Using roof water on a vegetable garden

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 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 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.

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7 Responses to Using roof water on a vegetable garden

  1. Jason Perez says:

    Here’ s some good info on rainwater quality tests using a variety of roof materials:

  2. Jolly M says:

    What are your thoughts on adding a layer of carbon?

    I am planning on building a slow sand filter and was wonding if it would be over kill, usless or counter productive to add a layer of carbon (like the type you can buy in bulk at the pet store for aquariums).

    I’m currently guessing that it would be useful but I’m worried that it would have to be changed but I’m also thinking that since most of the stuff would have been removed before it gets to the bottom layers that it could last quite a while.

  3. admin says:

    If I were to add carbon filtering to a slow sand filter I would use it on the output pipe outside of the filter inside of another larger diameter pipe; for several reasons. First if the carbon is outside of the filter it would be much easier to replace, second if the carbon is placed inside the filter bacteria will grow in the carbon eventually and this may or may not be a good thing. If the carbon layer is on the top it may interfere with the formation of the biolayer. If the carbon is below the sand then it will be very inconvenient to change if the need should arise. Pet store carbon granules or fish tank filter carbon is not recommended. Even if you have no intention of producing potable water I would still recommend using carbon granules approved for potable water.

  4. Pat Riot says:

    I live in the northeast where it is freezing 4 months out of the year. Would it be possible to operate a slow sand filter in my basement, obviously taking into account the weight and ambient humidity involved in having a barrel full of water and sand inside the house? Does the system give off any amount of foul odor if run properly? Are there any other concerns I should consider before constructing this?

  5. admin says:

    As long as the filter does not freeze, and has water flowing through it on a regular basis it will function even in a basement. A properly functioning slow sand filter should not emit any undesirable odor. Water, with oxygen in it must circulate through the filter to keep the biofilm alive.
    In the winter when it is below freezing (or below 0) outside, obviously pond water or creek water or rain water will not be readily available. At this location, during the summer months rain water, creek water and pond water are not available.
    The filters I have running here use a small pump to supply a trickle of water continuously recirculating from the storage tank. The flow is not much more than a “dripping faucet”; and this has worked for the past 3 years on filter 1 and the past 2 years on filters 2 and 3. I have a small dc pump drawing water from the filtered water storage tank. (the filtered water storage tank fills from the filter by way of gravity feed – the storage tank is lower than the output of the filter) This pump fills (pressurizes) a 5 gallon captive air tank with the filtered water. There are 2 lines running from the captive air tank (use a “Y” connector). One line goes back to the filter input through a faucet set at a slow drip, the other line goes to another faucet that is turned off until water is needed. The filtered water is constantly recirculated this way and it is still possible to draw pressurized water from the storage tank/pump arrangement. Since the filtered water has very little bacteria, unfiltered water must be supplied to the input of the filter at least once a week if possible. This set up may work in the winter in the northeast – but I am not sure – I have not tried it at that location.

  6. Dennis Condon says:

    A potion of my vegetable garden is directly underneath a portion of my gutterless roof so rain water goes directly into my garden. This year I have one tomato plant, one sweet pea plant and some potato plants being hit by this water. Should I be terribly concerned?

  7. filter_guy says:

    Capture some of the water from your roof and have it tested by an EPA certified testing facility. Until you know exactly what is in the water, there are too many unknowns for anyone, even an expert, to tell you whether or not you should be concerned. What is your roofing material made of; galvalume, composition, cedar, asphalt, tile, steel, plastic? The nature of your roof surface will greatly influence what is in the runoff. Know that different plants take up different amounts of substances and concentrate them in vastly different ways in fruit and leaves.

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