Waist Slab for a staircase is probably more used as a slang not as a professional technical word. This term refers to a slab of the stair that is slanting from the floor slab to the landing slab. This can be imagined like a ramp – not including the steps. Such a slab does not exist in reality: the stair is either cast with the steps making one unit or it is created by beams holding the steps. So, by our opinion this word is used as a slang referring the minimum thickness of stair. Waist Slab Staircase Meaning and Technical Details
The meaning of the word is something usually used but not in the right connotation. The images bellow will show you the technical elements of a thick staircase.

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