected. The crew here doing the work was considerate, patient with any questions we had, and kept us updated every day about what was going on. They did a really great job retrofitting our basement and crawl space and took care of some necessary repairs as well. We were lucky enough to have Evergreen recommended to us, so passing on the recommendation now! We would hire them again with zero reservations.

Amelia Apfel

Evergreen Seismic and Matt were such blessings to us during our seismic retrofit on a Bellevue home we just bought! Matt and his crew, Phil and Nick, were all equally tremendous and beyond helpful to us and my inlaws! We were hell bent on seismically retrofitting by removing drywall interiorally vs sidings exteriorally. And Matt was accommodating to our request. As a hedge though, we knew we wanted a contractor who knows how to retrofit by both methods should a need arises during the retrofit peocess! And Matt fits the bill with his extensive knowledge and years working in earthquake retrofitting. They say never open up drywall if you don't want to find out what deep dark secrets lie underneath. And boy, did we unearthed a structurally problematic framing job on most all the studs holding up an exterior wall of the house, which threw my husband and I in a loop thinking of the scope creep. But Matt stuck with us, helped to sister in all new 2x4s on the compromised studs, and best yet didnt charge us an arm and a leg to fix the framing issue because he truly cares about doing the best job at the most economical rate. The framing job along with the seismic retrofit all easily passed various round of inspections. What started as out as contractor/client relationship became friendship based on what transpired with our house framing issue. We can only hope to find contractors as knowledgeable and honest as Matt and crew!

Cindy Hedrick

I would hire Matt again in a heartbeat. Not only do he and his crew perform good work (in my case, seismic retrofit) at a fair price, he combines his core competencies with great people skills and clear, timely, winsome communication. I am especially grateful to Matt because he was ever so patient with me when I suffered long COVID a few weeks after signing the contract, prompting me to delay the project a few months while I recovered. Matt turned what I expected to be a stressful experience into a very good experience. I encourage anyone considering a seismic retrofit to invite Matt to visit you and give you an estimate. I expect that you, too, will be highly satisfied with the result.

Darrell Dorr

Matt did a fantastic job on the earthquake retrofit for our home. Our garage had never been so clean as it was after he was done with his work. Matt had the utmost professionalism through every step of the project. The contract clearly explained everything he was going to do, and he completed every step exactly as described. He thoroughly answered each of my questions during his initial visit in a way where he not only answered the question I asked, but gave background information to educate me on the topic of seismic retrofit. It's great to have the peace of mind that not only is my home now better able to withstand the "big one" that will eventually come to the PNW, but also that the work was done by a great company.

Matthew Offerman

Matt is knowledgeable and competent, and he communicates clearly before and during the job. He made sure that we knew what our options were, and that we understood them. Matt is a friendly, likable person and obviously cares about doing a professional job. He wants his customers to be happy. It was soon clear that we could count on Matt and his team to show up on time and also to clean the site each day before they left. They worked all day each day until the project was finished. I would certainly recommend Evergreen Seismic to anyone who wishes to retrofit their home. I have confidence now that my 1901 home is in much better shape to withstand a significant earthquake without coming off the foundation.

Catherine Wesley

Matt came out to see if our house needs a retrofit and I was absolutely blown away by his honesty, knowledge and professionalism. He advised against retrofitting our house due to the newer age of the home but did provide me with a tonne of useful information that I did not come across during my research and while speaking with other retrofit companies.

Ana Lenskiy

I was extremely pleased with the seismic retrofitting job Matt and his team completed for me. The entire experience was seamless. Matt was extremely informative during the bidding process and everything was completed on time and to a high standard. The work site was cleaned everyday and work plans and schedules were clearly communicated. It's obvious that Matt and his team really care about performing quality work. To echo another review, if I could hire them for all my home renovations, I would.

Sarah Weil

They did a wonderful job on the seismic retrofitting. Our 1926 house is noticeably more stable. The workers were very pleasant to have in our home, and they cleaned up shockingly well when they were done. If we could hire them for the rest of our home improvement job we would.

Allyson Lanter

It’s a matter of time, experts say, before the Northwest shudders under another massive tectonic plate earthquake or the Puget Sound region succumbs to a big quake from one of its faults. Wood frame homes generally survive the earth shaking. But there’s a risk of them sliding off the foundation. Split-level homes also may crumble along the pony wall, sometimes called a cripple wall, that supports the upper level. Quake risks were not that well known when my home was built in Bothell in 1975. In the summer of 2022, Evergreen Seismic upgraded the house with a retrofit. This included installing anchor bolts and hold-downs into the foundation and bracing the walls in places with ¾-inch CDX plywood. Owner Matt Robinson and his crew worked in a tight crawlspace. They also removed some outside siding to brace the pony wall, upgrading the insulation before replacing the siding. Matt handled all the required engineering plans, permits and inspections. Matt showed he is skilled in retrofitting. He communicated clearly about scope of the work and scheduling. The site was cleaned after each day and neatly finished at the conclusion. The retrofit improved the safety of my home. I have documentation that I believe adds to its value. Of various people we’ve had in our home from time to time for repairs or remodeling, Matt Robinson is one of the best. I recommend Evergreen Seismic for its quality of work and service. Doug in Bothell

Doug Esser

If you need to earthquake your home for seismic activity, then you can definitely trust these guys. Matt and his team at Evergreen Seismic are very knowledgeable and do a great job.

Jeff Sherman
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