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Q.Recently we contracted to do a house with precast foundation walls that were set out of level — up to 3/4 inch over 30 feet, in some places. What’s the best way to shim and air-seal the mudsill?

A.Tim Uhler, a lead framer for Pioneer Builders in Port Orchard, Wash., responds: Because out-of-level foundation walls really slow down production, I always make it a point to check new foundations before my crew begins framing; if there are problems, I immediately point them out to the foundation sub and general contractor. When the walls are more than 1/4 inch out of level, the foundation contractor typically should take care of the problem, either by grinding down the high spots or filling in the low spots.

Grinding down high spots isn’t any fun, but filling in low spots isn’t that difficult. One option is to use Quikrete’s FastSet grout (800/282-5828, quikrete.com), a nonshrinking portland cement–based grout that can be mixed to a fluidlike consistency, making it a self-leveling product. If the foundation sub can’t fix your problem, clamp 2x4s to both sides of the top of each out-of-level wall running the length of the foundation, snap level lines, and pour in the grout up to the lines. After a couple of hours, you can set your mudsills.

If the foundation is less than 1/2 inch out of level, we just set the mudsills normally — first installing a foam gasket underneath the sill — and tighten down the anchor-bolt nuts. If the low spot is longer than the rim stock we have available, we nail the rim together, set it and string the top, and then shim the rim to the line. If the low spot is localized, we just use a piece of rim long enough to span it. Then we install plastic or composite shims between the mudsill and the rim joist on the joist layout, leveling and supporting both the rim joist and floor joists at the same time. Expanding foam — from either a can or the insulation sub’s spray gun — seals the cracks between the mudsill and the rim joist.

Another option is to leave out the foam gasket in the area that is out of level and shim between the mudsill and the foundation (though this may not work if the anchor bolts are set too low). Shim every 4 feet or so to level the sill and install shims under the ends of each joist. After the sill is shimmed straight, use expanding foam to seal the low spots between the concrete and the mudsill.