Shed Foundation (reinforced concrete) (Ed.)

[jasonre]

Hi all, I'm going to build a 12ft x 10ft shed with 4-inch blocks. I have a foundation already with reinforced steel but it is only about 4 inches thick.
I know underneath is compacted with gravel, and the concrete is above ground. It's almost like a big slab.

My question is if I place another say 4 inches of reinforced concrete on top of the other 4 inches would this be okay to then lay blocks onto it?

Also, would it be okay to put down the DPM before putting on the concrete?

Any info is welcome.
Thanks

 

[Roy Bloom]

The slab is perfectly adequate for the floor as is, although damp proofing becomes a problem. The walls will probably allow some moisture through without some treatment to the outside surface. And there might be some humidity condensing on the inside of the walls.
What's the intended use of the shed? How important is the presence of some humidity?

For the walls, is it possible to dig a proper foundation around the outside of the floor slab?

 

[Ivor Windybottom]

Your intention is to make a raft, so it effectively floats on the soil as a single entity. This will require a very strong slab. There are probably tables available somewhere that state the required thickness for whatever size you want. But usually they still have a greater depth under the wall, which you don't have the option of adding as it's already there.

As stated above, the safer option would be to dig down beyond the topsoil around its perimeter and use the existing slab only as a floor.