20mm screed in a plain 1:4 cement/sand mix?

Quick sanity check:
I've just laid 20mm marmox in the bathroom and have left cutouts for the bog and bath feet (wide enough to marry with most standard baths) as suggested by Marmox technical as these are points of higher load concentration (bath moreso but the bloke suggested both).
I need to infill these with screed to level tomorrow - then give it a week curing and drying out time.
I *could* use an SBR mix, but I feel there is merit in allowing the substrate under the marmox to breath slightly. Anyone see any problems with a 1:4 basic mix at 20mm thick? It will be bonded to the substrate with cement slurry.
Ta
Tim
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Tim S wrote:

Bit on the thin side - but it will probably be ok. The other option would be a self levelling mix with some sharp sand it it.
--
Cheers,

John.

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John Rumm coughed up some electrons that declared:

That's an option too.
I know an SBR screed will go down to 10mm - but then I considered that I have <random stuff[1]> sandwiched between 2 impermeable layers (30mm bonded SBR screed underneath and marmox on top). It's a bathroom - so it is inevitable that water will get in sooner or later. So it seemed a good idea to allow some paths for it to get out, even if it's very slowly via just the grout lines.
[1] Random stuff is 10mm levelling compound (because I cannot dry screed level!) and tile adhesive - neither of which is really rated for continuous dampness.
I'm over worrying as usual but my philosophy is if a precaution can be added for little effort, it's worth doing...
Cheers
Tim
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