Is it possible to collect rain water from a roof for drinking use?
It turns out people have consumed rainwater for millions of years. So there's some precedent.

This is very well covered online, so grab a search engine and get reading. Here's a summary of concerns.

In the US, it may be illegal (look up "water rights") but for individual home use, that is usually ignored.

You'll get a lot of water from your roof really fast. It's pretty amazing. 

The biggest problem is the dirt on your roof. The rainwater coming down is great, but once it hits your roof, it's dirty. You can redirect the first bit of rain, and then collect once the roof is clean(ish).

The roof material matters. Some roofs will leach small amounts of toxins in to the water that you probably don't want to drink. Metal is probably the best choice for a roofing material.

For most homes, only a small portion of the water goes to drinking. Much of it goes to laundry and flushing toilets and irrigation. These systems don't mind slightly dirty water (including greywater). Use your collected rainwater there first. You can haul it by bucket or plumb something special. 

A problem specific to collecting rainwater for irrigation is that it always comes at the wrong time. You would have to save it from the wet season to use in the dry season, which implies a huge cistern. Being clever helps. For example, a cistern can go under a deck.

Any water storage should be protected against mosquitoes. You can put fine mesh screens over openings, or (my favorite) put goldfish in the tanks to eat the larvae.

Cleaning dirty water to be drinkable is harder. If you build a complex filtration system, you'll probably negate all the financial and ecological benefits of saving that small amount of water you were drinking. My favorite approach is the slow sand filter - it's simple, cheap, low energy, but it takes a lot of work. read here: http://www.homestead.org/TedPraast/SandFilter/Filter.htm