How to Build Concrete Block Foundations

Square the foundation walls by laying out the foundation corners and making sure they are square. Begin in one corner and pull a tape to mark your wall dimensions until all four corners are marked for your foundation walls. Cross check the corners by pulling a tape measure diagonally across the corners. The measurements across diagonals should be the same for both diagonals. If not, adjust the corner marks until the diagonal measurements are the same.

Snap chalk lines onto the footings to connect the corner marks for the foundation walls. This will be your reference point for the first course of blocks.

Lay all four corners of your foundation up to a height of at least 48 inches. This will allow you to stretch a string from each corner and fill in between to complete your walls. Lay four blocks in one direction from your 90 degree corner and three blocks in the opposite direction of the 90 degree angle. Lay down a 2-inch bed of mortar using a mason's trowel for this first row of blocks and butter the ends of the blocks where they butt against each other.

Stagger the proceeding row of blocks until you reach 48 inches, laying in each direction of the 90 degree angled corner. Butter the top of each previous block before stacking the next block on top and remember to butter the ends of each block as well. Use a 4-foot level to keep the corner plumb and a framing square to keep it square.

After all corners have been completed, stretch the nylon string from one corner to the next to fill in the center of the walls with concrete blocks. If you are going higher than 4 feet with your corners, repeat the corner building and fill in in the same way by stretching a string line.