Snap chalk lines that are a consistent distance ― 1 or 2 inches ― from each side of the blocks.
Use a pencil to mark the openings exactly, then double-check that the openings are the correct width. Use a framing square to check the corners. Dampen the slab.
Mix a batch of mortar and spread a layer about ½ inch thick in between the layout lines.
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Bed and level the first block
Set an end block or a half block in the mortar bed, centered between the chalk lines and exactly aligned with the end line. (You may need to scrape away mortar to see the line.)
Press it into place so that the mortar is about 3⁄8 inch thick. Check the block for level in both directions and check the face of the end for plumb. Scrape away excess mortar and use it for the next block.
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Butter a block end
Scrape mortar onto the end (or the end webs, if you are using webbed stringers) of the next block to form peaks as shown. On a long wall, you should set an end block at the other end first, then set up a string line and fill in between the two blocks (see step 7).
For a short wall like this, simply set the blocks in order.
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Check for straightness and level
Set the next block in place and push it against the first block so that the joint between them is about 3⁄8 inch. Check the block for level.
Scrape mortar onto the flanges of the laid blocks and set the next course of blocks on top. Start the next row with a corner block or a half block, so the blocks will be stacked one on top of two. Check this row for level and see that the blocks form a straight line. Scrape mortar from the joints as you work.
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Tie a corner together
If you don’t use corner blocks, you can use this method. Where two walls adjoin, use a circular saw with a masonry blade to cut channels in the tops of the blocks. Bend a piece of rebar to fit. Fill the cells with mortar and set the rebar in the mortar.
Do this every other course.
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Reinforce with ladder wire
To strengthen a wall, and perhaps a corner as well, add ladder wire every other course and under the top course. Spread a layer of mortar onto all the webs and set the wire in the mortar.
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Use a line guide for a long wall
On a long wall, set the blocks at each end, then attach mason's line to a corner block at each end and pull it taut.
Install the intervening blocks so they nearly touch the line. The line can serve as a guide for level as well as straightness along the wall’s side.
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Finish a long wall with a closure block
The last block on a long wall, called the closure block, is a bit tricky to install.
If you need to cut a block, cut the next-to-last block and then install a full-sized closure block. Spread a thick layer of mortar onto both ends of the closure block. Align it precisely with the center of the opening and carefully slide it straight down into place. If mortar peels off one of the ends, pick the closure block up, reapply mortar, and try again.
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Install an angle iron
At a lintel (the top of an opening), install an angle iron for support. If possible, plan the installation so you can set the angle iron on top of a block at each side of the opening.
If that is not possible, use a circular saw equipped with a masonry blade to cut a channel that you can slide the lintel into. Mark and cut carefully so the angle iron will be level. The masonry saw blade makes a cut that is just thick enough for the angle iron.
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Finish a lintel
Fill the cells on each side of the opening to cement the angle iron in place. Spread mortar and install blocks on the angle iron.
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Install an electrical box
Electrical conduit should run inside the counter, rather than through the block cells, wherever possible. To install an electrical box on the outside of the counter, cut an opening for the box using a circular saw and set the box in so it protrudes the thickness of whatever finish material you will use.
Drill a hole for the conduit using a masonry bit. Draw a circle for the conduit's hole, drill six or seven holes along its circumference, then chip out the hole using a small masonry bit. Run conduit to the box using an elbow to make the turn. Tap in shims to wedge the box in place. Stuff newspapers into the cell about 6 inches below the electrical box, then fill the area around the box with concrete to hold the box firmly in place.
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Strengthen a wall by filling it
A narrow wall section like this can be strengthened with concrete and rebar in one or more block cells. Mix a batch of concrete that is wet enough to pour easily. Pour in a foot or so of concrete. Cut a piece of rebar to the height of the wall minus an inch or so and insert it into the center of the cell. Fill the cell the rest of the way with concrete. For extra strength, you can use this method on some or even all of the cells.