Flat slab is a reinforced concrete slab supported directly by concrete columns or caps. Flat slab dont have beams. They are supported on columns itself. Loads are directly transferred to columns. In this type of construction a plain ceiling is obtained thus giving attractive appearance from architectural point of view. The plain ceiling diffuses the light better and is considered less vulnerable in the case of fire than the usual beam slab construction.The flat slab is easier to construct and requires less formwork. The thickness of Flat slab is minimum 8″ or 0.2m. This is a one of the types of concrete slabs.

It can be used where :

To provide plain ceiling surface giving better diffusion of light
Easy constructability with economy in the formwork
Larger head room or shorter storey height & pleasing appearance.
This kind of slabs are provided in parking
Flat slabs are generally used in parking decks, commercial buildings, hotels or places where beam projections are not desired.
Advantages:

It minimizes floor-to-floor heights when there is no requirement for a deep false ceiling. Building height can be reduced
Auto sprinkler is easier.
Less construction time.
It increase shear strength of slab
Reduce the moment in the slab by reducing the clear or effective span
Disadvantages:

In flat plate system, it is not possible to have large span.
Not suitable for supporting brittle (masonry) partitions.
Higher slab thickness.
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THERE ARE FOUR DIFFERENT  TYPES OF CONCRETE SLABS (FLAT SLABS):-

Slab without drop and column without column head(capital).
Slab with drop and column without column head.
Slab without drop and column with column head.
Slab with drop and column with column head.