How to Construct a Concrete Slab Foundation for a Shed

If you want your shed foundation to last forever and ever, a concrete slab foundation is the way to go. It may be a little more difficult to construct but trust me, you won't be sorry. The nice thing about a concrete slab foundation is that you can put it pretty much anywhere and you know it will be stable. The one thing you have to remember about a concrete slab foundation is that some counties and codes may look at it as a permanent structure and require different permits. Usually a shed that is freestanding does not require a permit unless it is a concrete slab. Just be sure you check your code listings before starting this project.

Obviously the first thing you want to do is lay out your shed and then dig away about 6 inches of soil, including the sod if there is any. Next, dig a trench about 8 inches wide around the edge about 12 inches deep, then slope the interior walls at a 45 degree angle so that the trench is about 18 inches wide at the top. Use level lines as guides to install 2x8 forms. Starting at a corner place one board and drive in 2x4 end stakes. Level the board so the top is at least 4 inches above the ground and nail it to the stakes. Install the second board and check the corner for square and reposition the boards as needed.

It is important to make sure the form is square, to do this measure across opposite corners. Adjust the positions of forms as needed so the diagonal distances are equal. Brace the forms every 2 to 3 feet with additional 2x4 stakes. If you want to add extra support, add a brace called a kicker, nailing 2x2 at an angel to a stake outside the form. Don't forget to add a commercial release agent to the forms so the concrete won't stick to the forms, this can be bought at any home improvement store.

The next thing you need to do is add a 6 inch bed of gravel to the site for drainage, this is especially important if you are putting the foundation in an area that has poor drainage. Unload the gravel into small piles with a wheelbarrow and spread it with a shovel and garden rake. Make sure it is level and even throughout the entire foundation area. To help compact the gravel spray it with water and then tamp it with a hand tamper or a power tamper. To make sure the gravel is compact make a series of parallel passes, then make a second set of passes at right angles to the first.

Once the gravel is in you need to add No. 4 (1/2 inch) rebars to reinforce the footing. Place two runs about 4 inches apart around the trench, supported on bricks. Overlap the ends of rebars at joints and corners, and tie them together with wire. Tie the runs to the bricks. Lay down 6x6-10/10 wire mesh to reinforce the slab, keeping it 1-1/2 inches or so from the edges. Prop the mesh about 2 inches below the top of the form on bricks and nails driven partway into the forms. Overlap sections by 4 to 6 inches and tie them together with wire. Hang two runs of rebars at the top of the trench about 2 inches below the wire mesh. Suspend the rebars from the mesh with wire and tie the ends at joints and corners so that the runs are continuous.

This should have you ready for the concrete, it is a tedious process but like I said earlier, it will be worth it when you are done. Depending on how much you know about pouring concrete this was either the easier part or the harder part. I actually think this is the harder part and pouring the concrete is easier but it just depends on how much experience you have. Either way, it's not an easy job, but you can do it if you take your time and make sure it's done right. Good luck.