J. Colton Hill- rain water

I thought it was very interesting when you talked in class about designing a system that catches the rain water that lands on your house and storing it to use for lawn and garden purposes. I am trying to figure out how to crete a similar system myself now.

My parents house have a set of four granary buildings like the one in this picture:

There are four of these round granaries in a row that lie adjacent to a large square garden approximately 30 by 30 feet in dimension.  We use quite a bit of water to keep the garden growing.

I was thinking that I could possibly attach some kind of metal lip on the outer perimeter of the top of the granary. It would stick up approximately half of a foot so that when water falls on the roof it collects at the lip like a typical gutter system. Each granary would have a gutter shoot on opposite sides of the roof from each other.  I could then route the pipes to converge to a single pipe that leads to a large tank on a large stilt like stand.  The rain water could then collect in the tank and be ready to use in the garden area.  I am not sure if the pressure will be sufficient so a small water pump might need to be added.  It was just a thought that started swimming around in my head after I heard you talk about something like that.