Personally I wouldn't use rain water, especially if it is collected off a roof. Rainwater is typically acid, pH of 5.7 or so, and it may collect pollutants, bacteria and parasites(birds and such use a roof as the outhouse....!)
http://forums2.gardenweb.com/discussions/1668154/help-ph-of-rainwater-is-alkaline
http://www.practicalfishkeeping.co.uk/content.php?sid=6199
http://www.ukpreppersguide.co.uk/can-you-drink-rain-water
Can You Drink Rain Water
Essential Prepping,
Water Filter Bottles,
Water Purification
November 13, 2013
by
UK Prepper
Can You Drink Rain Water and is it safe to do so.? 
Collecting and using rain water can be a great way to conserve resources at home and in a survival situation it may well be your only choice for drinking water.
However, rainwater is not quite as pure and clean as you may might think, so never assume the rain dripping down from your roof is safe to drink.
Even as the rain pours down it has collected many airborne particles and contaminates.
As it runs down and off your roof it has a happy time soaking up anything that just happens to on your roof.
Typically, any form of waste or bird poo will wash down from your roof and then easily be absorbed into any rainwater that passes it. So if you’re trying to collect it, you can guarantee it’s already contaminated with something that can cause you problems.
Bacteria, viruses, parasites and numerous chemicals can also be carried along in rain water – all of these can lead to an upset stomach at least and severe disease at worse.
There are, however, a few things that can make your rain water better or worse, namely your location and how you store that water. Also the temperature the water is stored at.
Whatever collection method you use the water will not be anywhere near clean enough or safe enough to drink – in a nutshell, stored water from collected rain water should only ever be used for watering plants or washing your car..!
From a preppers perspective and a survival situation you would most likely be collecting rain water for drinking and cooking purposes only. So for the purpose of cooking you will be boiling the water, making it safe to cook with.
http://www.cdc.gov/healthywater/drinking/private/rainwater-collection.html
Rainwater Collection
Collecting and using rainwater can be a great way to conserve resources
1. Some people use rainwater for watering plants, cleaning, bathing, or drinking. Rainwater collection is not allowed in all places.
Some states
consider rainwater the property of the state and prohibit its collection, so you should consult with your local government (for example, your environmental quality department or health department) before proceeding
2.
Germs and Other Contaminants
While useful for many things, rainwater is not as pure as you might think, so you can’t assume it’s safe to drink. Rain can wash different types of contaminants into the water you collect (for example, bird poop on your roof could end up in your water barrel or tank). Rainwater can carry bacteria,
parasites, viruses, and chemicals that could make you sick, and it has been linked to disease outbreaks
3-4.
The risk of getting sick from rainwater may be different depending on your location, how frequently it rains, the season
5, and how you collect and store the rainwater. Dust, smoke, and soot from the air can be dissolved in rainwater before it lands on your roof. Roofing materials, gutters, piping, and storage materials can introduce harmful chemicals like asbestos, lead, and copper to the water, though building standards minimize some of this
6-8. Dirt and germs can be washed into collected rainwater from the roof, especially when rain follows several days of dry weather
3-4,
9,
10.
Rain barrels, for example, do not provide any type of disinfection or filtration.
Adding chlorine or iodine to water does not protect against chemicals, and some parasites are very chlorine tolerant. Boiling the water will kill germs but will not remove chemicals. Using a simple device called a "first flush diverter" to remove the first water that comes out of the system (which may have been sitting) may help avoid some of these contaminants
8,
11.
Some cisterns, which are common in certain areas, can provide safe water supplies. Some people add purchased, treated water to the rainwater they collect in their cistern. This may make the treated water less safe. Cisterns that are intended for drinking water should be properly maintained
8,
12. Consider adding a screen to the water inlet or emptying the rain barrel in less than 10 days to prevent mosquitoes from using the rain barrel as a breeding site
9,
10. Water intended for drinking needs to be filtered, disinfected, and tested regularly
9,
13.
Preventing Illness
If you collect and store rainwater for drinking, you have a private, or
individual, water system and are responsible for ensuring that your water is safe. You should have your water and your system tested regularly and maintain the system properly
4-5. When rainwater is used as a supplemental water source, homeowners should ensure that rainwater cannot enter pipes containing safe drinking water
4,
6. Contact your state or local
health department for more information.
To lower your risk of getting sick, consider using rainwater only for uses such as watering plants that you don’t eat, washing items that are not used for cooking or eating, and bathing (keeping water out of your mouth and nose). If possible, avoid using rainwater for drinking, cooking, brushing your teeth, or rinsing or watering plants that you intend to eat. Instead, use municipal tap water if it is available, or purchase bottled or bulk water for these purposes.
If you have a weakened immune system, you should be especially careful when choosing your drinking water source. Discuss this with your healthcare provider.
References
- Managing Wet Weather with Green Infrastructure Municipal Handbook: Rainwater Harvesting Policies
[PDF - 16 pages]. 2008.
- State Rainwater Harvesting Statues, Programs and Legislation. 2012.
- Microbiological quality of roof-harvested rainwater and health risks: a review. J Environ Qual. 2011;40:13-21.
- Health risks associated with consumption of untreated water from household roof catchment systems. J Amer Water Res Assoc. 2002;38(5):1301-6.
- Monitoring of water quality from roof runoff: Interpretation using multivariate analysis. Water Res. 2011;45(12):3765-75.
- Rainwater Collection. 2012.
- NSF Protocol P151 – Health effects from rainwater catchment system components. 2012.
- Oregon Smart Guide to Rainwater Harvesting [PDF - 20 pages]. 2012.
- Virginia Rainwater Harvesting & Use Guidelines [PDF - 15 pages]. 2011.
- Rain Barrels – Truth or Consequences [PDF - 5 pages]. 2013.
- Quality assessment of rooftop runoff and harvested rainwater from a building catchment.Water Sci Technol. 2011;63(11):2725-31.
- Georgia Rainwater Harvesting Guidelines [PDF - 70 pages]. 2013.
- Evaluation of Rooftop Rainfall Collection-Cistern Storage Systems in Southwest Virginia [PDF - 49 pages]. 1998.
10th Year Anniversary
Bacteria, viruses, parasites and numerous chemicals can also be carried along in rain water – all of these can lead to an upset stomach at least and severe disease at worse.
