Levelling sub-base on garage conversation

Morning all, looking for some guidance please on correcting a garage conversion. I asked some time ago a similar question, however the job has finally come around and not everything is as I thought it would be previously.

Previous owners had installed a floating floor DPC/22mmT&G/100mmEPS/22mmT&G. I previously thought there were joists under the bottom layer of T&G, but that was only half the case, the sub base slopes, and basically had been levelled really poorly using joists, bits of plasterboard and all sorts of things, which is what I assume lead to the terribly springy and noisy floor (that also hadnt be glued).

That said, it's all been removed and i'm left with a concrete subbase that is 165mm at the shallowest sloping down to 265mm working floor height at the deepest. I intend finish the floor with CemFloor Therm Screed with UFH installed.

Due to that fact i've lost insulation depth due to the slope, i was planning on taking the following approach:

- Levelling Sub Base (leaves me with consistent 165mm WFH) - Unsure how, MOT or do I need a pour? Trying to avoid getting concrete guys out twice and also avoid having to leave a large drying time before I can cover it with XPS.
- 100mm of XPS (to improve efficiency & compensate for loss of depth)
- Wet UFH and 57mm of CemTherm Screed

Thoughts welcome, particularly on options to level the sub-base. Also considered sticking an few extra 75mm sheet in the deepest part before levelling, but i'm not sure that will achieve anything.

Thanks in advance
T

Morning

If you don't want to get the concrete guys out, and depending on amount, you could build this up in concrete or screed, mixed up yourself.
Failing that, you could use PIR in a couple of thickness to build it up in sections (25mm/50mm thick bits), just fill gaps with expanding foam and foil tape it up. Then screed as normal.

I'd be using foil faced boards, it works well with liquid screed.

I've worked it out to be about .79m3 to bring the lowest point up-to the highest with screed/concrete, which feels a bit of a heft to hand mix it all with a paddle

It's a 5.2*5.4m room and a 4" drop from one side to the other.

When suggesting building up with different size PIR, do you mean:

***Top***
Screed
UFH
XPS/PIR
Screed
PIR (To level)
DPM
***Bottom***

Or do you mean just bring the level all the way up with PIR in section and then UFH pipes ontop and final screed.

**Top**
Screed
UFH
XPS/PIR
**Bottom**

I dont see how I can do this without either having the top layer of PIR with ridges where the thickness change and angles (maybe this is ok? UFH pipes would run up and down and then correct with the final screed), or I keep the top level but end up with big voids in the lower sections?

Any issues with just levelling this out with type 1 compacted down on top of the current concrete slab and then a sand blind to protect the DPM?

This is the profile, so most of the drop is in the first 2 meters.

I was suggesting building up the slope in thinner sheets of PIR.
Like stacking 25/50mm sections until you get to nearly level, filling any voids with expanding foam.
You can then go over all of this with the 100mm and 65mm screed. The screed will take care of anything not level/ridges etc.
Add a polythene sheet layer before screed.

Depending on where you are in the country a cube of concrete is likely to be around £250+ (use a local volumetric concrete outfit)

I think using MOT won't be straight forward, you will need 20 to dust and where it's think will be difficult to compact, then you will need some sand blinding.