We need water up high on the roof, clean and insulated. This needs to be pumped from storage on the ground after all filtration has occurred (and it could still be subjected to UV light for keeping it clean.
This could involve a float switch in the top tank that activates a pump in the bottom tank and just pumps to keep the top tank full. Alternatively, a simple pressure tank like you use with a well could do the job:
Pump + tank - pump to pressurize, and reactivate when pressure drops... < $500