Pressure at the tap -- if pressure isn't being supplied by a pump, which I assume it isn't in this case -- depends on the height of the column of water. The rain barrels in my back yard are elevated 4 feet above ground level to increase the available pressure, and it's still much, much less than what I get out of the municipal water supply. I can run drip irrigation from it, or fill a watering can; I could not run a sprinkler without adding a pump to the system.
When doing this sort of thing, also remember that water is heavy. My two rain barrels approach 2000 pounds. Unless your proposed tank is small, make very sure that whatever is supporting it can handle that additional weight, or you may do serious damage to the building... And remember to allow for lever-arm effects as well as direct weight.