III. Structural Problems and Strengthening Methods
E. Weak Cripple Walls

Houses often have short wood frame walls between the floor joists and the concrete foundation. Many of these cripple walls are not adequately braced, and this weakness allows buildings to shift sideways and collapse downward in earthquakes.

The cripple wall can be strengthened by adding plywood panels. This reconstruction is usually done in the crawl space, so a low crawl space means greater difficulty (see Figure below for specific requirements). Strengthening can also be done from the outside, but this requires the removal of the exterior finish material.

Because the existing foundation sill is often wider than the stud, installation of plywood generally involves adding a new piece of blocking on top of and parallel to the sill, to provide backing for the plywood sheet. When the crawl space is two feet high or taller, the work can proceed easily.

Typical Cost: $15/lf for crawl space between 2' and 4' tall

When the crawl space is less than two feet high and the sill is wider than the studs, the difficulty is greatest.

Typical Cost: $20/lf

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