How to Earthquake-Proof Your House's Foundation How to Earthquake-Proof Your House's Foundation

Knowing that your home can withstand the extreme pressures of an earthquake is important for many homeowners. If your home was built more than two decades ago and has not been upgraded, then it likely needs a few earthquake updates. Luckily, making your home’s foundation earthquake-proof is a relatively straightforward process that usually will not break your budget.

Identifying Weak Points

Having a strong and secure foundation is critical in the wake of an earthquake. The forces of the earthquake should pass through the foundation of the home and not damage the structure above. Unfortunately, many older homes—and sometimes houses that are only a decade old—were not built to withstand an earthquake. The most common problem are foundations that are not connected directly to the upper portion of the house. You will need to connect the two structures to make your house earthquake-proof.

Inspect the Foundation

A sill plate on a house foundation.

The first thing you need to do is find out if the foundation of the home is securely connected to the sill plate. Go into your crawlspace or basement and look for steel anchor plates that connect the sill plate to the foundation wall. If there are no anchor plates in place, then your house is at risk of sliding off the foundation in the face of an earthquake. Luckily, installing anchor plates is an easy process that should only take a few hours to complete.

Anchor the Foundation

An anchor plate bolted to a concrete foundation for a house.

The anchor plates should be secured to the foundation with a combination of bolts and epoxy. You also need to use an electric hammer drill, as long as there is enough room for the tool. This will make installing the plates much easier. The plates should be secured to the sill plate with strong washers. It is not recommended to use round washers as they tend to split and fold under the stress of an earthquake. The anchor plates should be spaced every six feet, though this may vary depending on your local building code.

Bracing Cripple Walls

Some houses feature cripple walls that hold the weight of the house over the foundation. If this is the case, then the walls need to be adequately braced to withstand an earthquake. If they are not properly braced then they can sway side to side and eventually collapse. To brace the cripple walls, simply nail sheets of plywood to the inside of the walls. The plywood will prevent future access to the sill plate, so ensure the house is properly anchored before installing the bracing.

Tying Slab Foundations

If your home sits on top of a concrete slab, you should connect the concrete to the sill plate via bolts or metal straps. You may need to remove a section of drywall to determine if the slab is properly connected to the sill plate. You can also determine this by looking inside the garage and assuming the home was built in a similar manner. Fortunately, houses that are built on top of slabs are less prone to suffering damage in earthquakes.

Post-and-Pier Foundations

Post-and-pier foundations refer to houses that are held up by concrete blocks and wood posts. These types of foundations are extremely likely to collapse in the event of an earthquake and need to be adequately supported to prevent substantial damage. Bracing post-and-pier foundations usually consists of pouring concrete, which can get expensive. Always consult a structural engineer before making any upgrades to a post-and-pier foundation.

Unreinforced Masonry Walls

Masonry foundations are common throughout the United States and around the world. Unfortunately, these foundations are liable to break and collapse during an earthquake. The best solution is to either replace the masonry altogether—especially if it is old and weak—or to fill it in with concrete. You can also build a steel frame and tie it to the masonry for added structural support.

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