We used rainwater/snowmelt roof catchment for many yr. until we put in a well last yr. At the beginning, we had a cedar shingle roof, later went with a steel roof. The steel was far, far superior in water collection and cleanliness. The wood, as with asphalt and has been already pointed out, has many crevaces and rough areas that collect pollen and dust that then come off with the water. The steel also collects these things, of course, but it is washed clean relatively quickly. Now, steel roof is more expensive than asphalt, by a large factor. However, for water collection, it is quite superior. And once the runoff is clean, with steel or other material [I've read that tile is good, as is slate], the remainder of the water collected is clear and pretty clean.
We collected ours via gutter downspouts plumbed into a cistern I built from cinder block set on a concrete pad and lined with a plastic cystern liner [these come in potable materials if ordered that way]. Inside dimensions are roughly 9 ft. square by 5 1/2 ft. tall. This gives us 2250 gal. of water storage, which for us is several months worth as we don't have a flush toilet.
You will find that the rain collected is soft, which is nice. You will need to design or buy a way to pre-filter the runoff to keep out leaves, pine needles, insects, dirt, etc. that all collect in the gutters or wash off the roof. A screen of hardware cloth at the downspouts will keep out the larger stuff, though you need to keep those screens cleared or they'll cause a backup. I cut them from hdwr cloth, free basically. But you will need to also filter the runoff for pollen and dust and etc. somehow.
A friend of mine lives in N. M. in an area where many people use catchment/cistern systems. Wells produce water that is so hard and so nasty they don't want to use it. Many of them simply drain the cistern once a yr. or so, get inside and clean them out rather than filter. Too much maintenance for me, plus you are not collecting water and dumping what you have during that process. But it works for them.
The health aspects I've come up with include the bacteria etc. you will get off the roof [think bird dookey and dead insects and so on]. Then, the water will be sitting in the tank a while, where bacteria can grow. You can treat the water with chlorine or something like that. Of course, you'll have the taste/health issue from that. We put in a counter-top filter, carbon canister, and it worked great. We also had an inline filter prior to the carbon filter, one of those you can get at any hardware store and use those replaceable filters that cost $4 apiece to make sure we took out any tiny particulate material. Also, the inlet line from the cistern can be suspended a few inches or a foot off the floor of the tank to keep the major detritus from getting into the pipe. We have a screen and footvalve setup on our line also, as detritus is death [or earlier death] to household pressure pumps.
In any case, in 18 yr. or so I cleaned out my cistern maybe three times.
