If you want your shed foundation to last until you want to get rid of it, a concrete slab foundation is a way to go. It may be a little more difficult to construct but in the end, the trouble will be rewarded. The nice thing about a concrete slab foundation is that you can put it pretty much anywhere and you know it will be stable.
There is one thing to keep in mind about a concrete slab foundation and it 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 make sure you check your code listings before starting this project 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 so we can 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 that the trench is about 18 inches wide at the top.
Use level lines as guides to install 2×8 forms. Can also use laser if you own modern tools, but the old school will work as well. Starting at a corner place one board and drive in 2×4 end stakes. 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 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. When both opposite corners give the same measurement, you know its perfect rectangle shape. To 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 to add extra support, add a brace called a kicker, 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 the concrete won’t stick to the forms, this can be bought at any home improvement store.
Preparing ground for 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 are putting the foundation in an area that has 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 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.
How to place rebar in concrete
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 6×6-10/10 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 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 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. For making ready mix concrete, here is a quick guide. If the slab foundation is big enough, the concrete truck might be the best bet.
Pouring the concrete slab
No matter how the concrete gets to the shed form, pouring concrete itself should be a relatively easy process. You mix the concrete or get it ready-made, and pour 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 fill it we need to use a vibrating tool to let the air inside the concrete escape so we can avoid void under 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 towards the direction that is unfinished moving it a little bit from side to side. Then we pour more concrete and repeat the process again until we’re on the other end and everything has been gone through with vibrating tool and screeding level.
Now we can use concrete float to finish the surface after the screeding is done. If it’s a big shed, the pour might be better to be done in portions, but here I assume we’re doing a small one.
You should know the general guidelines for making a DIY concrete slab foundation for a shed. The key points here are level ground, sturdy form build around it, compact gravel, rebar, and carefully done concrete pour.
Sheds aren’t usually that big so it should be something that a beginner can achieve. Remember to take all the time you need in preparation work. The concrete pour in itself will go very fast. Compared to preparation, it will be a flash.
After that, you get to build the walls and roof for the shed.