Rainwater Harvest Study Finds Roofing Material Affects Water Quality

For the past few years, one of the most common questions facing the Texas Water Development Board (TWDB) hasn’t been over contentious water rights or proposed water projects; it’s been from homeowners wanting to know what type of roofing material is most suitable for collecting rainwater for indoor domestic use.

“Rainwater harvesting is becoming fairly widespread, at least in Central Texas. There’s interest born out of necessity because people are simply running out of water in rural areas or they’re interested in conserving water supplies and it’s good for the environment,” said Dr. Sanjeev Kalaswad, the TWDB’s rainwater harvesting coordinator.

But when it came to responding to residents’ questions about which roof collection surfaces are best suited for rainwater harvesting, TWDB didn’t have a good, science-based answer to give, Kalaswad said. That’s when the Cockrell School of Engineering came in to help.

With funding from TWDB, Cockrell School faculty and students conducted an in-depth study - recently published in the academic journal Water Research - examining the effects of conventional and alternative roofing materials on the quality of harvested rainwater. The study, led by civil, architectural and environmental engineering Assistant Professor Mary Jo Kirisits, showed that, of the five roofing materials tested, metal (specifically Galvalume®), concrete tile and cool roofs produce the highest harvested rainwater quality for indoor domestic use. The study also showed that rainwater from asphalt fiberglass shingle roofs and increasingly popular “green” roofs contain high levels of dissolved organic carbon (DOC). Although other potential pollutants can be significantly lower on green roofs (turbidity and aluminum), the high DOCs are significant where these roofs would be used for potable rainwater collection.

Water with DOC is not necessarily dangerous on its own, but Kirisits said when it’s mixed with chlorine – a common product used to disinfect water – the two substances react to form byproducts that potentially cause cancer and other negative human health effects.

“Someone who already has a rainwater system is probably not going to change their roofing material based on this study, but this information is useful for anyone who’s trying to make an informed decision about what material to use,” Kirisits said.

Over the course of a year, Kirisits, her co-Principal Investigators Professor Kerry Kinney and Research Associate Professor Michael Barrett and their engineering students examined water collected from five roofing materials: asphalt fiberglass shingle, Galvalume®, concrete tile, cool and green roofs.

The test sites included both pilot-scale and full-scale residential roofs — one of which was the roof on the home of Kirisits and her husband. The other roofs were located at or near the Lady Bird Johnson Wildflower Center, where her team had the expertise of the center’s director of research and consulting, Dr. Mark Simmons, who helped them interpret some of their findings.