Brewing with rainwater / tank water

I have the same problem. The only water I have available is tank rain water. The modern view is that it is not fit for human drinking but I've been drinking it for years and so do all the city people that visit. No one has had ill effects. I bring water from a near by town for beer brewing only because of all the negatives I've seen on chats. I am going to take the plunge and start brewing with tank water. I will let you know how it goes. I think city people are a bit over the top when it comes to tank water. It is much better to drink than chemically saturated town water. The only doubt I have ever had is that it is too pure (lack of calcium and very soft). I will filter out any particles accumulated in the tank and carbon filter first just to be on the safe side. As for all the comments regarding having it tested first surely drinking it for 25 years is good enough.

You probably want to get the water tested to ensure it is fit for human consumption. The rain itself should be fine, but the roof surface and storage vessels may not made of food grade/food safe materials.

Here is a list of water testing labs from the NZ MoH: http://www.drinkingwater.esr.cri.nz/mohlabs/labsfornzregionalpha.asp?NZRegion=NZNZ01

For Auckland you probably want to talk to these guys as they have the widest range of tests available: http://www.watercarelabs.co.nz/Pages/default.aspx

And here for a bit of fun is a link from sciencebuddies about how you can analyse the rainwater yourself: http://www.sciencebuddies.org/science-fair-projects/project_ideas/EnvSci_p015.shtml

These guys will also test the dissolved salts for you and that will give you a baseline for you to rebuild the water profile you are looking for.

=== Edit

As you mention in your comment: "The tanks and pipes used are specially designed for drinking water storage" you should be fine then to brew with it.

In which case you will likely be low on a number of salts.

I would add some calcium salts as these are required by the enzymes in the mash, and maybe a little sulphate, if you are looking for a crisp hop flavour. And may be a tiny amount of Calcium chloride, as the chloride enhances the roundness of flavour. You may also have to add some magnesium salts but I suspect you should get enough from the malt. Yeast likes to have some Mg++ ions.

Have a look here:

http://howtobrew.com/book/section-3/understanding-the-mash-ph/using-salts-for-brewing-water-adjustment

and here:

https://www.morebeer.com/articles/treating_homebrew_water

Here are some water profiles that you can shoot for:

http://www.brewersfriend.com/brewing-water-target-profiles/

Rain water can be very close to RO or distilled.

Rain is usually triggered by a solid particle (dust) and or atmospheric compression. Then as it falls it's collecting other particles from the air. Then again picking up particles from the collector.

I would have it tested not only for safety to identify potential contamniates but also to establish your water profile baseline.

Water chemistry plays a huge role in the quality of beer. If your rainwater is as clean as RO or distilled, you will need to build a water profile suitable for the style of beer by adding brewing salts. If you use straight RO or distilled your beer will be very lifeless and "flat" in flavor. I would liken it to the difference between a plane pretzel and a salted pretzel.

If the air is really clean, you have this part covered. But is it? It is not only about how clean it is where you live - that is, in your area, near ground level. Is it clean up to 2000 feet? Was it clean where water evaporated? What was in the air on the way? Start from reading about acid rains. And there is much more about this topic. Too much to put here. Rainwater most definitely is not the same everywhere.

Assuming it is clean when it falls, there are other factors. What is your roof made of? Glazed ceramic tiles are best for rainwater collecting as they do not contaminate water in any way. But if your roof is galvanized (zinc covered) steel, you may end up with too much zinc to your liking. If your pipes are made from brass, you will have a lot of copper and zinc. If your storage tank is made from plastic not meant for contact with food, you will have carcinogenic plasticizers in your water. And so on. And, of course, is your roof clean enough you would dare to serve dinner directly on it? And so on.

Maybe, just maybe, none of the above problems affect you. But you should check, carefully. In my country, chemical analysis of possibly drinking water is under $50. By our sanitary and epidemiological inspectorate. Find something similar in your area if you want to be sure.