In rural locations, gathering rain water from roofs and collecting it in cisterns has been a water source far before the term "green" meant "environmentally friendly." Nearly every kind of roofing material has been used, but metal and clay are preferred to wood shingles, asphalt shingles or other roofing types. Metal roofs vary in their composition and coating processes. Some are preferable to others. Some metal roofing materials should be avoided in rainwater collection systems.
Materials and Coatings
Most metal roofs are made of aluminum because of its resistance to rust. Others are made of steel, then given a coating for protection. The coating is the primary concern for water collecting. Because it creates a rust-proof barrier, metals will not leach out into the rainwater. But there is concern about chemicals from the coating process leaching into the water. Whether the metal is coated with traditional paint or a powder-coated dry paint system, concern has been expressed. The consensus seems to be that modern, smooth coated-metal roofs are not just safe, but among the best choices of roofing materials for collecting potable water.
There are some exceptions. Some metal roofs have pebbly grit adhered to the surface to mimic the duller surface of other roofs. It is the smooth surface of metal that makes it a good roofing choice, so by added a rough exterior, these types forgo one of their rain-collection benefits. Roofs are apt to have nearly everything from the environment blow onto them. If the surface is rough, it gives debris a place to settle and stay until wind or rain removes it. This is true of corrugated "tin" roofs that are not coated and will rust. The rust itself will run off into the water collection tank. And it gives other debris a slightly more porous surface to stick.
Pre- and Post-Filtering
Any water collection system can use a pre-filter. They are necessary with some types of roofs, like three-tab shingles, so the grit doesn't get washed into the water. The pre-filter is primarily for larger debris that may cause maintenance problems. Modern drinking filters should be considered, regardless of your roofing material, as even the best roofing materials are susceptible to contaminants being blown onto them.
What Not to Use
One material you shouldn't use is any flashing made of or containing lead. Use an alternative flashing. When your new metal roof is installed, discard water collected from it for at least one good rain. The rain will clean the roof of any residue left on the roof from manufacture or installation. If possible, discard a gallon or two of the first water collected in each rain as it may contain some impurities blown onto your roof.
- Photo Credit abstract red metal tiled roof background and chimney image by Avesun from Fotolia.com
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