I think you have laid the slab /roof. The best method is to finish the roof top with cement shullary and water proofing compound at the time of laying the slab.For this situation do bitumen painting on the roof surface, stick polythen sheet on the bitumen when it is hot and press fully.Then lay the layer of brick tiles and sealed the joints with cement mortar and cement pointing on tile surface. For more detail you should visit gharexpert. com website DAMP PROOFING SECTION where lot of articles above damp proofing help you.
If you do the process according to the instruction then the roof will be leak proof.