Hovis builds a garden office*

@TW Yeah well I could try that, but it wasn't earth shattering money and I'm still stuck with the slab. I'd rather focus on just figuring out the best way to move forward, it's structurally sound so it won't matter in the end.

@Dammit Interesting idea with the vertical rebar. I was just going to pour concrete straight onto the slab if I went down that route tbh. Will give it some thought.

I went out with my string line and its actually not quite as bad as I initially thought, 33mm droop over a 3m length. Enough to bother me but not impossible to manage.

I was just thinking that the issue you'd want to avoid would be the second/top layer slipping under load, hence you'd want to locate it so sideways movement was prevented.

i.e. the nightmare scenario would be the weight of the shed pushing the top-up layer horizontally along the main slab toward its lowest point.

An SDS would put a 10cm hole into the slab in seconds, squirt in some epoxy, drop in the rebar, then either adjust to height, or cut it to the correct height with a grinder when the epoxy has set.

In the UK the general rule of thumb is that rebar must have 50mm of 'cover',
the reinforcing steel must have 50mm of concrete between it and the atmosphere to prevent the steel being exposed to moisture and oxygen to prevent the formation of rust.
Rust is lower density than the parent steel, and exerts a concrete splitting force as it forms.

Best bet, roughen the surface of the slab, probably too late to just rinse off some the cement slurry which I hope your builder vibrated to the surface, so either acid etch it with Hydrochloric Acid, (which you used to be able to buy at around 36% acitve in Janitorial Supplies distributors),
or mechanically abrade it with a chisel end on a (sizeable) SDS drill on 'Chisel function'.
You want to expose the tops of some the larger aggregate in the concrete.
A reasonable layer of PVA, won't do any harm as a priming layer then apply whichever screed, (polymer modified and shrinkage compensated/dimensionally stable), you can find in bag form locally. A 25kg bag should be not much more than £30.

Overthinking it. Its a shed. Add timber and packers. Job done.

I did not know that, interesting - thanks.

In the UK the general rule of thumb is that rebar must have 50mm of 'cover',

the reinforcing steel must have 50mm of concrete between it and the atmosphere to prevent the steel being exposed to moisture and oxygen to prevent the formation of rust.
Rust is lower density than the parent steel, and exerts a concrete splitting force as it forms.

So you're saying use GFRC then...

Agreed.

LFGSS--overthinking fixies since, uh, since before they were cool.

Add a screed? You could float a 10mm - 50mm screed over the whole length and get it level that way?

GFRC = glass fibre reinforced concrete?
The ridges in rebar prevent the drying/curing concrete shrinking as much as it would like,
putting the concrete in compression, where it is much stronger than in tension.

I seem to remember reading about concrete being reinforced with materials other than steel in structures where light(er) weight was required, bridge sections.

For a concrete slab rebar and mesh is so cheap it is hard to argue against,
as long as you have adequate cover, the frameworks is sturdy enough,
and the wet/liquid concrete is vibrated enough to avoid voids.

3m bifold doors though right over the offending region. If I don't level it out I fear I'll get jammed up doors later.

I'm pretty settled on packing it up too desired height now. Either a bespoke length packer under each stud, or a long firring strip like wedge. Leaning towards option 2, but need to have a go at making one first. I'm glad for track saws for occasions such as this.

The other option is to leave the slab alone,
and,
claim it has been designed to give a storm water-shedding slope.

I was kidding! I've been looking at GFRC furniture recently.

Interesting to read that about rebar though.

Yes,
if you've ever been round the Mediterranean, especially the eastern end,
and, coastal Turkey, you often see 'unfinished' properties with some thin steel sticking up proud from the existing structure. I've been told that this conferred some advantage on the builder or owner in that the property was not yet finished, and maybe not yet subject to some property tax or other. Certainly in Turkey, the steel used by jobbing Builders until the major earthquake near Istanbul in the early 2000s was smooth, and laughably small gauge.
Maybe only 4/5 mm diameter and only 4 strands in a column.
So no compression effect as the concrete dried/cured, and typically without sufficient 'cover' to protect the steel from rusting, especially in a coastal environment.
In Turkey, that earthquake did force the authorities to start enforcing Building Regulations, and the used of proper ridged rebar.

