Table 2d is a summary of several different scientific studies that were done in several different areas of the world, on different kinds of roof materials. Trying to decipher the information is difficult because the samples were taken at different times during the rainfall, at different locations in the rainwater collection system, and the units of measure in some of the reports, simply don’t look right. A good conclusion is that no matter what you make your roof out of, you will need to face the fact that you’re going to have chemical and biological contaminants to contend with – just like any other water you collect from nature. Those of you that live in an area that has historically not had any modern agriculture, transportation, industry, manufacturing, mining activity, lead based paint, or domestic animal farming in perhaps ten miles of your home for the past 100 years, and you don’t have trees hanging over your roof, will have little difficulty if you drink your harvested rainwater without further ado. For the rest of us, we need to plan on cleaning the rainwater at least in some basic way. Ways to purify the water will be discussed in great detail in the chapter titled Purification.
There are some startling results that might be concluded from this table. It appears that gravel and asphalt shingles are a good choice for roofing material. Indeed, there is a study by P. C. Van Metre, and B. J. Mahler; http://tx.usgs.gov/coring/pubs/rooftops%20Chemosphere.pdf in Texas, as well as Jürgen Förster in Germany, that suggest that the fillers and texture of these types of roofs reduce heavy metals and PAH runoff. PAH's are Polycyclic Aromatic Hydrocarbons. There are about 100 chemicals that fit in this category. Most are carcinogenic and tend to be made from two to six benzene rings. The 2 and 3ring PAH's are usually from raw organic materials like oil, coal, and decomposing organic material. The 4 and 5 ring PAH's are from burning organic material like forest fires, home heaters, engine exhaust and even meat on your grill. It was widely thought that PAH's were coming from deteriorating asphalt shingles on homes (2 and 3 ring PAH's). Metre and Mayler have evidence that the PAH's coming off asphalt roofs are no more than what comes off of a metal roof. This suggests that PAH's are deposited on roofs and not sourced from roof material deterioration.
Roof Material selection Conclusion
If we accept the idea that, regardless of what the roof is made out of, our collected rainwater is going to have at least some amount of Fecal Coliform in it, we can greatly simplify the argument of which roof material is best. Each one has its issues. I realize that’s a tall statement, and I don’t make it lightly. After carefully reading these scientific papers, which you are welcome to read in their entirety via the links listed, I find that regardless of the roof material (except lead), the majority of the pollutants found in the studies come from deposits made upon the roof and have little to do with the roof material itself. That includes asphalt and asbestos shingles.
There are two sources of contamination. One is what lands on your roof from the surrounding area of your home, and the other is from what the roof is made out of. Wind, animals, and traffic stir up or track dirt that eventually winds up on your roof – a lot of it! Secondly, water is a phenomenally corrosive chemical that we don’t actually know much about. Scientists, as well as our own observations, tell us a lot about its behavior and effects, but we know little about why it behaves the way it does. The corrosive and erosive properties of water are trying to dissolve your roof the day you install it. Typically, the lower the pH of the naturally occurring rainwater, the faster the roof dissolves. The dissolved and eroded roof chemicals wind up in your cistern and you either drinking them or take steps to eliminate the pollutant; which is the subject of the chapter titled Purification. Sometimes, there isn’t a problem drinking the water without purifying it because the contaminants are so few and far between. If you live in an area that historically has not had modern agricultural, transportation, industrial, manufacturing, mining activity, or domestic animal farming within perhaps ten miles of your home, you can make your roof out of just about anything except lead, and probably safely drink your water without further ado. However, it is a good idea to test your water periodically to make sure it’s safe. We will discuss simple testing methods in the Water Testing chapter.
Reducing Collateral Damage
Metal roofs usually contribute high amounts of zinc to the environment. Fortunately, it takes a lot of zinc to hurt you, but it can add a lot to the burden that nature has to absorb in its streams. It can cause problems with bioassay, rainbow trout and insect larva.
Cement roofs would be a good choice but there are weight issues during design and construction.
Asphalt shingles also appear to be a good choice of rainwater harvesting roofing material.
One short note was found that indicated green roofs may be a good choice. Although there is a probable increase in fecal coliform contamination, the plants seem to hold on to some of the heavy metals and modify or hold on to hydrocarbon pollutants such as PAH's. The fecal coliform issue is relatively easy to handle with simple purification techniques.
No information could be found on thatch roofs. There were lots of off-hand remarks that they would be unsuitable, but I could not find any hard evidence. I have a suspicion that Mother Nature knows how to get rid of a lot of pollutants using plants and that we should be paying attention.
Research Needed on Roofing Materials for Rainwater Harvesting
Invent a paint for gutters and roofs that is not only edible, but actually adds vitamins and minerals as it is dissolved or eroded by rainwater. Make it so inexpensive that the poorest people in the world can us it; even if you have to vary the price form one, world location to another so that the wealthy can support the disadvantaged – on their journey becoming advantaged.
Make a global standard for how to collect roof water for testing and which testing should be done. Include the time that the samples were taken in mm of rainfall normalized to the size of the collection area and pitch of the roof. Include a detail discussion of the roof material composition.
Make a global standard for rainwater sampling and testing.
Normalize tests to millimeters of rainfall for time, days between rainfalls for incident frequency, parts per million or billion for concentrations, cfu/100ml for fecal coliform, and millimeters of rainfall per 100 square meters of roof shadow for volumes.
Conduct research on thatch roof and green roof runoff quality.
Conduct leaching analysis of most roofing materials using distilled water, pH adjusted from 4.5 to 10.0 in increments of 0.5pH.
Sample raw rainwater in most urban and rural parts of the world using a standardized protocol. Note the predominant industries in the area, especially pollution sources, their compass location form the test site, and predominant wind direction. back to overview