On Mon, 18 Jan 2010 16:39:15 +0000, "David WE Roberts" wibbled:
Might be worth standing back a bit now. Lots of replies about major structural rafts etc.
How heavy is this all likely to be? Will the wall blocks be lightweight (eg celcon) or concrete? What's the roof - timber+felt or tiled or metal?
At one extreme, if it were a wooden "shed", you could quite frankly stand it on paving slabs laid on a bit of sand onto well compacted earth and it's unlikely to go anywhere. And if it did, the timber wouldn't care that much within limits.
At the other extreme, if there is a lot of load on the walls and the ground is crap and liable to move or wash away, you could go for a trench of concrete for the wall base and a separate floating floor slab.
It sounds like looking at conservatory build guides might be useful, and cut the floor slab down a notch or two. The size and load of the walls are likely to be in a similar range and it is the walls you want to ensure integrity. The floor cracking a bit over time will not be as serious if you under engineer it, compared to walls going.