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Two types of houses built before 1980—"stem-wall” and “cripple-wall”—may benefit from retrofitting with foundation anchoring, which reduces the possibility of a house sliding off of its foundation in an earthquake. |
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“Stem-wall” houses have a wood-framed first floor resting directly on a raised concrete perimeter foundation. The stem-wall retrofit consists of bolting the house sill plate to the foundation using foundation plates. |
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“Cripple-wall” houses have a short perimeter wall between the foundation and the wood-framed first floor. A cripple-wall retrofit consists of anchor-bolting the base of the wood frame to the concrete foundation and bracing the cripple walls.
Foundation Anchors: Bolting Raised Concrete Perimeter Foundations
“Foundation anchorage” describes both foundation bolts and foundation plates, and either can be used to secure a house frame to its foundation. For detailed information about foundation anchors, including which method to use, visit Earthquake Brace + Bolt.
New foundation anchorage can be installed by a licensed contractor according to a standard plan set for seismic retrofits, if your community has adopted such a plan set and your foundation meets plan-set requirements. Typically, Plan Set A is used in Northern California, and Standard Plan Set 1 is used in Southern California.
If the house does not have a raised continuous concrete foundation, or if it is not a wood-framed house, the homeowner should hire a licensed structural or civil engineer to advise on the retrofit efforts.
Strengthen Your House
CEA encourages owners of older (pre-1980) houses—in other words, houses that are more vulnerable to earthquakes—to retrofit their houses to strengthen them against earthquake damage. Homeowners with properly retrofitted eligible houses can receive a premium discount of up to 25%.
The Earthquake Brace + Bolt (EBB) program offers grants of up to $3,000 to homeowners in certain higher-earthquake risk ZIP Codes for houses that qualify.