Securing Cinder Blocks to a Concrete Floor

Ensure that the concrete floor is extremely clean and free and debris. If the floor is dirty and stained, clean it with a concrete degreaser. Apply the degreaser to the floor with a paint roller and allow it to remain on the concrete surface for a short period, but do not allow the degreaser to dry on the concrete. Scrub the floor with a stiff brush to remove stains and embedded debris, then rinse with copious amounts of water, until the water runs clear.

Mark the location for the cinder blocks on the concrete floor using a chalk line, ensuring that the line is extremely straight. To ensure the line is straight, have one person hold the end of the chalk line at the exact location of the corner while the other person goes to the other corner location with the chalk box attached to the string. While holding the chalk box in place, the second person sharply pulls the string up to "pop" it, leaving a straight chalk line. If there is a time lapse between marking the chalk line and laying the blocks, mark the line with a grease pencil so it cannot be brushed or washed away accidentally. The levelness and straightness of a wall depends upon this initial chalk line.

Lay a dry run of the cinder blocks along the chalk line, without using mortar. Place the blocks on the concrete floor, leaving a 3/8-inch space in between each block. Cutting a piece of 3/8-inch plywood into small strips and placing the strips between the blocks makes appropriately sized gaps. Mark the corners of the end blocks with a steel square so you can properly duplicate the line with mortar. Remove the blocks.

Mix the mortar. Combine one part masonry cement with three parts sand in a five gallon bucket and thoroughly mix them together with a hoe. Form a pit in the middle of the dry ingredients and slowly add up to two gallons of water, pushing and pulling the mixture to blend the ingredients together. Continue mixing the mortar until it is the consistency of mud and forms a soft ball when molded in your gloved hands.

Spread a 1-inch thick layer of mortar on the concrete floor using a steel trowel. Place the point of the trowel on the floor and pull it toward you, moving it in a side-to-side motion. This causes the mortar to slide off the trowel in a straight line. Spread the mortar to fill in the chalk lines marked earlier. Begin at one end, covering enough space for three cinder blocks.

Place a cinder block on the bed of mortar and tap the top of the block lightly to embed it in the mortar. Spread a 3/8-inch layer of mortar on the end of the block and place the next block into position. Push the second block gently toward the first to ensure a good bond. Lay three blocks at one end, then move to the other end of the wall and lay three blocks on that end. Fill in the middle, spreading enough mortar to lay three to four blocks at a time and mortaring the end of each block.

Position subsequent layers of cinder blocks by staggering them so that the end of one block is at the halfway mark of the block below it. This ensures a solid, strong wall that does not have weak joints. Always place a 1-inch thick layer of mortar between layers of blocks. Upon completion of the wall, allow the mortar to cure for three to seven days before allowing it to get wet.