Sub-base area for shed base?

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When you lay a slab for a shed base, do you compact the hardcore and THEN put the formwork for the concrete down on top, or is the formwork laid and then hardcore compacted down within the boundary of the formwork?

If it makes any difference, 'm planning on 75mm MOT1 followed by 75mm concrete, for a 10x12 shed. Only the top 25-50mm of the concrete will be left showing above the finished ground level.
 
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I know this is dragging up an old thread but I am embarking on something similar and wondering the same things?

I thought for my 3.5m x 4m shed I would need a slab of 3.8m x 4.3m and a sub base of 4m x 4.5m...effectively like a pyramid so it's all supported effectively?!
 
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depends what you want to keep in your shed! but I built a workshop 10 years ago with the slab 25mm (an inch) bigger than the outside wall size, 9" concrete under the walls and 3-4" concrete for the slab. All this was on a few inches of hardcore very well compacted. I dug out, put the levels in, fille dwith hardcore and then compacted...filled with concrete, levelled, first layer of blocks, dpc then built up from that...flat felt roof

Stood well, no leaks cracks or movement for 10 yrs. Then I moved out so presume it's still standing... had benches, lathes, mills etc etc all in there no issues with base cracking...
 

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