The run off from the shed roof should be all you need. I was going to use the garage and or house roof for my collection but after thinking about it I figured it would be cheaper and easier to just put a simple roof over the work bench I use for storage.
I have not added it on yet but one day I did see a piece of gutter in the trash. It was wider than standard house size gutter and already had a gutter guard added on and was a bit longer than what I needed. When I rebuild my gardening bench it will all come together.
