How to Build a Concrete-Block Foundation

Solid-concrete blocks foundation is one of the simplest and strongest types of on-grade shed foundations is made out of solid concrete blocks. It’s easy to build and adequate for all but the very largest sheds. This type of foundation is best when building on relatively level sites, but is also suitable on slightly sloping sites. Simply stack the blocks on the low end to level the floor frame.

Step 1: Get the Supplies

Here are the tools and supplies needed to build a simple on-grade foundation with solid-concrete blocks:

Tools

• Shovel
• Garden trowel
• Pickaxe
• Measuring tape
• 4-foot level 
• Mason’s string and four wooden stakes
• Hammer
• Wheelbarrow

Supplies:

• Straight 8-foot-long 2×4 x
• Concrete blocks
• Asphalt shingles
• Gravel

Step 2: Prepare the Building Site

Start by clearing the building site of rocks, shrubs, or anything else that would interfere with laying out the foundation. Also, be aware of overhead tree branches and power lines that may be too low.

Step 3: Create the Shed Perimeter

Hammer a wooden stake into the ground at each corner of the foundation. Run a string around the corner stakes, then install one stake in the center of each side of the outline.

To determine if the string outline is square, measure the opposing diagonal dimensions, from corner to corner. When the two diagonal dimensions are identical, the layout is square.

Step 4: Set the Concrete Blocks

Set the solid-concrete blocks into place, aligned flush inside the string perimeter. Then, dig out the grass and about 2 inches of dirt from beneath each block. Add 2 inches of gravel and compact the gravel with a hand tamper or 4×4 post. Set the concrete blocks back into place on top of the gravel.

Step 5: Level the Blocks

If your site has a slope and it is not possible to place blocks on the same level, stack solid-concrete blocks on the low end. Use 2-, 4-, or 8-inch-thick blocks to raise the low end of the site level with the high end.

These are the basic steps to building a concrete-block foundation. Now you’re ready for the next step: building the floor frame. For more information on floor framing, click here.

Joseph Truini is a host on the Today’s Homeowner TV show and co-hosts the weekly Today’s Homeowner Radio Show. He has written three best-selling shed-building books and lives in Roxbury, Connecticut.