Shear Pleasure

Depending on the house’s construction, there may or may not be a cripple wall. This feature is a short wall about 36″ high between the foundation and the first-floor framing that is commonly found on late-19th-century houses on the West Coast, especially row houses. The cripple wall is the weakest part of the house, usually relying on nothing more than diagonal bracing to keep it from collapsing into a parallelogram. The way to greatly improve a cripple wall’s resistance to lateral forces is to reinforce it with shear panels of plywood. Because the mud sill is typically wider than the 2×4 uprights, you must add blocking between each stud, as well as at the top and bottom, to which the plywood can be nailed. The blocking is typically pieces of 2×4 and needs at least four 10d nails per piece. Pre-drill the holes to keep the nails from splitting the short blocks, and don’t use pressure-treated green wood for blocking. For shear panels, plywood is preferred over OSB (oriented-strand board), and the sheets should be five-ply material at a minimum of a 1/2″ thick (or 15/32″, which is what passes for a 1/2″ thickness these days). For optimum strength, the panels should be installed horizontally rather than vertically using the longest piece of plywood possible.

Old houses come off their foundations in earthquakes and floods because they are unanchored. Adding bolts through the mud sill keeps the building in place.