The basic idea of waterproofing of UGWT is:
- Externally so that external pollutants should not enter the tank.
- Internally so that internally stored water does not leak out.
I would suggest doing the waterproofing treatment to the complete tank including ceiling as water vapor tends to stick to the ceiling of the tanks and eventually over a longer period of time will cause corrosion of slab steel of the tank so if you are quality conscious get it done to the full tank.