Bricks, Masonry, Asphalt and Concrete - Concrete slab help 24' x 20'

I am cosidering building a workshop on a Concrete slab that I want to do myself. I have some do-it-yourself experiance but have a few questions. First because it would be fairly large, ( 24' x 20' ) do I need to pour footings first, then pour the slab inside? What is the best way to prepare the ground? I believe I have good draining soil but how do I know for sure? I've seen slabs prepared so that around the edges the concrete is a good 12'' the rest of the pour is gradual to 4''. Does this sound right? Can I get away with 8'' at the edges? If not what should I use for forms? 2x12's maybe or something else a little cheaper. I live in the Mountains in Canada, so cold weather is a consideration. Last thing I need to now is, can I get away with useing wood stakes for the forms or should I try to get a hold of some steel ones? :confused:

About the only 'slab' foundation you can use for your climate is a 'frost protected shallow foundation'...otherwise you will need to build full foundations to below frost line depth for your region and that can be a considerable problem and cost a lot more money.

With a Frost Protected Shallow Foundation, you use foam insulation to make up for the lack of depth.

Here is a helpful link:

http://oikos.com/esb/43/foundations.html

A conventional slab foundation may not be practical for your location because of frost considerations.

You can get all the information you need about your soil type and how to address it, how to construct a conventional or frost protected shallow foundation, and information about frost depth and other considerations by contacting your local building code enforcement office.