This question is related to the situation in an apartment in a reinforced concrete building that is approximately 30 years old.
The ceiling is almost perfectly straight and level.
However, the floor from which the existing screed was allegedly removed is lop-sided or crooked at many places, having differences between spots in the same room that go to 4 cm.
Is it possible that there were multiple layers of screed and only one was removed? Is there any way to determine if what I'm looking at is the reinforced concrete slab itself or another screed?