Laying floor with tar/bitumen screed

Hello,

I am currently renovating our bathroom from 1979. We now want to install a walk-in shower. I have removed the bathtub and leveled the floor as best as I could, meaning I removed the loose bitumen, etc.
Now I am reading on the internet that before laying screed, you absolutely have to remove the tar and bitumen residues 100%. Is that really the case?!

The screed will be about 2-3 cm high and has an area of about 1.5 sqm, it is supposed to serve as a level base for an XPS shower board.

I had hoped that there is some kind of primer or something similar, or that I can lay screed directly.

Best regards Neubauer

Hello Mr. or Mrs. Neugebauer. Unfortunately, internet searches often reveal error messages, half-truths, old-fashioned things that once represented the state of the art – or did not. Therefore, one should distrust everything that is not communicated currently. If the bitumen felt ([Tar was certainly not installed indoors in the last 100 years]) has been largely removed so that it does not interfere, then that is fine. Bitumen residues must only be completely removed during flooring work (which also includes parquet and tiles). However, the screed cannot only be 2-3 cm high unless it would become a composite construction with the concrete underneath. If that were the case, then indeed all loose components would first have to be removed. Is the dark layer firm, or can it be scratched? If it is firmly attached, then one can also create a suitable substrate for a composite screed with a reactive resin primer, which is fully sanded in the wet phase. If the black layer can be scratched, well, then it must be removed. A primer is an adhesion promoter. If the substrate does not allow construction on top with it, then one must remove or grind down to the "solid meat" (as one colloquially says in the construction industry). ------------------------------------ Regards: KlaRa