Shuttering question

Planning a 6m x 3m shed on a 4" concrete slab with 5-6" of sub base, I have a question on the shuttering. Is it normal / best to put in shuttering for the whole of the depth of the base - ie around 10" in my case - and then compact the sub base within the shuttering? Or is it best to lay and compact the sub base, then put in 4" shuttering on top of the sub base to contain the concrete?

Thanks..

chris

a thickened edge is useful, as this is where damage or failure is most likely to occur. Additionally, by filling a boundary trench with concrete before putting in the sub inside it, you can prevent it being exposed or moving at the edges as it might do if unrestrained.

Thanks for the reply - what do you mean by a thickened edge exactly? The whole of the sub base will be below ground level so constrained by the earth - although I accept thats not the strongest thing in the world!

As I understand it, the other way is to dig a trench round the area that the sub-base will occupy and fill that wil concrete before you put the sub-base in to keep it in place - corrrect? That sounds like quite a bit of extra work - and concrete! Is that a bit overkill for a shed slab?

Still curious to get views on the shuttering question..

Thanks

Chris

crushed hardcore has a tendency to migrate sideways, especially under wet conditions, or if rodents find a way in. The "thickened edge slab" gives a foundation for the walls as well as a (thinner) floor for the outbuilding.

The floor is usually raised above ground level, which means the subbase will be very close to level

I would install the shuttering, making sure top edge was at required level of concrete base, put the crushed hardcore in the rectangle tamp it down but try to 'V' the edge next to the shuttering, when the concrete is poured, the edge nearest to the shuttering will be thicker, and smooth, while retaining the hardcore in position.
When concrete is set, strike off the shuttering for a nice smoothed edge slab.

Wotan