Stop before you get to potable water. Stop well before potable water. Rain, stormwater, precipitation, or whatever you want to call that falls from the sky is relatively pure compared to water from the surface of the earth. Well, that's probably true unless (until?) we get air moisture harvesters, of which I have no clue about. But the keyword is "relatively".
The moment that rain hits a surface and runs along it, it collects and carries contaminants. Remember what you wash your hands with? That great universal solvent is only as clean as what it touches. If you are thinking about rainwater harvesting, then your catchment surface is most likely a roof. It may be the ground if you have some specific use in mind that doesn't involve cleaner water. Note that rain can be contaminated with other substances in the air on its way down - think acid rain. Unfortunately, I am not an air pollution expert, so good luck!
So, let's assume that you're using your roof. What is so wrong about it?
Think about what your roof is made of. Not just the roof itself, but also the conveyance system like the gutter. Residential homes may use asphalt shingles, metal sheeting, clay tiles, and whatever people come up with. The conveyance system (i.e. gutter) may be painted aluminum, copper, or something else. None of these materials are food grade materials nor were they ever thought of as things to eat off of. What you need to know is that they may leach toxic substances into your water. Given that risk, it is generally not recommended that you drink rainwater collected from your roof, or use it for any potable purposes. This means don't wash your dishes or cook food with it.
And I haven't even gotten into your neighbor's BBQ ashes; the birds, raccoons, squirrels, cats, and whatever local fauna are in your area that have let surprises on your roof; and everything else in the air that decided to settle and attach itself to your roof. The moment it rains, that will all be washed into your rainwater. This is exactly why all rainwater systems need a "first-flush" system, which just gets rid of the first batch of water to prevent it from getting into your storage tank. But we all know that anything short of a pressure washer will not remove stuff like bird crap from any surface. The result is continuous leaching of contaminants into your water, though at a diluted concentration. It's good to be paranoid when you work with drinking water...
Having put the fear of god into you, I'll let you know that there is lots of literature on the interwebs concerning building rainwater systems. They usually include the same warning I just gave, and also some hints on what some of the safer materials are.
With that out of the way, we can get started!
Legality of Rainwater Systems
Check your local laws, by-laws, ordinances, or whatever. Many jurisdictions in North America have no laws relating to rainwater harvesting and many that do couple it with greywater, which is usually dirtier. Long story short, check if it's legal to set up a rainwater system or not. If it is, check out what you need to do, be it permitting or labeling. Many localities require non-potable water outlets to be labeled properly so no one accidentally drinks from them.
Last I checked, Illinois was drafting legislation, and Ontario was putting some wording for it into the next building code. I believe Oregon, California, Texas, and other "dry" states have some firm legislation in place. They are great sources for guidelines and "how to do" publications. Check them out and use rainwater at your own risk.
How much water can I get from rainwater?
Your water supply with rain is a function of how much it rains in your area, how big your catchment area is, and your storage capacity. There are others I probably can't remember right now like evaporation, leakage, etc. Unless you live in a desert or have a really small house, storage capacity will probably be your biggest concern.
If you're Canadian, then thank the government and your tax dollars because Environment Canada keeps records on precipitation through Climate Normals. This will give you average annual or monthly precipitation quantities in rain and snow form. Another perk of being Canadian is snow and sub-zero temperatures except for probably parts of British Columbia (*cough* Vancouver). Try to figure out how to get a volume using information like that, or refer to one of the available guides on the interwebs.
