Is it okay to harvest rain water from my rooftop where I also have a small vegetable garden?

Roof top rain water collection from terrace garden space


I have a roof top terrace garden that I have tended to for many months now. I don't use any chemicals for the plants and the only addition to the soil is compost once in a few months. Is it okay to harvest rain water from the same place where I have this garden?

I was adviced by one person that i should clear the terrace space if i want to harvest water from there. Is that so and are there any other ways to do this (any creative designs/ filters that can take care of the garden contamination?)