Dear Paresh
The thickness of RCC slab should be 4'' to 5'' according to design/ span.In gharexpert web site construction section you will find lot of article about leakage protection in dampness & roofing sub section. See the link below.
http://www.gharexpert.com/articles/Roofing-Ceiling-1998/Roof-Leaks--Causes-Repair_0.aspx