New floor advice for old house

20 Feb 2021
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United Kingdom
I have purchased an early 1900s stone built mid terraced house and it needs a new floor completely, old concrete slab is broken and wafer thin.

I have begun digging out said old floor which contains no hardcore and about 1-3 Inch of concrete. I was expecting to replace the floor with:
Hardcore -> Sand -> Damp membrane -> 100mm concrete -> 100mm insulation then underfloor heating in a ~75mm screed.

But i have been told by an ex builder in the family that i can go without the 100mm concrete and it should be fine...this didnt sound correct to me as there would be no truly solid floor. Though it would save time and expense filling it with more hardcore I would rather do it properly as I certainly dont want to revisit this in the future.

Any suggestions much appreciated.
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I would echo sxturbo's comment about including UFH. Why not at this stage? Here in France there is a big push on Heat Source Pumps (air or ground source, with almost 100% grants awarded, depending on criteria.) and UFH lends itself to these sorts of heating systems.
Only one issue to consider really, IMO. include the concrete slab in the 'heat sink' or to exclude it.
If the concrete slab is part of the heat sink, i.e. the UFH heats up the concrete slab, the insulation is placed below the concrete. A mat is used to protect the pipes from the concrete. But the heating cycle is more like a four hour cycle.
Whereas if the heat sink does not include the concrete slab, the insulation is placed above the concrete slab, and the UFH pipes laid on that insulation. This apparently gives more like a two hour heating cycle.
The screed in both types is unaffected.
There's no obligation to complete the UFH system with the heat source, but at least the pipes are there for the system to be completed at a later date.
Obviously, the manifold and pressure testing must be completed.
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