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How to Make a Concrete Block Crawl Space

Jim Wildman

The foundation is extremely important in new construction. Everything follows the foundation of the house, so if there is a minor error in the foundation it can become a serious error before the house is complete. With this in mind, laying out the concrete block for a crawl space must be done accurately. The do-it-yourself handyman can do this when he follows guidelines and double-checks his work.

Typical concrete block wall
  1. Place two constructions stakes at each end of the wall at least 2 feet away from the edge of the foundation. This allows room to work. Place a third stake between the two others, forming a big "H," and nail the stake into place to serve as a cross member. This makes what is called a batter board. Run a length of builder's twine between the two sets of batter boards. Adjust the string until it marks the location of the outside of the foundation wall exactly. Tie it in position to the crosspiece of each batter board and mark the crosspiece at this spot in the event the string comes loose.

  2. Repeat Step 1 for the remaining walls of the foundation. Be sure to check that they are square to each other. Use the 3, 4, 5-foot rule. On one string place a mark 3 feet from where the two strings cross. Place a mark at 4 feet from the crossing point on the other string. Have a helper adjust the opposite end of the string on the batter board until the distance between the two marks equals 5 feet.

  3. Set a plumb line at the point where the two strings cross. Mark the location of the plumb line onto the foundation. This is the exact outside corner for the CMU block crawl space.

  4. Place a CMU block at the exact corner of the walls. Run a twine line between the two end blocks. Place dry CMU blocks to fill in between the two end blocks. Use a small piece of 3/8-inch plywood to represent the mortar joint. Cut any blocks that are needed.

  5. Check the architectural drawings for the specifications of the rebar steel needed to tie the footing to the CMU block wall. Mark the footing through the center cavity of the CMU blocks that are specified to have rebar steel.

  6. Drill and epoxy the rebar as specified in the detail of the architectural drawings. Use a hammer drill and a masonry drill but.

  7. Place the CMU corner blocks into 1-inch thick bed of mortar at both ends of the wall. Check that the blocks are level across the top and aligned with the builder's twine. Double-check the blocks, this is the most crucial step because all the other blocks are set to the corner blocks.

  8. Pull a piece of builder's twine between the two corner blocks. The twine must be set to the outside edge of the blocks. The string acts as a guide line for the remaining CMU blocks for that row.

  9. Pull a piece of builder's twine between the two corner blocks. The twine should be set to the outside edge of the blocks to serve as a guide line for the remaining CMU blocks for that row. Trowel a 1-inch bed of mortar the width of the CMU block about 6-foot long onto the footing. This is enough mortar for three CMU blocks. Use the tip of the spade trowel to create a furrow down the center of the bed of mortar. Apply enough mortar onto the end of a CMU block for a 3/8-inch mortar joint. Place the CMU block onto the mortar bed and level the block. Repeat for the remaining CMU blocks for the row. Check that the blocks are level in all directions as you work.

  10. Repeat Step 9 for the next two rows of CMU block. Rotate the corner blocks 90 degrees between rows. This strengthens the corners and makes the CMU blocks' mortar joints stagger between rows.

  11. Add a layer of CMU block wire onto the top of the CMU block. This wire ties the blocks together and strengthens the wall. The wire needs to be added after every three courses of block. The wire comes in 8-foot sections and is welded together in 8-inch widths. To install the wire just lay it on top of the CMU block before adding the mortar to the top of the block.

  12. Install the last layer of CMU block using header blocks. Header blocks have a groove cut into them for rebar to run across the block wall. They also have a solid bottom so that they can be filled with mortar. This creates a solid reinforced top layer of CMU block for the crawl space.

  13. Place anchor bolts per the architectural drawings into the mortar before it dries.The anchor bolts are pushed into the wet mortar so that 2-inches of the threaded end remains above the surface of the mortar. Keep the anchor bolts as straight as possible when you install them.

  14. Tip

    Building codes require a minimum of 24 inches of clear vertical space inside the crawl space.


    Wear gloves when handling CMU blocks. The rough texture of the blocks can cut your hands.