rain drops are dropping on my head

So, with all grain brewing, the ph and water chemistry is one of the most important keys to brewing good beer. History tell us that different countries brew different styles of beer dependent on the natural water chemistry of their particular area. Rain water has a ph of around 5.6 with some trace elements. So to start with keeping the water ph between 5.2 and 5.5 when you mash in with your malt would be tricky. I would probably use pilsner malt and maybe a touch of specialty malt and I guess some chalk to help raise the mash ph.
You will also need to get the rain water analyzed so you know what youve got to work with and use your software.
Me personally,I think it could get quite tricky and limiting your brew variations.
Not saying it cant be done but I would stick to your town water supply. Gash will have more of an idea. Very good question mate. Cheers

Had a very interesting and informative chat with a now retired master brewer today regarding using rain water to brew with. When it comes to malts and ph it can get very complicated but basically malts these days have some sort of buffer and tend by their nature and the way they have been modified to manage to get into the ball park with ph. for conversion.
Having said that, your rain water would need to be analyzed and treated with minerals and salts to get it to make good beer. He gave an example of using ,I think he said double distilled water to make beer once and adding no additions and the beer turned out terrible.
It was such an awesome discussion today, and he told it in such an interesting way. Just a little bit of info, but not too much. Such a brilliant topic OZ. Thanks for throwing it out there. Cheers.

Excellent. Its often the way with this brewing . You can gather all the information you like but until you actually brew it then you know for sure. Well done. Cheers