First it depends on how much money you are willing to spend for your piece of mind for a level floor. If it were mine, and i repeat, if it were mine, i would put a wood floor over the concrete. Here is how i would do it, pour the footer for the end walls only. The floor itself would be constructed of treated wood and layed out the same (16" on center) as you would if it were built untirely of wood, only tapering the joists from the high point to the low point and make sure everything was tapconned to the concrete. I am not the best at explaining but if you have a question send me a PM. OMT -- i agree with WoodnThings on his method of leveling the floor except i would get a quote on the price of concrete versus the cost of the wood only because of the amount of drop you have 8-1/2" from high to low point and the sheer size 20'x40' with an average of 4-1/4" depth on the concrete. LOL, i am only about 150 miles away and i work cheap, just kidding.
Last edited by garryswf; 11-02-2009 at 10:59 AM.