How to Build a Concrete Block Foundation Step By Step

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Here’s how everything looked after the concrete cured and a little bit of rain.

Step 1: Mix Mortar
The guys used a gas-powered mixer to mix sand, water and type-S masonry cement. Then they would shovel the cement in to large tubs for the block layers.

Pro-Tip: Wet the tubs before adding the mortar to help prevent it from drying out.

Step 2: Measure Length
From the blueprints, Victor and his crew determined the location of the foundation walls. Next, they would mark out each section and calculate the number of blocks per section.

Pro-Tip: Concrete blocks measure 8″ x 16″ including the joint.

Step 3: Clean Footer & Strike a Line

Step 4: Set Corners

For each section, Victor’s crew would start at the corners and work their way into the middle. For that reason it was really important that the corners be perfect because they act as a reference for the rest of that section of wall.

If you look closely, you’ll notice the corner blocks are slightly different with a finished outside corner.

Step 5: Run String Guide
Once the corners were in place, Victor’s crew would run a string along the top corner of the block and use that as a guide for the rest of that section.

Pro-Talk: The bracket for securing a string is called a dogbone or line-stretcher.

At times they also used a wooden corner block.

Step 6: Lay First Course
Using the chalk line as a reference, the guys laid a bed of mortar and began laying block.

Pro-Tip: Joints should measure 1/2″ and the width of a finger is a good approximation.

After each section, excess mortar and broken pieces of block were used to fill the corners for additional reinforcement.

Step 7: Turn the Corner

Keeping the corners square is very important, and the guys employed a little trigonometry (3-4-5 Pythagorean triple) to double-check their measurements.

Step 8: Wall Ties

Every 2-3 courses, the guys used wall ties and nails to anchor the block wall to the existing foundation.

Step 9: Cutting Block

It was often necessary to cut the last block of a section or in order to stagger the joints on the next course of block, and they used two different methods. The first method was to use a brick hammer along the cut line, and after a few well-place whacks it would break.


When precision was necessary, the guys used a concrete saw to cut the block.

Next, the guys hammered stakes at all the corners.

Setting the laser detector onto the stake allowed the guys to mark a consistent, level line. Measuring off that line enabled them to make sure each course of the foundation was level with the rest.

Finish
If you’re looking closely, you’ll notice that not all the blocks are filled at this point. The guys use left over mortar to eliminate waste, and even though I’m calling this finished, they still have some filling left to do.
The next step is to call in an inspection before backfilling around the foundation and let the plumbers complete their rough-in.