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Improper Shimming

The wall shown in Figure 4 is built directly on top of a poorly formed concrete stem wall that went up and down like a wave.



Figure 4.

To correct a wavy foundation wall, the framers drove wedges between the top plates, effectively breaking the connection (photo). The best solution would have been to wedge between the bottom plate and sill, then tie the connection together with metal straps or plates, or with well-nailed, continuous sheathing (top illustration). The in-place wall can also be corrected with metal twist ties that connect the trusses directly to the studs, and with metal straps that tie both top plates to the studs (bottom illustration). When it came time to frame the roof, the framers noticed that the trusses were out of plane. To bring everything back to level, they drove wedges between the two top plates. Unfortunately, the shims lifted the upper plate so much in many places that the nails pulled nearly all the way out of the lower plate, effectively breaking the connection. This put the entire truss roof at risk from wind uplift at the overhang. What the framers should have done was shim the wall directly under each stud between the bottom plate of the wall and the sill. The shims can be steel, or wedges made out of the same material as the plates; a softer wood, like cedar, would crush under the load. The two plates would then have to be connected with metal straps or plates. Alternatively, the exterior sheathing will serve to connect the shimmed plates so long as it is installed continuously from the sill to the top plate of the stud wall. Use nails 6 inches on-center along the entire shimmed portion of both plates and at all edges. While this is the simplest and cheapest fix, the wall shown in the photograph can be fixed using metal twist-tie connectors, such as Simpson's LTS/MTS series or USP's LSTA/MSTA series, to fasten the trusses directly to the studs. Since the framing intervals for the trusses and studs differ, this would provide a solid connection every 4 feet. Where trusses fall over stud bays, a metal strap could be run up each stud and across the top of the plates, then fastened securely to both sides of the stud. This would effectively reconnect both plates to the studs.