questions on slab for shed

[Snipe Hunter]

i was out lookin at the spot the shed is going yesterday. since we have 1- woodchucks that dug under the shed, and 2- moles that have trenched everywhere, i have some leveling and packing to do.thinking of leveling out, packing the soil, then raise elevation with crushed limestone or ??

Build your form with 4 to 6" of it above grade. I'd dig the pit around the perimeter and about 4 " deep in the center and 3 times that around the edge. I'd use 2x6s for the form and probably use tie-wire through the form, across to the other side to stop the form from bowing. Probably only need a couple of them each direction. Your forms will be nailed/screwed to wood pegs driven into the ground. If you use nails, leave the heads sticking out a little so you can easily pull them when removing the form. The forms won't be sitting on the ground in the pit. That makes them easier to remove and allows concrete to flow under them, adding more strength. That mess will get covered when you back-fill. Limestone? Whatever your landscape supplier has. I would probably use is #1 crushed stone (2" to 4"). Spread and tamp your stone as you fill. That way you have thicker concrete all around the edge to add strength and help prevent cracking. You would have 3-4" deep concrete in the middle of the form and maybe twice that around the perimeter. You could add some rebar around the perimeter if you wanted. I might do that just because I'm a little anal that way but it probably isn't really necessary. You really should set some anchor bolts in the perimeter standing proud about 2-1/2 inches to bolt down the sill plate of the walls. I'd also put down a sill seal to isolate the pressure treated sill plate from the concrete to prevent moisture wicking up the walls.

So put in a couple inches of stone in the pit. Then set your form above it so the edges of the slab are on stone too. Then put the rest of the stone in the center and tamp it flat leaving 3-4" between it and the top of your forms. This is a monolithic slab. If you were to do a non monolithic slab, you would want at least 4" and I'd probably do the mesh. A monolithic slab over stone will settle (sink) less than a non-monolithic slab. Either way, I'd do it on a bed of stone.

The stone does 4 things for you:
* Adds strength to the concrete
* allows water to settle below the base of your slab to help prevent wicking into the concrete
* acts as an uncoupler so the concrete isn't directly attached to your soil to help prevent cracking with earth movement
* groundhogs don't like digging in stone

Monolithic slab