How to Build a Cinder Block Shed Foundation

Installing a cinder block for foundation
  • 3-20 hours
  • Intermediate
  • 0-500
What You'll Need
Construction stakes
Construction twine
Plumb line
Scrap (lumber or plywood)
Hammer drill
Cinder blocks
Spade trowel
Carpenters level
CMU block wire

A cinder block shed can be an excellent place to keep all the extra things for which you have no more room inside the house. Usually, the outdoor shed becomes the receiving end of such objects that can no longer fit inside your house. However, since cinder blocks are lightweight and may not be very stable unless a strong foundation is provided to them, it becomes all the more important to pay extra attention to the foundation. The foundation should be strong enough to prevent any cracking and weathering over a long period of time.

Step 1 - Set the Parameter

Along the wall corner, just parallel to the footings at the corner, drive two construction stakes. Right in the middle of these two stakes, drive in another one. Take a piece of twine and tie the stakes together. This now represents the outer parameter of the foundation. Set up more stakes on the adjacent corners to finally make a square. Tie twines on all of the stakes to set up the entire perimeter. At the point of intersection of the two twine pieces, place a plumb line. Do this at every corner to mark the foundation corners.

Step 2 - Lay the Course

Lay out a rough course for the laying of the concrete foundation. To do that, place the cinder blocks along the wall’s full length, using corner blocks, specially earmarked for the corners. Use the scrap pieces of lumber and plywood to fill in the spaces between the cinder blocks to make sure you have the right layout in front of you. If some blocks have to be resized, mark them along the edges to the size they need to be reduced to. Also mark the foundation center so you know where the steel rebar will have to be inserted.

Step 3 - Lay the Foundation

Once the markings have been made, drill the steel rebar that is designated to connect the CMU block wall to the foundation. For this, you may have to refer to architectural drawings. To attach the rebar, you will need an epoxy and a hammer drill. Now set the corner blocks at 90 degree angles to start laying the foundation.

Make sure the corner blocks at both the ends are plumb. Use a level to check if they are at a straight level. Since all other blocks align with the corners, it is crucial to have them positioned correctly. Now lay a thick layer of mortar into the bed of the foundation. The mortar layer should be at least one inch thick. Apply enough mortar for three blocks. Carefully begin to place the blocks on the mortar, working from the corner down. Once an entire layer of blocks has been placed, again repeat applying the layer of mortar. Repeat this till three layers of cinder blocks have been laid. For the last row, use header blocks.