62.4lb/ft^3*7 ft (vertical distance from tank to faucet, roughly)=436.4 lb/ft^2
436.4lb/ft^*(1 ft^2/144in^2)=3 psi
I think that explains why people don't use tanks on the roof.
Now there is some velocity to factor into the equation once it starts moving, but I think that gives you a pretty good idea why people don't use tanks on the roof. You just don't get any pressure. Without the pressure you will have to make up for it with volume (like to get soap out of your hair) and we just don't have the resources to do so.
Typical bathroom type fixtures are designed to run on about 40 psi and are restricted such to deliver the proper spray pattern and strength at that pressure. If you drop below it I think you will be disappointed.
What you CAN do is something similar to what I did and just pressurize your water tank with air. My water pressure is NOT consistant as a result and varies within 1 psi of what the air pressure is in the tank, but it's silent and easy to deal with. Realistically, 15-20 psi is more than sufficient for most purposes, but you'd need to have your water tank mounted 35 feet above your bus to achieve this.