Three Critical Steps to Seismic Retrofit a Home

In our first blogpost inthis series,weidentified why older homesaremore vulnerablethan newer homesto earthquake damage. Wedescribed what retrofitting is and how it strengthens your home’s structure. And lastly,we posed five questionsyou should ask yourself when deciding if your homeneedsretrofitting.

One of the questionswe askedwas, “Is my homebuilt on a raised foundation?”History,abetted byresearch,has shownthatwoodframedhomes built on raised foundations (withcrawlspaces and basements)prior to1985arethemost vulnerable todamage fromearthquakes. That’s because the structure under the house wasn’t built to resist the resulting sliding and racking forces.

This second blog postexamineshow you can protect your home from these damaging forces by retrofitting or strengtheningthestructure anditscritical connections in three key locations.

What do I do,you ask?A good first stop is your local building department,which mayprovideprograms and information about how toperform a home retrofit and secure a building permit. Many building departmentsprovide prescriptive plan sets (consider a plan set likea cookbook for contractors and DIYers) thatcan help guide youfromstarttofinish on whatquestionsto ask, who can help, what to know when working withthe building department, and whetheryour home needs further evaluation from a structural engineer.

Thereare three areasthatare critical in retrofitting a home:

  1. Is your house properly bolted to its foundation?Check to see if your perimeter walls are attached to the foundation. The wood that rests on the concrete is called a mudsill or sill plate.Building codes require that anchor bolts be placed at certain locations to prevent the home from sliding off the foundation.The anchorbolts may require a bearing plate between the top of the sill plate and the nut. Check with your local building department forspecificrequirements in your area. Finally, make sure your bolts areundamaged and rust-free.

  2. Does your house have cripple walls?Ifyes, thentheseperimeter wallswhich arelocatedbetween thesillplate and thefirst-floorframing— mustbe reinforced to prevent the house from racking to one sideduring an earthquake(think Leaning Tower of Pisa). Older homes didn’tuse modern woodsheathing. Instead, sidingor shiplapboards were used, and thesearent adequateto prevent failures. The building codes recommend the installation of wood structural panels(plywood or OSB)to the inside face of the walls in strategic locations to strengthen the perimeter walls.
  3. Is thefirst-floorframing connected to the top of the cripple wallor sill plate (if no cripple walls are present)?On top of thecripplewall rests the floor system, which must be attached to the rim joist. Around the outside of the floor system and the perimeter wall system is what is known as the rim joist. The rim joist should be attached to the top of thecripplewall with metal connectors.If no cripple walls are present, then the rim joist must be secured in a similar manner directly to the sill plate.The connectors should be located at 16″o.c.for three-story, 32″o.c.for two-story and 48″o.c.for one-story homes.

In the next blog wellfocus on what products are needed, what you need in order to work with your local building department,andwhetheryoushouldchoose to use a contractor or do it yourself.

If you want more information, please visit the linkbelowfor thecity you live nearest to.These links will provide you with resources to help you plan, prepare,and perform a retrofit on your home,whether youchoose todo it yourself or have a professional contractor perform the work.

Seismic Retrofit Resources: