Dear Ravi,
There are a lot of treatments available to fix leaking roof slabs, which treatment would be applicable in your case would depends on the extent of leakage, the type of waterproofing treatment that was used on the roof during construction, the way it has failed, the reason why it has failed, has the reinforcement been damaged etc. Problems due to leakage can be of different types and so treatments vary from case to case. So it is very difficult to suggest something based on the limited information you have provided.
If cracks are clearly visible on the top of the roof slab, one commonly used treatment is to drill holes at close intervals along the crack and grout it using a non-shrink cement slurry or epoxy. The crack also should be widened, cleaned and sealed with appropriate repair material. If the reinforcement has corroded, it has to be cleaned and treated before patching with repair material.
If cracks are not clearly visible and the leakage is limited to a small area, the damaged area should be clearly marked on the roof and the existing waterproofing treatment should be removed and fixed.
If the leakage is extensive, the waterproofing treatment whether it is a brick-bat waterproofing or a bituminous waterproofing might have to be completely removed and completely redone.
But, remember any leakage from outside of the structure should be fixed from the outside, fixing leakages by removing concrete or plaster from inside of the structure will fix the problem temporarily but after some time the leakage will reappear.
