How to Build a Cement Block Foundation

Use a stiff-bristled broom to clean the top surface of the pre-poured and cured concrete footings to remove all dirt and debris.

Replace the string lines to the temporary wood corner markers, called “batter boards,” used to layout the concrete footings. If the batter boards are no longer in place, use a small sledgehammer to drive a wood stake into the ground approximately 3 feet beyond the ends and/or corners of the planned cement block walls, in line with the planned exterior wall line.

Drive a second wood stake at a 45 degree angle to each of the stakes placed in Step 2, and attach these to the first stakes with a 8d duplex nails. The second, angled stakes will act as braces to prevent the vertical stake from moving. Repeat this staking procedure at both ends of each foundation wall line. These stakes will hold the string lines used to verify the correct placement of the cement blocks.

Tie a length of nylon string between each pair of vertical stakes to use as a reference for the exterior wall lines. Make sure the string is taught and unobstructed.

Use a builder's level to mark a plumb line from the string lines set in Step 4 onto the top surface of the cement footing at each end of a foundation wall section. Snap a chalk line on the footing surface to connect these marks. This will be the guidelines for the first horizontal row, or “course,” of cement blocks.

Prepare premixed mortar in a wheelbarrow or mortar box according to the manufacturer’s instructions. The mortar consistency should be fairly stiff and moist.

Place two parallel lines of mortar 8 inches apart along the first 2 feet of the chalk line marked in Step 5. Make these lines approximately 1 ½ inches wide and ¾ inch thick. Gently lower a cement block onto the mortar lines, and use a torpedo level to level the block. Tap the top of the block lightly to move the block to level, keeping a minimum ?-inch mortar joint at the base of the block.

Use a builder's level to check the alignment of the block’s outside face with the string line above. Check that the end of the block aligns with the end or corner of the wall line. Collect the excess mortar that squeezes out of the mortar joint and return to mix for reuse.

Repeat Steps 7 and 8 for each cement block along the entire first course of the wall section, checking for alignment and level. Make sure the block ends are tight against each other. Use a straight-edged board against the top outside edge of the blocks to check alignment every 6 to 8 blocks along each course.

Follow the method described in Steps 7, 8 and 9 for the second and third courses of cement block walls. Be sure to offset the vertical joints 8 inches between adjacent courses of block, similar to standard bricklaying.

Allow 24 hours for the mortar joints in the walls to cure prior to filling the block cells with concrete. As you fill the wall sections, use a trowel or straight board to scape the concrete flush with the top surface of the block wall.

Install foundation anchor bolts in the top course of the foundation wall as you fill the block cells with concrete. Center the bolts in the filled cells of the top course at a maximum of 48-inch spacing.

Allow the filled block walls to cure 24 hours prior to beginning any wall framing to be built on top of the foundation walls.