Using water from flat roof
I've been meaning to ask this for ages. We've installed water butts on our downpipes - however the water in the butt which takes the run off from the flat garage roof tends to be brownish and foams when you fill the watering can.
This isn't something I've come across before.
Last summer I restricted it's use to ornamentals but was I being over cautious? What's causing the discolouration and foaming and is it ok to use on the veg patch?