The thickness calculation is a matter of structural design. However some guidelines are given below.• Calculate the weight which is born by slab i.e. self weight of slab+ weight of lean concrete+ weight of floor+ live load. If slab is at roof level, total weight can be calculated by adding self weight of slab, weight of earth, weight of tiles and live load. • Calculate bending moment by taking weight born by slab and effective span. Care should be taken that the span length should be effective span not clear span.
After this depth is calculated in respect of bending moment and tensile stress in steel, the depth available from calculation is effective depth and not overall depth of the slab. To find overall depth you have to add thickness of cover in effective depth.Thanks