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Frequently Asked Questions


I am collecting water for my garden. Is it OK to use the water from the old asphalt shingle roof of my garage?

Yes, and be thankful you have an old roof. Most of the new fibreglass or asphalt roofs contain a fungicide to reduce moss growth. Some of the metals and salts used will leach out of roofs when they are new and provide a toxicity your plants may not thrive with. For these new roofs we recommend installing a First Flush Diverter (FFD) to reject the first (most polluted) water running off the roof. See the FFD section of our Rainwater Components page.

What is the best type of roof to install on my new house? We plan to use the water for our indoor potable water supply.

In general you are looking for a smooth, easy to clean surface that will not emit toxins into the water. The ease of cleaning will save you time and money in the future, and the lack of any harmful metals or other chemicals minimizes the expenditure on water treatment equipment.

The most commonly used roof for potable rainwater harvesting systems is factory coated enamelled steel. Zinc coated, galvanized metal, terracotta and concrete tile, and properly glazed slate are also appropriate. However, buyer beware applies to all roof materials. There are no water quality standards for roofing in Canada and very few products carry water test results or assurances of safety – as is mandatory in New Zealand for example. Appendix 6.6 in the RDN Best Practices Guide provides further information on this topic.

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Maintaining Your RWH System

Why do some people stop collecting rainwater in the Spring? Do I have to do this?

It’s all about pollen–especially the pollen from fir and cedar trees that starts in late march or early April and that lasts for 2-3 weeks.

Yes, you should definitely divert water away from your tanks during pollen season if you are collecting potable water to use inside the home and for drinking. We also recommend that you stop collecting water during pollen season for garden systems with substantially sized tanks – especially if those tanks are exposed to sunlight.

Pollen can be very small – from 500 microns (the size of a human hair) to smaller than 40 microns (the smallest particle that can be seen by the human eye). This makes it hard to remove with the pre cleaning filters used in RWH systems.

Pollen is very rich in nutrients. Add water and sunlight and it grows almost anything. For example algae blooms in poly tanks exposed to the sun are common when pollen is present. Want to see it for yourself? Add some pollen to a clear glass vase or jar and leave it in the sun for a few days. You will see why rainwater harvesters stop collecting during pollen season, and then clean up pollen residue from gutters and piping before they start collecting again.

Is rainwater really safe to drink?

Absolutely! IF it is properly collected, stored, and IF it is properly disinfected on the way to the tap.

These are important IF’S. Potable RWH systems require more careful design, attention to every material used (e.g. potable certified piping and pumps), and installation and maintenance of water disinfection equipment to deal with harmful viruses, bacteria and cysts. Potable water, roof to tap water systems will cost more to install, and because they have more equipment, they require more time and expense to maintain.

In short, roof top collected rainwater can be a high quality and safe source of potable water if you are willing to accept the responsibility for administering your own water system.

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