Letter Re: Concerns About Toxicity in Water From Roof Catchment Rainwater

Mr. Rawles,
I searched the forums to no avail on this topic. In the Pacific Northwest, a common roof setup is untreated wood shakes with copper ridge caps and flashing. The rain hits the copper which leaches chemicals onto the wood shakes to inhibit moss growth.

On other sites I’m reading conflicting thoughts on whether this amount of copper renders the water unsuitable for rain catchment into a water barrel for vegetable garden irrigation or a cistern for human consumption. Some folks seem to think there is more danger from the concentrated nitrogen in the water (bird droppings, raccoon feces) than the minute traces of copper. Others believe any amount of copper renders the water useless, no matter what purification steps one might take (filters, Aerobic 07, etc). – What is your take? – Scott J

JWR Replies: The copper itself is not the issue, since after all most residential water pipes are copper. But if lead solder was used for any joints, that would be cause for concern. Personally, I’d be much more concerned about any wood preservative treatment chemicals impregnated in the shingles. Some of those chemicals, such as copper naphthalate, are nasty! But if your roof was built with plain, untreated cedar shingles and copper sheeting without lead joints, then the catchment water should be fine for domestic use, once it is properly filtered and treated for microbes.