I too had experience same seepage issue at one of the place where i reside in resent past.
To stop the seepage from the ceiling, i did following;
When there is rain or water is stagnant on the terrace, i sprinkled the cement dry powder on crack areas.
After sprinkling of the cement powder on crack area, the action of seepage will also attract the particles of cement through cracks and once the particles enters into it, they get locked and forms a gel to seal those cracks.
This is very effective way of treatment and also the cost effective one.
You might need to do the sprinkling twice or thrice at same crack area to stop the leakage completely.
You may also do brooming on terrace after sprinkling of cement or pouring of cement slurry on terrace.
There is one more way of treating it, but need to be done every year before start of mansoon.
Take Pidicrete URP and Cement, mix them in 1:1 proportion to form a brush-able compound and apply it like paint on terrace floor with brush. this will form a elastic layer on the roof and will be unbreakable during monsoon but as soon as heat starts increasing, the disintegration might occur in it.