Wonky concrete shed base

Advise please if you will.

Having removed an old rotten shed to replace it with a new one I find that the previous owner had cast 2 x 6" deep concrete slabs next to each other which made a base for a 6 x 4 shed.

One is perfect (level and square etc) the other has dropped / sunk by 2" at one end and tapers up to almost nothing at the opposite edge.

So I need to level it up but don't want to dig out a 6" deep slab and cannot 'jack it up' and underfill it.

The 'wedge' I need to lay on top of the sloping base is about 4' x 4'.

I think the best way is to paint first with 1:1 water / PVA mix, then lay concrete straight away on top.

Would it be acceptable to lay postcrete (like Wickes sell) straight onto it and then just sprinkle with watering can ?

It doesn't need to look pretty - just needs to be strong enough to hold 7x5 shed (plus any junk inside) and withstand the British weather.
I have very restricted garden space available so not really able to mix up own concrete.

What do ya think chaps and lasses ?

Think you'll have problems feathering up to the existing flat surface with postcrete. Easier if you mix up sharp sand and cement (3:1 mix) and lay it wet - you don't need the strength of concrete since you already have a base to lay it onto. A slightly better solution would be to overlay the whole area with slabs on a wet mortar base - the 400 by 400 from wickes are easy enough to handle and cut.

Alternatively, since your new shed is larger than the existing base, you could just lay timber bearers beneath it and install an osb floor after erection, if you catch my drift

Cheers dextrous,

I like the slabs idea (never even entered my head) but the existing concrete slabs (both 4' x 4' in size by the way) are already proud of the surrounding paving / paths.
So although good idea it would mean a big step up into the shed and ramp etc (and would look a bit odd ).

I think I'll go with the 3:1 SS / C mix and PVA to make it stick to old base.

Ta v much

What's 'osb' ?

Ah - found it . . . . . .

Oriented Strand Board flooring - OSB Floor is the daddy for residential and commercial construction, including prefabricated and modular home applications.Suitable for carpeting and hard-flooring, the rigid sub-floor it forms is recognised by many major hard-flooring manufacturers and associations.


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Slight concern about how you intend to enlarge the base area footprint to allow for the larger shed - what are you intentions? The mortar mix I suggested will suffice for levelling the concrete, but not really to extend it.

The extended part has already been compacted and filled with hardcore, sharp sand. Am gonna 5 blob 10 x 18" square flags in 2 rows to widen the edge that's gonna be visible. So it looks a bit like this from above >

_____________
|__|__|..............|
|__|__| a...........|
|__|__|-----------|
|__|__| b...........|
|__|__|_______|

Excuse the crude formatting !

a = the C / SS mix to level it to same as 'b'
b = already perfect
Flags will go down left and front side facing garden / patio

You seem to have it in hand - the only thing I'd do different would be to go for full wet mortar coverage under the flags - ridged (as bricklayers do) prior to laying them, knocked down with rubber mallet. You probably already done this - but it's worth erecting temporary shuttering around the whole of the intended base to help get slabs and mortar top up exactly flat.