I recently received an email regarding using “roof water” on a vegetable garden:
“. . . . I am writing about your article on roof water harvesting using slow sand filtration. I am working with a group on a community garden in Portland, Oregon and we are having a discussion about the safety of using roof water for our garden. Several members are worried about contaminates in the water coming out in the food we are growing.
While I know your research is on making roof water safe for drinking, I am hoping that you can point me in the right direction for research on the matter of unfiltered roof water for gardening. I have had a hard time finding any solid results one way or the other.”
To make a long story short, generally, I would not advise using unfiltered roof water on a vegetable garden. There may be exceptions to this, however. If you have a non-toxic roof surface and the air quality is always good in your area, and no animals, birds, or insects have access to your roof surface, and you use a first flush diverter; you might be safe using the water from it for a vegetable garden, but have the water tested first and keep in mind that any surface is subject to contamination at any time, and unfiltered roof water may be contaminated easily at any time.
The website slowsandfilter.org suggests some of the most common contaminants that should be tested for in roof water: To start with – know what the roofing contains, what animals are present, the air quality, the time of year and the temperature range at the time of collection.
Again, I must stress that the research I have done does not and should not imply or otherwise suggest that any of the filters I describe can guarantee to provide “pure drinking water” in all cases. Your local health department is the final authority on drinking water. The filters I have operating at the location of this study do work to provide clean water that could be used in an emergency situation and is used to water the vegetable garden. But know that all of the information I provide is free and there are absolutely no guarantees. Every situation is different and each owner of a water system is solely responsible for the water quality. Contamination can come from anywhere; and water quality will vary. There is no way anyone can assure purity of harvested roof water without proper testing at the site. In order to know what to test for it is necessary to carefully consider each situation for all possible contaminants.
As far as watering a vegetable garden; be aware that roof water can contain just about anything and its purity will likely vary considerably depending on what is on any particular roof surface. Unfortunately, to actually identify exactly each individual microbe or type of microbe that exists in a particular water source is nearly impossible. Some pathogens are particularly potent and it only takes a few of them to cause illness; and others do not cause illness unless consumed by the thousands. This whole situation is further complicated due to the fact that older persons, children, and people with weak immune systems can be more susceptible to the harm from infections. The World Health Organization (sometimes referred to as the WHO) has a study that explains this in detail. There are thousands of types of bacteria, viruses, helminths (microscopic worms), molds and spores that can be in water and some are very dangerous to people; and furthermore all water contains bacteria, viruses and other organisms that are not harmful. I suggest reading through this WHO document at the above link completely to gain a proper understanding of the nature of biological contamination in water. As for the chemical contamination; each source will be different. It will be necessary to know what the water comes in contact with in all cases to determine what non-biological contamination may be in the water. So to summarize; I cannot tell anyone what might be in roof water they harvest – it could be quite harmless, or full of pathogens and toxic chemicals. The FAQ page on slowsandfilter.org has most of the info I have found on roof water purity, along with links to scholarly articles that contain more detailed information.
All this does not mean that harvesting roof water is a bad thing or that having rain barrels to store roof water is bad. Quite the contrary. Instead of using drinking water from your public water supply for outdoor watering; irrigate flower gardens, and decorative trees and shrubs and non edible plants with non-potable roof water from a rain barrel – this conserves drinking water and reduces runoff during heavy rain events. If your roof uses commercially produced cedar shake roofing, check for copper sulfate and/or arsenic compounds in the runoff; and some composition roofing contains zinc compounds intended to kill moss, but these are exceptions.
Know that water is just about the best – if not the best – solvent known to exist. If you are far from any freeway or major highway, and there is no industry nearby, chemical contamination from air pollution may be minimal and biological contamination along with chemical contamination from the roofing material may be the most important concern. Cedar shake roofing almost always contains powerful poisons including arsenic and copper sulfate, and additionally cedar contains naturally occurring tannins and oils that are toxic (that’s why cedar does not rot as quickly as other types of wood). Lots of composition roofing contains poison that kills moss, along with petroleum hydrocarbons and associated sulfur compounds, that are toxic. Slow sand filters will not completely remove all toxic chemicals however my tests have shown that a slow sand filter will remove petroleum hydrocarbons. Galvalume roofing will supply fairly clean runoff, as will tile roofing, but still the water is only as pure as the surface, and there can be lots of nasty stuff on the surface of a roof.