At first, collecting rain water sounded good as it rolls off our roof. However, the materials on the roof could be dangerous in the long run from chemicals that leach off the roof with the water. Is this a concern? Are there types of roofing materials that should not be used if wanting to collect rain water for gardening? Has any one looked into this? What about dust in cities that collects in rain gutters and then gets washed into the rain collecting system with the first rain falls of the season? What is in this dust?
I used unpainted metal because I was concerned about the paint. I found instructions online for building a simple “roof washer” out of pvc pipe. Essentially, the gutters fed into a 6″ pvc pipe with a removable, screw on cap at the bottom. Water only went into the storage tank after the “washer” was full. So, the dust and any leaves or bird droppings or bugs went into the washer. After each rain I unscrewed the cap and let all the yuck flush out. Put the cap back on and you’re ready for the next rain.
I can only comment on this with regards to what I saw in my water distiller. I distill all my drinking water.
I used to use tap water in my distiller. And boy was the left over filthy. Disgusting actually. Brown smelly muck was left in the bottom of my distiller.
I then experimented with rain water. The distiller was left a lot cleaner. Not 100% cleaner, but noticeably cleaner. There was a thin layer of white dust left, as opposed to the brown sludge.
So when you look at the water you use in your garden, if you use tap water, there’s a lot of stuff in there that’s disgusting. You just don’t see it.
Rain water from your roof is as close as you’ll get to rainwater from the sky.
When harvesting rainwater off your roof, just make sure you have all the relevant filters that capture all the coarse stuff. There will be other materials that are small enough to pass by your filters, but they are also usually large enough that they deposit on the bottom of the tank.
If used for drinking, I’d always either filter or distill water to be safe.