Typical Method of Securing a Sill to a Concrete Block Foundation

Anchor Bolts

• Anchor bolts inserted into the mortar or concrete that fills the voids in the concrete blocks offer superior strength for securing the sill plate. These J-shaped bolts slip into the mortar or concrete while it’s still wet, with the “J” end downward and about 3 inches of the bolt top sticking out. When the mortar sets, the anchors will be firmly entrenched.

Positioning the Sill

• On a standard 8-inch-wide concrete block wall, use a treated 2-by-6 as a sill plate. Because this plate serves as the base of your exterior wall framing, it must be perfectly square. To ensure this, measure from the outside corners of the block foundation and snap a chalk line 5 1/2 inches in, along the length of the block wall. The backside of the sill will be flush with this line. While you could use the blocks as your guide, they may be slightly uneven, so using a chalk line is more accurate.

Marking the Holes

• Before you can slip the sill plate over the anchor bolts, you have to drill holes that correspond with the placement of the bolts. The bolts are not in a perfect line but they should be about 2 feet apart. The distance of the bolts should meet local building code, and will depend upon whether you must fill all the voids in the concrete blocks with mortar or only some of them. By holding the sill plate on top of the block wall, next to the bolts, you can make a mark on the edge of the sill by each bolt. Then, measure the distance from the chalk line to each bolt and transfer that measurement to the sill plate to give you the location for each hole.

Installing the Sill

• Many building codes require the installation of a moisture barrier between the sill and the masonry foundation. This may be a strip of polyethylene foam or a brushed-on sealer. Once the moisture barrier is on, slip the sill over the anchor bolts. A washer and a nut comes with each bolt. Put these on, the washer first, and tighten the nuts to secure the sill. Now, you’re ready to start framing the walls.