A flat slab is essentially a slab with no drop beams. Think of it as a ‘sheet’ of concrete extended in both X and Y directions and supported on columns. Sometimes, flat slabs have a perimeter beam, they call it a ‘spandrel beam’.
There are two types of flat slabs:
1. Flat plates – just like I described, a sheet of concrete sitting on columns and that’s it
2. Flat slabs – also as described, but with an added ‘drop panel’ at the columns. A drop panel thickens the slab and resists what we call ‘punching shear’
Note: A flat plate is every architect | interior designer dream because it lets them go wild with partitioning the floor plans and all.
Now for the solid slab;
This is a normal slab used in traditional design. It is generally a slab surrounded by drop beams. Solid slabs are less expensive because they require less thickness – however, are not a favored solution by architects since the drop beams stand in the way of creating a good open space.