In a week, I'll be building a 10x12 shed.
The plans call for a on-grade foundation with 6x6 skids. My property
is sloped. From high pt to low pt is about 8 inches. I called out a
guy and asked about pouring a slab. After I got over the price
objections ($3k), he and I talked about cheaper suggestions.
$3000 for a 10x12 slab? Ouch. Even if I built simple forms and dug it down
so it was a full 12" thick all around it would cost less than $800 for 5
yards of concrete here (Washington State).
A slab that size is not difficult to do yourself (with a part time helper).
Just build the forms, make sure they're square and level, and have some
concrete delivered. Use a board to work out any air bubbles during the
pour, then use a long board and your helper to screed off the concrete
level with the forms.
You're not building a house, so I probably wouldn't worry about frost
lines. Just let it float on top of the ground. It may move up and down with
the freeze/thaw and end up slightly out of level, but so what? It's a shed.
For small pours like this, I prefer to use companies with trucks that mix
concrete right on site. There's less waste and you only pay for the
concrete you use.
Last time I priced an 8x12 shed, it was cheaper to pour a (4") concrete
slab than to build a wood floor with 2x6 floor joists and plywood. Concrete
is also nice if you're storing outdoor equipment such as mowers or tillers,
since you don't need to worry about water running off onto the floor.
I would make the upper edge of the forms with 2x4's, then add filler boards
underneath on the low end to accomodate the slope. You don't need to dig
the whole area down to the same level, but I would probably try to dig out
flat stepped areas so there's minimal risk of movement on the slope. I
would also dig a little deeper around the perimeter to make "footings", how
deep is up to you.
Plan to install some "J" concrete anchor bolts after screeding the concrete
so you can bolt the shed to the slab once it's set up.
Even if the shed shifts over time, or you decide you want it in a different
location, you can always move the shed and pour a new slab like we did:
http://www.mountainsoftware.com/projects/2005shed.htm
Anthony Watson
www.mountainsoftware.com/anthony.htm