A concrete-slab foundation may require a bit more work to build than other types of shed foundations, but it’s perhaps the strongest and most durable. It’s close to the ground for easy access, and it’s ideal for storing vehicles and heavy machinery. And, if you’d like, you can always top the slab with some other type of finished flooring.
Step 1: Prepare the site
Before you begin, be certain there aren’t any buried cables, pipes, or other obstructions beneath the building site. If you’re unsure, call the Before You Dig service.
Step 2: Mark the Foundation
Mark the outline of the foundation using either stakes and string or grounding-marking spray paint.
Step 3: Excavate the Site
Start digging out the area for your foundation. The top of the slab should be about 1½ inches above the ground, but you must also accommodate about 2 inches of gravel. So, for a 4-inch-thick slab, dig down 6 inches. Cover the excavated area with 2 inches of gravel and compact it flat.
Step 4: Install Form Boards
Use 2x6s to build a perimeter form to hold the concrete. Screw the corners together and drive into the ground several 2×4 stakes to hold the form boards straight and level, and to prevent them from bowing out under the weight of the wet concrete.
Step 5: Reinforce the Concrete
Lay several pieces of ½-inch diameter (No. 4) metal rebar on top of the gravel. Wire the bars together. Mist the surface with a little water.
Step 6: Pour the Concrete
Mix the concrete to the proper proportions, then pour it into the 2×6 forms. Overfill the forms slightly, then use a long board as a screed to smooth and scrape off the excess concrete. Finish the surface with a bull float and hand trowel. Then, if desired, lightly drag a broom across the concrete to create a slightly textured, slip-resistant surface. Keep the concrete moist for several days as it cures.
Poured concrete-slab foundations are great for any size shed. It can be a little more difficult to build, but once you’ve figured out how to make it, it will be a good, strong foundation. When you’re done with that, you can move on to the wall framing.
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.