There are a number of important health and safety concerns when storing run-off water for garden use. Mainly, non-potable run-off water can harbor bacteria and contain concentrations of heavy metals from the surfaces they run off of. You should always have your water tested each season, multiple times if you change your run-off water source.
Dean of Green - Roof Water Runoff (05/04/2012)
Also, before making any grand plans, you'll want to make sure you live in a state/province that does not prohibit rainwater reclamation (as completely ridiculous as that may seem). For U.S. citizens, a map of which laws go where can be found here. The stated concern is that health and safety issues arise from private, undocumented, and unlicensed reclamation; the crunchy version of this argument usually refers to rights infringement and government over-reaching. Either way, if you don't want trouble, just verify you aren't breaking any laws or city ordinances, and you'll save yourself the worry.
Capacity
I called around and found a source for large 55 gallon food grade plastic barrels. The people at Mercury Brewing of Ipswich we friendly and very willing to give these things away. All that were missing was the caps. Some had one side, but not the other. I could live with that, since I didn't need them to hold a seal, but simply one side to be water tight.
Capture
I bought a rain barrel diverter kit, which is simply a unit that fits to existing downspouts and redirects some or all rain run-off from your roof and gutter system to the rain barrel. Easy enough, right? Well, your gutters need to be relatively debris-free, and the height of where you install the diverter needs to follow the instructions that come with the kit. With a little bit of adjustment here and there, I got it working well, reclaiming about 40 gallons with just an inch of rain overnight.
Distribution
Between professional and family duties, I knew it would be nearly impossible to remember to water every time I should. So I decided to buy a water timer and drip irrigation system (1/2" parts). Because my roof is slanted downward at about 5 degrees, the roof barrel fed the drip system with enough PSI to keep the 1 GPH emitters flowing fine. However, I failed to check the minimum PSI rating on the timer (10 psi) which I had to compensate for by re-purposing my PC water-cooling pump into a post-barrel, pre-timer addition. This I put on a digital outlet timer on a schedule equivalent to the outdoor water timer. I opted out a few containers from this setup, those containing plants that either needed less vigorous watering schedule or ones that I wanted to baby throughout the season.
(1/2" tubing recommended profusely by this guy).
Application Effectiveness
A drip system is only as effective as you make it to be, though it's way more effective than my watering can every other day. Placement of the emitters with spaghetti tube to help direct flow and control over/under dampening, where the emitters are pointing slightly downwards and the tube goes right to the plant base, seems to be the most effective use of the water. A mulch cover (such as landscaping fabric) also helps to reduce soil evaporation and control weeds, though I will still manually apply nutrients every month or so as required.
Reference:
Q: How much rain is "one inch"?
A: Approximately 5.5gal per square yard
For my 20x20ft back roof, this is approximately 45yds x 5.5gals = 247.5gals per hour
...
in a perfect world. My diverted is not 100% efficient, it is not connected to this roof, and the roof leaks from all sides, not just the one with the gutter. Also, its connected to a different roof than the one I grow veggies on.
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