How to Lay Concrete Block in an Existing Wall

Remove any concrete blocks that overhang the gap where the missing blocks used to be. Use the chisel and hammer to break the mortar holding them in place. Each row should have a gap as wide as the one below it, or a little wider. This will prevent you from having to find a way to wedge new blocks into the gap and still apply a proper coat of mortar to the joint.

Mix up a small batch of mortar according to its manufacturer’s instructions, to a pudding-like consistency. Use a bucket or wheelbarrow and hoe, instead of a cement mixer. For small loads like those needed for repairs, the cement mixer is not worth the trouble and expense.

Spread the mortar 1 inch thick on top of the blocks at the bottom of the gap in the wall, with the masonry trowel. Set a block into the mortar on the left side of the gap, 3/8 inches to the right of the block on the left side. Tap on the top of the block until the mortar beneath it is 3/8 inches thick, and scrape away the excess with the trowel. Check that the block is lying perfectly flat, with the level.

Fill the gap between the block that you just laid and the one to the left of it with mortar. Continue laying blocks along the bottom of the gap until you reach the right side of it. Then lay another row on top of the first, following the same procedure, until you completely fill in the gap.