How to Fortify a Wood Frame House for Earthquakes

Enter your crawlspace or basement and inspect the masonry foundation and wooden sill plates on which the house rests. Look for crumbling masonry and rotten wood. These must be repaired or replaced before doing anything else. If the masonry and wood are sound, look for anchor bolts that fasten the wooden sill plate to the masonry foundation. These bolts must be at least 1/2-inch in diameter, spaced less than 6 feet apart and should not be severely rusted. Otherwise, they need to be replaced.

Install new anchor bolts, if necessary, to secure the sill plate to the foundation. Use a handheld drill to bore a 5/8-inch hole through the sill plate and about 5 inches into the masonry foundation. Install a 2-by-2 inch square steel plate washer onto the sill plate, insert a 5/8-inch expansion anchor bolt through the washer into the hole and tighten the bolt with a wrench. Secure perimeter boards and joists to the sill plate with angle brackets nailed or screwed in place on every third joist.

Anchor the house from the side if you do not have sufficient headroom to drill through the sill plate. Use steel right-angle strap anchors or steel angle irons that are about a foot in length. Drill 1/2-inch holes through the iron, two at each end. Drill 1/2-inch holes into the side of a supporting joist then into the side of the masonry foundation. Fasten the strap anchor or angle iron to the foundation wall with masonry bolts and use regular bolts to secure the other end to the joist. Install an anchor strap on every other joist. Several companies make steel plates for anchoring sill plates to foundations if you lack clearance to install anchor bolts. Install these steel plates, according to the manufacturer’s instructions.

Reinforce “cripple walls,” 3-foot-high, wood-framed walls that create a crawlspace beneath the house. These are a weak structural element in earthquakes and must be reinforced to prevent collapse. If the cripple walls are not secured to the foundation footings, install anchor bolts as described in Step 2. Otherwise, cut 1/2-inch exterior plywood sheets to fit the height of the cripple wall and nail the plywood to the studs, header and footer of the cripple wall. You must cover at least 50 percent of the cripple wall area with plywood. Cut 2-inch holes into the plywood at each stud span, one near the top and the other near the bottom, to allow moisture to escape.