Harvesting Rainwater from a composition (asphalt) roof

New information is available about rainwater harvesting from composition roofs. The slow sand filter described on slowsandfilter.org has been shown to remove petroleum hydrocarbons from water down to less than 1 part per ten million by weight; and this is when it is operating at its LEAST efficient rate of purification (at 32 degrees F). Also, petroleum hydrocarbons are present in roof water from the composition roof in the study in potentially harmful amounts if consumed over a long period of time (2.9 parts per million by weight). If a first flush diverter is used the, concentration is reduced to almost acceptable levels (less than 1 part per million by weight). The hydrocarbons may, in fact, be originating partially from local air pollution, because diesel fuel traces were found along with heavy oils. The implication here is that composition roofing is indeed a viable means of collecting rainwater, if proper filtering is used. A first flush diverter can be built easily with some pvc and recycled containers, and a slow sand filter can also be put together for minimal cost using recycled containers.

Another important fact uncovered is that rainwater from a roof may indeed vary WIDELY in the amount of Coliform bacteria present. The study has shown that water from a roof can contain about 60 cfu/100ml of coliform bacteria (relatively harmless) to 50,000 cfu/100ml (very likely to contain harmful pathogens and not safe to use), and that a slow sand filter will vary somewhat in its ability to remove pathogens; and also that a first flush diverter will remove a considerable number of the pathogens, but not enough to be safe. The slow sand filter has been shown repeatedly, by other studies (see the lit cited page on the above mentioned website) to remove coliform, ecoli, and fecal coliform to a safe level – from none detectable, in the best case, to 10 cfu/100ml in the worst case. In all cases the water from the filter is very non-turbid and should work well in conjunction with a UV filter
followed by an epa approved point of use tap water filter.

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13 Responses to Harvesting Rainwater from a composition (asphalt) roof

  1. Compost and rain collection are important. Do your part.

  2. Trance says:

    Hi, my name is Trance and I think the same thing. I even wrote about it on my blog at http://www.trancecommunity.com/Forum/ a while ago. Anyway, I’m not necessarily into this stuff but Harvesting Rainwater from a composition (asphalt) roof Biological sand water filters and roof water harvesting just happened to grab my attention there in the sidebar, while I was browsing through.

  3. Having a rain barrel can help you harvest lots of water during the rainy season. Learning how to make a rain barrel can help you pay less for your water bills. For you to water your garden and the lawn, you need lots water which can be quite costly. Making a rain barrel is not expensive. That’s some of the best rain barrel information I’ve seen online. I tried to make a tutorial too, hope it helps.

  4. Roofing says:

    Very easy to fit rainwater harvesting system too, suprised more people don’t try it. At http://jrmarlroofing.co.uk we have fitted over 50 systems in the last 2 years or so. Work a treat.


  5. This is such a great resource that you are providing and you give it away for free. I enjoy seeing websites that understand the value of providing a prime resource for free. I truly loved reading your post. Thanks!

  6. Paul says:

    In your drawings of the first flush diverter, you show a 1 1/4″ coupling but off holding a 1 1/4″ pipe that is used in conjunction with a table tennis ball to stop flow when enough has been flushed. Why is the coupling cut in half? Is it attached in any way to the couplings below it? I have a small roof I collect water off of and am thinking of just using a 4″ PVC for the flush storage instead of a barrel. Do you see any issues with this?

  7. admin says:

    The 1 1/4 inch coupler cut in half holds the 1 1/4 inch beveled pipe in place (the pipe frictions into the coupler tightly and can go up any further because the coupler keeps in there) so the table tennis ball does not push it (the beveled 1 1/4 inch pipe that is beveled on the ping pong ball end) out when the water level rises. Essentially, it is what allows to ball to seal the pipe off when the water fills up the barrel.

    As far as what you use for the reservoir of the first flow water ( I use the 55 gallon barrel because this roof gets very full of nasty stuff and the rain is usually light), the main thing is that it has enough volume to make sure the runoff that eventually flows into your system has had enough chance to wash all the nasty stuff off of the roof. It will depend on the nature of precipition in your area. Here precip, is usually not heavy so it takes a lot of water to wash the roof surface clean. In an area where it rains heavily, the roof may wash off faster. Another thing to consider is how much air pollution there is in your area. If there are lots of cars, trucks, and coal fired power plants, there may be lots of hydrocarbon pollution on your roof. On the other hand, if you live far from local air pollution, your problems may only be dust, and biological contamination.

  8. Paul says:

    Thank you for the prompt response! I actually work in air pollution for my city, so I’m pretty knowledgeable in that area. In addition to meeting EPA’s national standards, my city has gone the extra mile to regulate toxins as well.

  9. Jason Dryper says:

    Ive been a long term advocator of rainwater harvesting, asphalt roof harvesting is a great idea when used in conjunction with a water filter.

  10. Jon Brady says:

    Should I use the north side of my roof that has moss to filter my roof water before it goes into my attic cistern tank it is a shingle roof, or should I take it off the south western side that has no mods, to prevent freezing? My place is on the mountain top in South Western PA, where winter is more severe than Pittsburgh?? Thank you all for comments. Jon

  11. filter_guy says:

    I’m not familiar with your roof situation, so I cannot be of much help. Have the water tested, that will tell you which source is the cleanest.

  12. Dawn Volz says:

    We have a 20,000 gallon rain water catchment system that provides our only water source. We have the entire house guttered. We would love a pool and are wondering about adding another tank for that purpose. We have a sloped asphalt driveway that has 2 drainage pipes that run under the deck. Would that water be clean enough to filter and use in pool? The water would come down the large sloped driveway made of asphalt and some concrete, plus run through the dirt on way down (we have 2 acres of land it would run through part of that). Not lawn just country trees etc. Would that just be too much organic matter? Maybe some way to filter that out? Risk of what is brought in on tires?

  13. filter_guy says:

    We are working on the website now. We will reply in detail as soon as we have the website work done.

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