Extending concrete slab

28 Nov 2016
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United Kingdom
Hi, i have a concrete shed base slab that is 6x4, but i want a 6x6 shed. The slab is ok except for one edge where it is only two inches thick, so dont want to remove it, as its only for a shed.

Whats the best way to extend the slab, ie cast a 6x2 slab next to it? Can i sbr bond the edge of the old slab, then cast the new?

Regarding the 2inch thick bit, i would prefer it to be a bit thicker for the shed. I will be building a brick plinth on the slab, then building the shed on the brick wall (which will be three courses). I can see two options:
1) Dig under the 2inch edge of the slab and pour a little extra concrete. How deep would this need to be under the old slab, and how far under the slab?
2) Or, i dig under the old slab by the required amount (from q1 above) but also pour a 4inch wide strip alongside, so that i can lay the bricks on this part of the slab, which i can then get to a minimum of four inches, at the same time as reinforcing under the old slab.

The old shed was resting on bearers running the whole length of the slab, the new shed will be on a brick plinth, around the perimeter, so i assume the loading of the slab will change from a fairly uniform loading, to just loading around the edges, hence my desire to reinforce the edge.

Finally, would i be better off building the brick plinth off of the slab, or digging new footings around the slab and building off that?

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No you can't joint slabs with SBR like super glue.

Lay a few 2x2 concrete paving slabs instead of messing about with concrete for no benefit.

Yes you can lay bricks on it, on the eadge but it would be wiser to support the whole floor on intermediate supports across the slab.
Thanks for the reply, i'll look into slabs instead, probably easier too. Could you explain how a 'day joint' works, isnt this the same principle?

Could i lay slabs over the old concrete base, to give a more uniform appearance inside?
A day joint, presumably you mean a construction joint, is just where a poured bay is finished where it will be continued at a later time. You are just adding a bit more concrete to some that's already there.

Yes, you can slab over concrete.
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Thats what i was thinking, which i was wondering why i couldn't just extend my slab, in the same way?


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