I wouldn't, not unless I ran a battery of tests on it. I would buy distilled water at Walmart.

I'm not going to use this roof water on my CPs unless someone can tell me they've done it without killing their plants. The good thing is that my plants got plenty direct rainfall the last two days and more rain is expected soon.

Rain and snow is good. I have used runoff but who's to say that my roof is comparable to yours. I use a combination of rain, snow, distilled water, and water from a nearby creek.

Hi
I use roof runoff on all my plants even an automatic water change for an aquairum. Think you're talking an Asphault roof ?? Unless the roof is very new don't think you'll have a problem..
One caution though do you ever have acid rain?? Easy to check though, rainwater should have a pH of 7 or slightly less. gary

Acid rain for mostCP's would actually be a bonus.. so long as it isn't on the order of HCL or H2SO4.... then we'd all be in trouble!

I used water runoff from an asphalt-shingled garage roof for years, with no problems. I now prefer to melt snow and save as much snowmelt as I can for the summer, but still use roof or deck runoff in late summer. One trick is to not collect the very first runoff in a rainstorm as it will have lots of grime from accumulated dust, pollen, soot, etc. But after a bit of rain has washed the grime off your roof, the runoff will be reasonably clean.

don555 might have an idea. I was initially appalled at the idea because I was thinking of mineral-rich dust on the buildings (not a problem living in a place with rain/snow.)
A couple of summers ago I almost lost all of my Sarracenia (>100.) I could NOT figure out why they suddenly all were sickening. Then I had a day off and realized they were all covered with a thick layer of concrete dust...the city was digging up/sawing all of the concrete driveways and sidewalks on the other side of the road, and enormous clouds of concrete dust were produced over the period of weeks. Washed them off, replanted in fresh media and only lost a few.

For what it's worth, I caught some runoff from the latest showers here in SoCal. When I transferred the water to clear plastic bottles I noticed it had a yellowish tint. I think that's because the bucket I used under the drip line had some of the local clay dirt at the bottom.

I have been using roof run off rainwater for several years now, NO PROBLEMS at all.

Use it.