How to Install a Cinder Block Wall on a Concrete Slab

Apply a coat of concrete degreaser and stripper to the concrete surface with a paint roller. Allow the degreaser to remain on the surface for two to three minutes, but do not allow it to dry. Scrub the concrete with a stiff brush to loosen embedded debris from the pores of the concrete. Rinse the surface well with water until the water runs clean. Allow the concrete to dry for about six hours before continuing.

Mark the location of the cinder block wall on the concrete using a chalk line. To get a proper line, ask an assistant to hold one end of the chalk line at one end of the wall while you hold the other end on the opposite end of the wall. Sharply pull the chalk line upwards to make it pop, which will result in a red chalk line on the concrete. Trace over the chalk line with a grease pencil in case the chalk fades. If the line is not straight, the cinder block wall will not be straight.

Cut a piece of 3/8-inch plywood into 2-inch strips with a power saw. Place a line of cinder blocks along the chalk line without using mortar. Slide a plywood strip between each block to act as a spacer. Mark the concrete at the corners of the end cinder blocks by tracing along a steel square with a pencil. This will help you positively duplicate the line when you lay the blocks.

Combine three parts masonry sand and one part masonry cement in a wheelbarrow, pushing the dry materials around with a hoe to mix them. Make a small pit in the middle of the wheelbarrow and slowly add approximately 2 gallons of water, mixing the ingredients as you add the water. The concrete mortar is the proper consistency when you can form a soft ball with your gloved hands, and the ball holds its shape.

Place the point of the steel trowel on the concrete surface, and drag it toward you in a slight back and forth motion to make the concrete mortar slide off the trowel in a straight line. Spread a 1-inch thick layer of concrete mortar inside the chalk lines of the wall, using a steel trowel. Work from one end to the other, spreading enough concrete mortar to place three to four cinder blocks at a time.

Position the first cinder block at the end of the wall and tap it lightly with a hammer to embed it in the concrete mortar. Using the steel trowel, spread a 3/8-inch thick layer of concrete mortar on the end of the block. Carefully set the next cinder block into position, pushing it gently against the first block to ensure a proper bond between the two. Lay a third block and then lay three blocks at the other end of the wall. Fill in the center of the wall with blocks, mortaring between them.

Place subsequent layers of blocks in the same manner, staggering them so the end of a block is at the halfway mark of the block below it to ensure a solidly built wall with no weak joints. A 1-inch thick layer of concrete mortar is required between each row of blocks for maximum strength of the wall. Allow the concrete mortar to cure for three to seven days before allowing it to get wet. This curing period increases the strength and durability of the wall.