How to Figure the Mortar for a Block Foundation

Masonry amounts are determined by number of blocks.

When you build a foundation out of concrete blocks, the strength comes from the fact that the blocks are stacked in a staggered configuration, effectively locking the entire wall together, and from the strength of the mortar that is put between the blocks. Correctly estimating the amount of mortar you will need for the entire job can save you the expense, hassle and lost time that results from having to make multiple mortar deliveries.

Instructions

1. Measure the length of your foundation wall and multiply the number of feet by 0.75 to determine how many blocks you will need to complete one course. For example, if the circumference of your foundation is 120 feet, you will need 90 blocks to go around it once. The standard block size is 8 inches, meaning 7 5/8 inches plus space for the mortar.

2. Multiply the number of blocks in one course by the number of courses you will need. Assume each course is 8 inches high, the standard block size. If your foundation wall is 80 inches high, you will need 10 courses of blocks. Using the example in Step 1, you would need 900 blocks to complete your foundation wall: 90 blocks per course multiplied by 10 courses.

3. Purchase 2.5 bags of mortar and 600 pounds of sand for every 100 blocks in the foundation, writes Tim Carter in Ask the Builder. Using the above example, you would need 22.5 bags of mortar and 5,400 pounds of sand. Rounding up to be sure that you have enough materials, you would buy 24 bags of mortar and 2.75 tons of sand.

References

• Kight Home Center: Laying Concrete Blocks
• Ask the Builder: Sand and Mortar Quantities; Tim Carter

Resources

• Home Building Mentor: Laying a Block Foundation That Won't Crack