Rebuild the floor from scratch

Hello everyone,

I am new here in the forum and a proud owner of a home. However, I have a question that’s bothering me and hope someone can give me a tip here. The house is from the year 1890 and built with bricks as well as half-timbering on the upper floor. I am planning or rather already in the middle of gutting it, and have almost brought it to a shell condition. The actual plan was to remove the screed down to the floor slab, insulate, and install underfloor heating. But what I had to find out is that there was no floor slab, or rather one part stands on a vaulted cellar and is filled with ash and the other larger part on clay. This is a bit "uneven" or I have now removed about 40 cm on one side and about 20 cm on the other side. Now theoretically I would have a lot of space to insulate, but how if there is no concrete slab. I thought about using a filler material to bring it to one level, maybe an insulation board like XPS and on it a studded mat for the underfloor heating and then pour an anhydrite screed on top, but does that hold? The heating engineer is unsure whether the insulation boards can withstand that. He said the screed would weigh about 75 kg / m²? How do you then compact the fill under the insulation boards? I would almost have said a vibrating plate but I can imagine that it might cause cracks in the load-bearing walls. Does anyone have any ideas here?

Best regards

I'll follow up to see if I understood correctly: the part that concerns you is the non-basement section of the ground floor, whose rammed earth floor you have leveled to an approximately flat level, and which now lies on bare earth with an unknown (and probably uneven) thickness, resulting in an unpredictable structural situation?

In that case, I would probably try to create a compacted subsoil using rammed concrete.

Regarding mechanical vibrating, I share your concerns.

Good morning, I have not removed any of the clay soil; it was probably made that way over 100 years ago. For the last xxx decades, there has been a concrete screed with various additives on top, presumably whatever was available. The question or thought I have is, with what can I level the surface, and can I put about 10 cm of insulation on it, or will that be too unstable? What load can such an insulation board bear? Thank you very much

For the last xxx decades, there has been a concrete screed with various additives on it here, presumably whatever was found.

For a house from this period, I assume that on a (constructed in multiple layers) rammed earth layer, there was again one like this in rammed concrete. In layman’s terms, that ("various additives ...") would be exactly the recipe.

You can google both, they are making a comeback for historical restorations.

What can I use to level the surface, and can I put about 10 cm of insulation on it, or will that be too unstable? What load can such an insulation board carry?

Do I understand correctly: You want to use an insulation board, which would be placed under the slab in new construction, here as a direct base for a screed with embedded underfloor heating?

I don’t know what compressive load such boards are suitable for. Additional forces would occur here if the substrate is not sufficiently compressive-resistant. I would fear forces that would exceed the elasticity of the heating installation transversely.

Therefore my thought is to recreate a floor like the one previously used (only level). For that, you could measure a height with battens and finally finish with self-leveling screed. But that is just an idea; I have not built it myself.

Yes, that could be rammed concrete. The plan is really to properly insulate the screed slab, as an idea 10 cm XPS or if foam glass gravel e.g. 20-30 cm is sufficient, here the underfloor heating and the screed on top. Had hoped to be able to save on the concrete.

Presumably, it will be difficult or only by luck to find the right solution in the forum for such a case (from a distance). Don’t you have any problems with the ceiling height given the age? I still remember my mother’s parental home, also from 18xx, where the ceiling heights were just over 2m, walls made of clay that were probably “smoothed” with the bare hand, and a little brook ran through the cellar. In such a case, hoping for a concrete slab foundation is, in my opinion, very optimistic. Can the current floor not be excavated as far as possible, then some kind of separating layer (gravel, crushed stone, etc.) applied, and then only a thinner reinforced concrete slab or possibly made of WU concrete, then also without gravel, be poured, on which everything else can then be built?