Geometry 'sticks together'.
Draw a face.
Draw another face that touches it.
Select just the first face and Move it.
Parts of the connected face will distort as you do so, because they are also connected to the first face.
However, had the two touching faces been inside their own groups then they do not interact.
As Dave says, you have given the 'slabs' insufficient thicknesses.
Walls are anywhere from ~4"/100mm to ~14"/350mm depending on what they are made from and their function.
Floors and roofs will be about 1/10 of their 'span', down to about 1/20 depending on their materials and function again...