Mortar Techniques for Concrete Blocks

General Purpose Mortar

• For the most effective results, you need to use the right mortar for the job. Type N is a general purpose mortar that is appropriate for garden and patio walls and anything that doesn’t have to bear a load higher than 750 pounds per square inch. Mix cement, lime and sand in a 1 to 1 to 6 ratio to get a type N mortar.

Specialty Mortars

• If you are looking to build a retaining wall, a foundation wall or any structure holding no more than 1,800 pounds per square inch, then you want to mix up a type S mortar. This requires cement, lime and sand in a 2 to 1 to 9 ratio. For heavy loads requiring up to a 2,500 pounds per square in capacity, you need type M mortar. Type M mortar is made from cement, lime and sand in a 3 to 1 to 12 ratio. If you are trying to repair historic brick or stone work, you may want to try a straight lime mortar of 1 part lime to 3 parts sand, to match the look more closely. However, it is not meant for load-bearing structures.

Mortaring the Blocks

• Cover the top of the concrete blocks with 1 inch of mortar before setting the blocks of the next row on top of them. Set the blocks of the next row into the mortar and tap them down into it with a hammer until the mortar between the rows is about 3/8 of an inch thick. Scrape away the excess mortar with the trowel. Fill the joints between blocks with 3/8 inches of mortar as well.

Using a Level

• Use a level as you work. Keep each stone in each row as level as you can. A small unevenness in the lower rows can create bigger deviations later on. Check the level repeatedly as the mortar hardens to ensure the blocks aren’t settling unevenly.

Calculating the Volume

• To determine how much mortar you need to make for each row, multiply the width of a block by its height and length. Ensure all of these measurements are in feet. Multiply this result by the number of blocks in the row. Multiply that result by 0.03125, which is the thickness of the joints in feet and you have the total volume of mortar between the rows will require in cubic feet. Multiply the width of a block by the height, in feet, and multiply the result by 0.03125. Multiply this result by the number of joints between adjacent blocks in the row. That number is just the number of blocks in the row minus one. That result will be the total amount of mortar, in cubic feet, that you will need between blocks in the row. Add the two totals together to determine how much mortar you will need for the entire row.