Project 2: Bolting the sill plate to the foundation

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detailed drawing

Consult your building department to determine whether a permit is required to perform seismic retrofit work.

Before beginning work, survey the entire foundation. A foundation constructed of unreinforced brick or stone masonry is usually too weak to resist earthquake forces. Consult with a structural engineer to correct the weakness. Periodic vertical cracks in a concrete foundation are typically not significant unless offsets have occurred. The cracks were probably caused by shrinkage when the concrete was hardening. If the foundation appears to be weak, attempt to install two or three test bolts first to determine whether nuts can be installed without pulling the bolts out or cracking the foundation.

1. Using chalk or a crayon, lay out and mark spots on the wood sill plate for drilling holes every 6 inches. Also mark spots for drilling holes 9 inches to 12 inches from the end of any splice or step in the sill plate. Holes should be made along the centerline of the sill plate.
2. Drill holes into the sill plate and concrete using a rotary hammer drill with a carbide drill bit. This drill can be rented from a tool rental store. Drill the hole vertically and as close as possible to the center of the sill plate. It is important that the holes in both the concrete and the sill plate not be oversized. If your foundation is reinforced and you hit reinforcing steel when drilling, shorter but more numerous anchor bolts may be required. If the foundation cracks during drilling, move the drill to another location at least 12 inches away and drill a new hole. If the drill suddenly moves very easily through the wood, this may indicate a pocket of fungus infection. The portion of the sill plate containing the infection must be cut out and replaced. Temporary shoring is required. Contact a professional pest control agency if you are not sure how to proceed.
3. Clear the drilled hole of any concrete dust by blowing it out with a piece of plastic tubing or a turkey baster inserted in the hole. Be sure to blow all the dust out of the hole.
4. Insert the expansion bolt with a nut and washer in place and drive it into the hole with a sledgehammer. NOTE: using chemical (epoxy) anchors is also possible and is recommended when installation of expansion anchors causes surface cracking. Epoxies that require manual mixing should be avoided. All manufacturers' recommendations should be followed.
5. Once the bolt has bottomed out, tighten the nut down firmly.

NOTE: These instructions and specifications are for one- or two-story homes with a clearance of no less than 20 inches between the sill plate and subflooring material. With less space, drilling anchor bolt holes will be difficult. (Special right-angle drills do exist.) In such cases, it may be necessary to devise an alternate detail, which could include bolting to the side of the foundation using 1/4-inch thick metal plates or premanufactured hardware. For footings with sloped faces, wood-shim plates and beveled washers should be used. Consult the building department or a structural engineer for assistance.