Many of you who follow this blog also follow Being Awake, my alter ego. Sometimes a posting fits the parameters for both blogs, so I cross-reference. This is one of those posts. Your rooftop can provide low-cost (over the life of the project) additional water for your outdoor use. In an emergency, this resource can supplement your domestic use with some 'home treatment. There are lots of ways to capture your rainwater. If your home already has gutters and downspouts, you have a lot more options because you can place a larger volume collection tank right under a downspout.
In addition to the types of collectors in the Being Awake post, you can use almost anything that will hold water. A wide opening is best for lots of reasons of you aren't equipped with gutters and downspout. Before we installed our system in the back of the house, I put a laundry tub under the drip edge of the house. It collected a fair amount of water, but because it wasn't covered, I had to use it all within 3 days or the mosquitoes started maturing. If you have some sort of tub to collect, you could go one step beyond what I did in those days -- get an old screen, from a window or door and put it over the top of your open collection tub. Then you can use the water over a longer period.
If you are blessed with regular rainfall, you can have the pieces of this system identified and implement it only during emergencies to supplement stored water. For non-potable uses other than watering fruit and veggie plants, you'll need fabric to strain the water, a bucket to strain into, a bowl or smaller bucket for the transfer and a way to disinfect the water. Boiling the water works and can yield potable water, but takes a lot of fuel. If your need is for bathing or clothes washing, a little chlorine bleach may do the trick. Chlorine products tend to dissipate over a relatively short time, so let the water sit an extra hour or two before washing dark clothing or they may not remain dark for long.
Depending on the geometry of your roof, or if you have a flat roof with canales, you may want a rain chain to help direct the water to your catchment. If you do research and decide you really need one, here's a tip: make your own. The commercial ones are lovely, but I sure don't need $200 worth of lovely. There are a lot of DIY designs on the Internet, like this one and this one. Or you can just use a big rust-resistant chain you already have. It just needs moderately large links to help direct fast flowing water.
I chose a simpler method. To make this kind, you need pre-rolled wire or use a form and roll your own. I used a form (I used a 2 foot section of 2" PVC pipe), a way to cut metal (I used a hacksaw and some tin snips) and some soft metal wire or tubing. You'll be bending it, so nothing stiff or it will be a real ordeal. The process is a lot like this, except I didn't solder the rings together.
Decide how long a chain you need. For a 2" form, you'll get a little over 2 chain links per foot of material. I had a couple of coupons and a gift card from Lowe's, so I went nuts and bought soft copper tubing -- the stuff that's already in a roll. You may want to use a few separate pieces of the wire just to make wrapping easier.
Wrap the wire around the form so that it forms a single layer, like you are making a big spring. Then use the hacksaw to cut the 'big spring' along a line that cuts each twist of the spring on one place (A single long cut parallel to the long direction of the tube). If the hacksaw can't get through the full diameter of the metal, use the tin snips to finish off each link ( hence using soft metal). You may need to slide the 'spring' off the tube to make the final snip. You should now have a bunch of links.
Loop these together one at a time and bend each link slightly to close the opening as you go. After a few links, you'll start to see your chain emerge!
So you can use transform your pesky roof runoff into a useful resource with stuff around your home, or go hog wild with a fancy system, depending on your budget and how much you need the resource!