Foundation bolts

Was your home built before 1980?

Homes built before 1980 used un-plated foundation bolts which have may have rusted, resulting in a weakened connection to the foundation. The diagram at right shows how the wood mudsill is connected to the concrete foundation with an anchor bolt. Moisture can accumulate here hidden from view and over time cause the bolt to rust, reducing its strength and weakening the house’s connection to the foundation.

This foundation bolt was found in a 1940s El Cerrito house. The bolt shows the danger of hidden bolt deterioration that can occur over time, especially in houses having a history of wet or damp crawl spaces. The observed bolt was rusted almost completely through in the space between the mud sill and the concrete foundation.

The bolt was discovered when the homeowner asked us to tighten some loose mud sill bolts. When the tightening attempt was made, this bolt merely twisted off in the rusted area. Less than one-quarter of the original half inch diameter remained in the mid-section of the bolt. Needless to say, the bolt tightening procedure was discontinued and new foundation bolts were installed. Depending on how many of the original foundation bolts were rusted in this manner, a hidden condition, the original connection strength of this house to its foundation was severely reduced.

The term “rusty bolt” is used as shorthand reference to this condition of deteriorated foundation bolt strength. It is not meant to imply that bolts showing surface rust are necessarily reduced in strength. The observation of surface rust, at the top of the bolts, however, can be an indication of hidden problems below. There is no way to visually determine if there is damage, or the extent of the damage. Since 1980, however, mud sill bolts have been zinc-plated as a rust retardant and to reduce the loss in strength.

It is also important to recognize that retrofitting of foundation bolts addresses considerably more than just this possible loss of original bolt strength. When bolt failure during seismic loading is of concern, equally important is spacing of the bolts, bolt size, avoidance of over-sizing in drilling for the bolts, and use of square washers large enough to resist being pulled through the wooden framing members. The direct observation of rusty bolt conditions under a house, thus, may be indicative of many reasons to consider bolt upgrading.