ᐅ Floor Structure in a Renovation Project from 1935 – Questions About Concrete Slab and Waterproofing

Hello everyone,

I’m currently in the middle of renovating the living area of an old farmhouse built in 1935. The ground floor living space has been completely gutted and is being rebuilt. The old concrete floor has been removed, and right now only the dry, compact sand soil is exposed.

The exterior walls have already been newly insulated and clad with facing bricks (cavity wall construction: 11.5cm (4.5 inches) calcium silicate blocks + 20cm (8 inches) insulation + air gap + 11.5cm (4.5 inches) facing brick).
Now it’s time to start the new floor construction – specifically the concrete slab. The following points are currently unclear for me:

• Blinding layer or plastic sheet?
Some say a polyethylene (PE) sheet directly on the sand is sufficient. Others recommend a thin concrete layer (5cm (2 inches)) to create a clean surface. How did you handle this?
• Waterproofing below or above the concrete?
The old horizontal damp-proof course in the masonry is located about 20cm (8 inches) above the future concrete floor level. Now I’m wondering:

• Is waterproofing underneath the concrete enough?
• Or should I also add waterproofing over the concrete (below insulation)?
• How thick should the concrete slab be?
I’m not planning any load-bearing walls on the slab. My gut says about 16cm (6 inches) of C25/30 concrete should be sufficient. Or should I go for 18–20cm (7–8 inches) instead?
• Should the concrete be tied into the walls or not?
One contractor suggests cutting a 20–30cm (8–12 inches) wide slot every 80cm (31 inches) into the calcium silicate block walls so the concrete can bond in. Another says this unnecessarily weakens the wall. I’m leaning towards not doing it, since the calcium silicate walls have stood on their foundation for 90 years already, and these “concrete fingers” in the walls might actually create weak points, right?
What do you think? Has anyone done this?

I look forward to your experiences or tips. I’m open to all suggestions – including example constructions or photos.

Best regards,
Ole