Earthquake Proof Your Home: Strengthening Your Foundation

Homes constructed more than two decades ago may need earthquake strengthening upgrades to their foundations. Here’s how to earthquake proof your home and resist the seismic forces.

If you have a poured perimeter foundation, look for steel anchor plates in your basement or crawlspace. These tie the sill plate to the side of the foundation, and they are easy to install. Image: SoundSeismic.com

If you live in earthquake country, which includes at least part of most states, a few relatively simple earthquake strengthening upgrades may be all you need to keep your house from sliding off its foundation in a quake. You can tackle the work yourself for as little as $500, or hire a contractor for about $2,000. Either way, you stand to avoid repairs that might cost tens of thousands of dollars — if repairs are even possible.

That’s because damage from an earthquake can be so extensive—in some cases, the only remedy is to completely rebuild. Earthquakes can exascerbate existing home foundation problems. If you don’t have earthquake insurance, the financial consequences can be dire. Even if you do carry earthquake insurance, simple eathquake strengthening measures could still save you tens of thousands of dollars, since earthquake insurance usually carries a hefty deductible.
 
You also gain peace of mind, and it may make your house easier to sell. At least one state (California) requires sellers to fill out a checklist specifying whether earthquake strengthening measures have been installed. If you buy earthquake insurance, which can range from a couple hundred dollars to several thousand dollars a year, retrofitting may save you 5% each year on premiums.

Check with your local building department

Houses built to today’s building codes should be strong enough to keep people safe during an earthquake. But older houses, even those built a few decades ago, might need strengthening. Your local building department can tell you what’s required in your area, and whether you need design help from a structural engineer.

If you need just simple upgrades, the department might have free plans you can use. Whether you do the work yourself or hire a contractor, get a permit. If you’re planning to insulate or finish the basement, tackle earthquake strengthening first before you close off access to walls.

Poured perimeter foundations

If your house sits on a wall of poured concrete, with perhaps a few posts in the center under beams, there may be nothing but a few nails and gravity to hold the house in place.

To check whether your house needs earthquake strengthening, go into the crawl space or basement and look for thick bolts along the top of the sill plate, and for steel anchor plates that tie an edge of the sill plate to the side of the foundation. If you find neither, they are easy to install.

If you have enough room to use an electric hammer drill, drill straight down through the sill plate and 4 inches into the concrete—you can rent a hammer drill for about $20 per day from a tool rental center. You may need one ½-inch-diameter wedge anchor or bolt with epoxy each 32 inches to 6 feet, depending on the recommendations of your local building department.

On top of the sill plate, add hefty square washers (often called bearing plates) and nuts. Avoid standard round washers because they may fold and split the sill plate during a quake. If there isn’t enough room to drill straight down, use ties that fasten into the foundation from the side. You’ll find them at a local building-supply company.
 
If the first floor sits several feet higher than the perimeter foundation, the short “cripple” walls on top of the sill plate may need strengthening. To check, look between the studs—if you see diagonal boards or plywood on the outside of the studs, the cripple wall will be braced properly.

Add bracing by nailing plywood to the interior side of each wall. The bracing blocks access to the sill plate, so be sure your house is bolted down first. Your local building department can tell you the specifics about the type of plywood and the nailing pattern.

Unreinforced masonry foundations

If your house sits on a perimeter foundation made with concrete blocks that are completely filled with rebar and concrete, retrofit it as if the foundation was made of solid poured concrete (above). But if the blocks are hollow or if the foundation is unreinforced brick or stone, you’ll need a structural engineer’s advice.
 
You might learn that your foundation is sturdy enough and you just need a creative way to fasten down your house. Or, you might learn that the foundation is at risk of collapsing in a quake. Expect to pay $500-$700 for an evaluation and recommendation from a structural engineer.

If your foundation consists of more than three rows of concrete blocks and is in good shape, you might be able to fasten the sill plate to the foundation by drilling slightly oversize holes into hollow parts of the blocks and then inserting mesh sleeves, epoxy, and threaded bolts. Tightening the bolts causes the epoxy to squeeze through the mesh and mushroom out inside the hollow cavity, holding the bolts much like drywall anchors work to hold screws in walls.

This relatively new option costs as little as $5,000 for a one- or two-story house with a footprint of 1,000 square feet. That’s about half of what it would cost for the more traditional method of cutting into the blocks and installing rods that tie the sill plate to the foundation footing.

If the foundation consists of only a couple of courses of blocks, or if the walls aren’t in good shape or are made of brick or stone, you might need a new foundation. A new foundation costs about $40,000—more for a house with a basement.

Short block walls are more vulnerable to collapsing in an earthquake than walls of four or more courses because short walls have fewer mortar joints. Fewer joints means more stress concentrated in each joint. In a taller wall, there are more joints to share the seismic forces. “I know it’s counterintuitive,” says Leif Jackson, owner of Sound Seismic, a retrofit contractor in Seattle, “but that’s what the engineers say.”

Slab foundations

For a house that rests directly on a concrete slab, metal straps or bolts should tie the sill plate to the concrete. If you have an unfinished garage, you can check sill plates there and assume the house is built the same way. If the garage walls are closed in, check by removing a section of drywall or siding in an unobtrusive spot.

Those who are not inclined to open up a wall can take comfort in the fact that a slab house probably won’t collapse since it doesn’t have far to fall. Of course, if you are removing siding or drywall for another purpose, that’s the perfect time to check for straps or bolts and add them if they’re missing.

Post-and-pier foundations

Some houses are supported by upright posts that rest on concrete blocks or piers. During an earthquake, these support posts are especially vulnerable to back-and-forth seismic movement and may collapse. Repairing collapsed posts starts around $20,000, if the house can be salvaged.
 
The preventative solution may be as easy as bracing the posts at a cost of about $1,000, or as expensive as adding a new foundation for approximately $25,000. Between these extremes, you may be able to pour short L-shape concrete foundations around each corner and securely attach them to the floor framing. Seek the advice of a structural engineer.