Once you"ve established two square lines as flush as possible with the exterior edges of the stem walls, snap reference lines on the stem that represent the inside edge of the sill. From here on out, take all layout measurements–for the whole building–from one of these two lines.
Check the top of the stem wall for smoothness first by sighting along the tops looking for high or low spots. You"ll need to compensate for them with the sill.
Use a builder"s level or a laser to find the highest point of the stem wall, taking measurements every few feet around the perimeter; mark elevations on the stem as you go. Make sure to mark them on the wall or on the inside of your chalk line so they don"t get covered up by the plate. The highest spot on the stem wall will be the bottom elevation of your sill, which, after you lay out and drill the sills, you should bolt down first. Then work your way around the foundation reading level and shimming as necessary. And, since this is framing, not interior finish, a sill plate that"s within 3/8 inch to 1/4 inch of dead level meets acceptable tolerances.
Correct a low spot by packing out the foundation under the sill plate with steel shim stock.
For spots lower than 1/4 inch in the stem, slip steel shims under the sill plate to fill the gaps. (Use steel because wood shims can be crushed.) Place the shims, then tighten the anchor bolts. Sight-down the finished sill to make sure it"s shimmed level. If the stem has a wickedly low spot–I"ve seen them an inch or more too low–use non-shrink grout to fill the space.
If the stem wall is fairly level but has one severely high point, you may have the concrete sub saw or chip it down to the proper elevation. Otherwise, you must raise the sill plate on the whole building to compensate for one high point. And, unless you can get a blessing (in writing) from the project engineer, using a narrower sill plate compromises the building"s structural integrity.
Start Off Right
I cannot over-emphasize the importance of starting off square and level. If the foundation is off and you do nothing to correct it, you, and every trade that follows, will fight the problem down to the last shingle. And, even after all the extra work, your product will still be lacking. Build it right from the start.
Use a transit or laser level to check for high spots or dips every few feet around the stemwall and at every anchor bolt.
– Michael Davis is president of Framing Square in Albuquerque, N.M., and is a contributing editor to Tools of the Trade.