33mm droop over a 3m length.

I may have missed this, but is the floor going to be the concrete base? I'd assumed you were putting some sort of joists with a wooden floor over the top.

From a position of no informed knowledge I'm surprised that topping it up over a roughened surface wouldn't work well enough for a shed.

@mespilus - they do the same thing in Peru to avoid property tax.

Our old house was 8 cm lower on one side, over approximately 8 m. Bothered me at first but then I forgot about it. Was handy if anyone lost their marbles, always knew where to find them 😉

Jordan too. I'd imagine it's a fairly common practice.

Glass fibre reinforcement can be regularly seen in tunnel soft eyes in retaining walls where a TBM will be breaking through.

It's going to be PIR insulation panels on top of the concrete, then 18mm chipboard T&G, then finally engineered wood flooring.

I'm sure from a floor perspective I won't notice the sag. I just care about having square walls and a square opening for the bifold doors from an ease of construction POV. If i screw the stud walls down to the concrete base as is it will make it that wall and the connecting side wall trapeziums, and I'm sure later on will lead to annoying issues with sheathing and the internal wall panels, having to cut tapers etc etc. Just a ballache.

My wife suggested I mirrored my design to put the bifold door on the right where the slab is flat. I think I'll do that, and then do the packer / wedge thing for the left panel, and then maybe I'll use a floor leveling compound to raise it up to the rest after I've done the walls if I can be bothered.

Definitely agree on the ease of getting it level now. If it were me, I'd roughen the surface up (making lines/groves with a SDS sounds sensible) then just chuck some extra concreate onto it

But like I said I might be incredibly naive here....

...in my head, putting a weaker/less strongly adhered layer of 3cm concreate/leveling compound/similar on top of a strong base, which is then sandwiched down with the pressure of insulation panels, chipboard, followed by a wood floor sounds fine. It's a work shed, not a +2 story house.

What actually is it, in reality, that will go wrong?

The extra layer crack/breakup from downwards/sideward pressure of footfall, then shift over time, out from under the floor (or just one spot), resulting in that section of the floor sagging? Is that likely? If so can't you get some sort of slightly more flexible concreate to mitigate it?

What actually is it, in reality, that will go wrong?

The problem is that to make it level the new pour would be ~3cm thick at one end tapering down to nothing over a few meters.

If you were able to deal with the bonding issues it you might want to make it level by making the entire thing a minimum of 5cm higher (i.e. 8cm at the lower end).

Personally I'd be making it the problem of the person who was responsible for laying it. They need to fight it out between themselves and the pumping contractors (who are also partly responsible due to the problems they introduced.)

tunnel soft eyes in retaining walls

I can comprehend all the individual words,
but, I'm not certain if a predictive text 'correction' has crept in here.
IANAG
(I am not a Groundworker).

Right. But would that thin sliver section actually cause any issues once it's covered with a load of floor materials squashing it down? Just curious to understand more from folk with a bit of knowledge.

But good point on raising the whole thing up.

Any cementitious screed is a judicious mixture of cement and graded aggregates to get sufficient density and interlock to give adequate compressive strength and impact resistance.
A flowing grout, (Can be as thin a 'screed' as you can find), may 'work' down to just a couple of millimteres thick, but would be too expensive for filling thicker layers/voids where larger aggregates can be used within the thickness.
You also need to consider de-aerators within the screed to ensure that air is not incorporated during the mixing process leading to voids that can encourage water penetration, (and eventually carbonation of the concrete as CO2 dissolves into the water, forming weakly acidic hydrogen carbonate anions that slowly reduces the pH of the concrete and leads to conditions favourable to rust formation),
and,
shrinkage compensation, (standard cementitious materials shrink as they dry/cure).
And you want the screed layer to adhere to the existing slab to prevent a plane of weakness.
(This is why I suggested upthread a mechanical 'key' for any post-slab screed).

Have you considered paving slab plinths on beds of cement so you then sit your shed on these multiple points? Like some sort of miniature mobile home. You could get it level then if you started at the high end and adding more muck as you get to the low end. Then nail a skirt of wood boards around the base sealing it up from the elements.