DIY concrete slab foundation for a shed

DIY concrete slab foundation for a shed


If you want your shed foundation to last until you want to get rid of it, a concrete slab foundation is the way to go. It may be a little more challenging to construct, but the trouble will be rewarded in the end. The nice thing about a concrete slab foundation is that you can put it anywhere and know it will be stable.

There is one thing to keep in mind about a concrete slab foundation: some counties and codes may look at it as a permanent structure and require different permits. Usually, a freestanding shed does not require a license unless it is a concrete slab. Before starting this project, check your code listings, or it might cause unwanted trouble.

How to build wood concrete forms for the base

The first thing you want to do is lay out your shed for forming. You can use the 3:4:5 right triangle rule to get all the corners 90 degrees. Then dig away about 6 inches of soil, including the sod, if there is any. We want it deep enough to add enough gravel for good drainage.

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 the trench is about 18 inches wide at the top.

Use level lines as guides to install 2×8 forms. You can also use a laser if you own modern tools, but the old-school will also work. Place one board and drive in 2×4 end stakes starting at a corner. Level the board so the top is at least 4 inches above the ground, and nail (or screw) it to the stakes. Install the second board, check the corner for square, and reposition the boards as needed.

It is essential to make sure the form is square to this measure across opposite corners. When both opposite corners give the exact measurement, you know its perfect rectangle shape. Adjust the positions of forms as needed so the diagonal distances are equal. Then, we can brace the forms every 2 to 3 feet with additional 2×4 stakes.

If you want extra support, add a kicker brace, nailing 2×2 at an angle to a stake outside the form. Don’t forget to add a commercial release agent to the forms, like concrete form oil, so that the concrete won’t stick to the forms; this can be bought at any home improvement store.

Preparing the ground for the concrete slab

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 put the foundation in an area with poor drainage. You can refer to other concrete slab guides on this site as well.

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 tamp it with a hand or power tamper. To ensure the gravel is compact, make a series of parallel passes, then make a second set of passes at right angles to the first.

How to place rebar in concrete

Once the gravel is in, you must 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 6×6-10/10 wire mesh to reinforce the slab, keeping it 1-1/2 inches 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 to keep the runs continuous.

This should have you ready for the concrete. It is a tedious process, but as 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 more accessible or challenging.

This is the more challenging part; pouring the concrete is more accessible, but it depends on your experience. For making ready-mix concrete, here is a quick guide. The concrete truck might be the best bet if the slab foundation is big enough.

To avoid void under concrete slab, vibrate the fresh concrete well

Pouring the concrete slab

No matter how the concrete gets to the shed form, pouring concrete itself should be a relatively straightforward process. Mix the concrete or get it ready-made, and run it inside the form, starting from one narrow end and moving towards the other.

So, you start filling the form from one end to the top of the foundation form. While we serve it, we need to use a vibrating tool to let the air inside the concrete escape to avoid a void under the concrete slab. That way, there won’t be hollow points left in the final product.

When the form is filled to the top, you can use a screeding level to screed the extra concrete from the top of the form. You move it in the unfinished direction, moving it slightly from side to side. Then we pour more concrete and repeat the process until we’re on the other end and everything has been gone through with the vibrating tool and screening level.

Now, we can use concrete float to finish the surface after the screeding. If it’s a big shed, the pour might be better in portions, but I assume we’re doing a small one here.


If you’re planning to create a DIY concrete slab foundation for your shed, it’s essential to know the key points that will help you achieve a solid and durable base. Firstly, you must ensure that the ground where you plan to construct the foundation is level and free from debris. Next, you must build a sturdy form around the area using wood, plywood, or steel. This form will help to contain the concrete as it sets and dries.

You should then add compact gravel to the form, which will help to improve drainage and prevent moisture from getting trapped under the foundation. Rebar, or reinforcing steel, should be included in the foundation to provide additional strength and support. This will help to prevent the foundation from cracking or shifting over time.

Once you have completed the preparation work, it’s time to pour the concrete. This is the process’s most critical stage, and you must be careful and precise to ensure that the foundation is level and correctly set. It’s best to start pouring the concrete from one corner and work across the form. After pouring, you should use a trowel to level the surface and smooth any bumps or ridges.

After the concrete has been set and dried, you can remove the form and build the walls and roof for your shed. Remember, preparation work is crucial to achieve a successful outcome, so take your time and pay attention to detail.