If you have a cinder block fence and need to heighten it to increase your privacy or to keep your dog from escaping, add one or more extra rows of cinder blocks. Matching cinder blocks can be affixed with mortar. Pre-mix mortars are widely available and easy to use. Alternatively, you can make your own mortar with sand and cement. When adding to an existing wall, it is essential to properly clean the top of the wall to remove stains, dirt and grime and to ensure adequate adhesion of the mortar.
Clean the top of the wall with a biodegradable, mild etch concrete cleaner. Pour the cleaner onto the top of the wall and wait a few minutes while the etching agents go to work. Flood the top of the wall with fresh water to rinse the cleaner and neutralize the etching agents.
Mix your mortar, if you are making your own. Use 3 parts sand and 1 1/2 parts cement. Gradually add water until you have a thick, goopy mix.
Start at one end of the fence, or at a corner if your fence has a corner. Use a trowel to spread a 3/8-inch thick layer of mortar for the first block.
Press the first block onto the mortar. Use a level to make sure it is plumb and even on the top. Adjust as needed. Use your trowel to scrape away excess mortar that oozes out.
Spread mortar on the top of the fence for the second block. Also apply a 3/8-inch thick layer of mortar to the end of the previous block where the next block will be abutted.
Set in and level the second block. Continue to lay blocks until your fence extension is complete.
As the mortar starts to dry, smooth down the mortar joints with a jointer tool to create nicely finished seams.