Selection and Consequences of Roofing and Gutter Materials for Rainwater Harvesting
Rainwater is the cleanest source of water that is available in nature today. As it falls from the sky, it is safe to drink, anywhere in the world. The problem we face with rainwater harvesting is keeping it that way - all the way to our spigot. As soon as the rain hits the roof it is subject to contamination by pollutants that were deposited on the roof by wind, animals, insects, or by the leaching and dissolving of the material that the roof is made of. If you go outside and look around your house, you’ll get a good idea of what is deposited on your roof between rains. Leaves, dirt, fertilizers, chemicals that you put on your land, animal waste, nearby industrial discharge and just about everything else around you will be blown or dropped onto your roof in some form or another over time. I suggest that you use whatever roofing material that you want except lead and perhaps thatch (I can find no data on thatched roof rainwater runoff quality). Use rubber, galvanized metal, or aluminum for flashing and vent seals, and stay away from lead. Finally, plan on discarding the first few gallons of rainfall at the beginning of a rain and then purifying your harvested rainwater in some basic way before you drink it. Methods of purification will be discussed in detail in the Purification chapter.
Discussion – Fecal Coliform and Chemical Contaminants in Rainwater
The question of which roofing material to use can be complex. There is no one roof material that is best. All of them have one problem or another. Scientists have done studies on the pollutants that rainwater picks up as it travels from the clouds to the roof, gutters, and storage tank. Most would agree that rainwater, as it falls, collects little in the way of pollutants from the atmosphere, at least in concentrations that would hurt anyone – even acid rain. However, once it hits the roof, the rainwater either collects pollutants that the wind or critters brought, or it begins to dissolve pollutants out of the roofing material. Table 2a lists the results of rainwater tests that were conducted in Newcastle, NSW, Australia. This is rainwater that had not touched anything but a test tube.
Notice the guideline in the far right column. You can wade through the WHO guidelines at this link http://www.who.int/water_sanitation_health/dwq/GDWQ2004web.pdf . All of the tests are well within the Australian guidelines for drinking water except pH. That means their rainwater is a bit more acidic than most places on earth. It also means that the roofs in this area of the world are going to want to dissolve more of their material into the rainwater as it travels across the roof.
Table 2b lists the results of the same type of tests except this time they collected the rainwater as it exited an unclean gutter from one house with a modern, color coated, pitched, metal roof. Samples were collected after different amounts of accumulated rainfall (rain depth is in millimeters - as a reminder, 1 mm is only 0.04 inch or about 1/25 of an inch of rainfall; so we’re not talking about big rainfalls here). The water quality looks pretty good except for Fecal Coliform (warm-blooded animal waste) which remained high even after a long rain. This house had dirty gutters that had not been cleaned in two years. You would expect that the pollutant concentrations would go down as more rain fell and washed off the roof. Indeed, that’s what the test indicates; for the most part.
Table 2c is results of the same types of tests on rainwater from two different houses that had modern galvanized metals roofs with clean gutters. These samples were taken about 0.3 m (1ft) off the bottom of the storage tanks rather than directly from the gutter as in table 2b.
Notice this water looks very good except for a slightly low pH (it’s a bit acidic) and is ready to drink. However, we don’t know how many days had passed from the last rainfall to the taking of the samples. If it was more than three days, the stored water would have had time to self sterilize which would explain the excellent Fecal Coliform results. We will discuss self-sterilization in great detail in the Purification chapter. We will discuss other roof materials below, but for now there are a few things that should be pointed out about this or any other roof.
Rainwater by itself is very clean and easily meets most government guidelines – any government’s.
Rainwater doesn’t get contaminated to the point of being a questionable health problem until it hits the roof (or ground).
If done with care and maintenance, roof catchment of rainwater is a very good source of drinking water.
Fecal Coliform can be well above guidelines (all government guidelines) regardless of roofing material.
The quality of water coming off of a roof during a rain tends to improve as the rain continues.
Most studies agree that it is a good practice to eliminate the first few gallons of rainfall that is collected from the roof. This is called the first wash and will be discussed in detail in the First Filtering chapter. The highest levels of most pollutants come off of the roof during the first one to two millimeters of rainfall. However, there are several studies that indicate the concentration is more a function of time between rainfalls, and the intensity of the rainfall. Regardless of the reasons; it is a good idea to avoid collecting the first bit of rainfall. Most guides recommend eliminating the first 10 gal per 1000 ft2 (41L per 100m2) of roof (shadow). It only takes about 1/64 of an inch of rain (0.41mm) to collect this much water. But, Table 2b suggests that the roof isn’t very clean until after the first 1mm to 2mm of rainfall; which means the 10-gallon (41 Liter) rule-of-thumb might be too conservative. We’ll discuss this more, in the First Filtering chapter.