Mid-Project Report on Rainwater Control

House roofs and gutters are not my favorite subject, but they are turning out to be key elements in managing our rainwater. When we bought this place, rainwater was simply an inconvenience to be kept out of the house, collected and disposed of, but it is now in addition a source of nourishment for the garden at a slight cost savings. Every drop that is saved for the garden is one we don't purchas from the municipality, which is not only grateful for the reduction in demand, however slight, but is helping cover some of our costs in order to cut its need for stormwater treatment.
We are still working out our system but what we've figured out so far:

• Metal roofs provide cleaner run-off than composite shingles, because rainwater doesn't pick up anything significant from the paint, whereas in running over shingles it picks up grit and gunk that you really don't need in your garden. Metal roofs, while more expensive up-front than composite shingles, are fair better in the long run because they don't have to be replaced every 20 or 30 years.
• Our photovoltaic cells don't seem to interfere with rainwater collection, so long as the lowest point of the installation is over a part of the roof that drains well into the gutter. The cells are covered in glass and the framework holding the cells are aluminum, both of which are pretty neutral as to rainwater.
• Our downspouts had to be re-routed to drain into our cisterns, rather than into our lawn. As a result, the flow of the gutters has to be checked and adjusted. 
• It is especially important to keep the gutters clean so detritus (mostly pine needles in our case) don't block the drains into the cisterns.  Here's another place that metal roofs beat composite shingling: the gunk you get in the gutter off a metal roof can go into your compost because it's all organic leaves, pine needles and probably some bird poo; in contrast, the gunk from off a composite roof can be laced with bits of shingle you don't want going in your food.
• Invest in a good-quality ladder so you can periodically clean your gutters and generally check on your system. A cheap ladder is poor economy since it takes only one fall to eat up all the cash you "saved".
• Formerly we had four downspouts releasing water either directly into the side sewer or into the yard, to flow eventually into the ally and eventually the storm drains. Now we have two downspouts draining into cisterns with overflow into our rain garden. Our replacement downpipes are roughly twice as wide as the originals, either to ensure that they can handle the extra flow or because that's now the standard size. We have a good quantity of old-school downspout pipe ready for use on some other project and are open to suggests as to how to re-used it!