Rain Fed Gardens designs and installs rainwater harvesting systems and native & edible gardens in nj, pa, ma, ny
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There is a common thought that asphalt shingles should not be used for rain collection due to leaching of contaminants into the water. Almost any roofing material is suitable for rain catchment for irrigation purposes. Asphalt shingles have, after proper filtration and disinfection, even tested appropriate for potable water.

The most recent studies have shown no appreciable difference among common roofing materials. In 2013 Rutgers tested rain barrel water quality for irrigation.

Results of this study showed that overall the water quality of the rain barrel water was very good. Heavy metals were well below federal irrigation standards for reclaimed water and posed minimal risk for irrigating a vegetable garden. PAH's were not detected in any samples. Results also showed the majority of water samples to be below recommended irrigation guidelines for E. coli.

Link to referenced study
Rutgers test of Rain Barrel water 2013
More studies can be found in the links section

Collected rain water is actually high quality irrigation water

It is soft (slighty acidic), chlorine & chemical free (unlike city water), and your garden actually benefits from the organic debris that gathers on your roof and enters your catchment system. This organic matter breaks down and becomes liquid fertilizer for your plants. It's the rain catchment version of compost tea.

 While subsurface irrigation is an ideal way to use collected rainwater, 
drip tape and soaker hoses work well when the collected rainwater is filtered first.

Rainwater collected in rain barrels, or any other non-treated system is NOT intended for potable use. This water can be used for irrigation, but should NOT
be applied directly to edible surfaces within a week of consumption.

Even though the UV rays of the sun will eradicate any bacteria within
that time, it is always advisable to wash vegetables before consumption.

michael@rainfedgardens.com ~ 908.752.7465
Rain Catchment Systems for Garden and Greenhouse Irrigation ~ Rain Barrels ~ Self Watering Gardens ~ Rain Gardens
Stormwater Management ~  Native & Edible Garden Design ~ Habitat Creation ~ Rain Fed Gardens currently offers design and installation services in eastern Pennsylvania, New Jersey, southern New York, and the western half of Massachusetts