There are 2 thought about pouring the slab after the walls are fitted or before.
As I have moved house I need to build a new shed as well. My old shed had the concrete poured after which filled all the corrugation and keeps out all the bugs and spiders etc. The concrete was screened to a waterproof smooth finish and sprinkled with black oxide and neat concrete. It won't soak in water or
oil etc. It's easier to sweep and hose out. Even though it's level, with a high pressure cleaner, you can just keep spraying towards the door and it won't leave much water on the floor. Any small puddles can be swept away, and it will dry overnight.
The disadvantages they tell me is that the sheets will rust. My old shed was 30 years old and didn't rust. Maybe the steel these days isn't as good as they won't guarantee against rust if you pour the slab after the sides are put up. So they suggest the lip that muzza was talking about. The z flashing sounds interesting, I haven't been told about that, as they told me to rest the sheets onto the concrete. But as the concrete won't be perfect, you'll still get gaps. So I might consider the z flashing. Also interesting about bolting the frame after. Great thread. I wish I could get a shed that big. I need 4 meters high for my hoist. How tall is yours?
JD