Internet-Zero Townhomes Profit From Vegetated PV Roofs
This suburban Washington, D.C., undertaking proves there’s no want to decide on between a inexperienced roof or a photovoltaic roof: By Ted Cushman
Developer-builders Jessica Pitts and John Miller of Flywheel Improvement had a aim and an issue with their Perry Road Townhouses undertaking. Their aim was to construct inexpensive net-zero townhomes in suburban Washington, D.C. Their downside was stormwater administration: Guidelines in Prince George’s County, Maryland, required them to detain the runoff from heavy rainstorms on-site, slightly than divert it into municipal storm drains. The poorly draining website soils and the tight website precluded conventional ground-based stormwater finest administration practices, in order that they regarded to a vegetated rain-detaining roof for an answer. However Pitts and Miller additionally had different plans for the roof: They wanted solar energy to satisfy their formidable vitality objectives.
In the long run, Pitts and Miller reached each of their aims with an progressive mixture: a vegetated roof whose rising medium captures important quantities of rainfall, with photo voltaic arrays blended in with the vegetation and supported by modular frameworks which might be anchored and ballasted by the green-roof rising medium. Manufactured by German agency Optigrün and Klima Roof, the PV array assist system is imported from Germany, and the pair say this was the primary time it had been deployed commercially in the USA. However Miller says the concept is a “no-brainer” financially: “As a result of the photo voltaic racking is ballasted by the inexperienced roof media and the vegetation that maintain it from blowing away, that proportion of the inexperienced roof that’s ballasting the panels can also be eligible for the federal tax credit score for PV.” The roof can also be efficient as a stormwater administration system.
“The photo voltaic inexperienced roof is designed to detain between 1.8 and a couple of.4 inches of rainfall,” Pitts says. “There’s little to no runoff from a small storm of about 1.2 inches or much less, and runoff will increase because the rainfall will increase.”
The method began in the course of the design section. Says Pitts, “This could go on any form of deck. Our constructions are wooden framed. However you need to account for the load that you simply’re going to be carrying with the engineering, so it’s vital if you happen to do that, to have a thoughts to do it firstly.”
“Wind uplift is a important calculation on the photo voltaic,” Pitts continues, “and the depth of the inexperienced roof media is set by wind uplift calculations. We truly had a 3-foot parapet wall on three sides of the roof, and the PV system is down under that degree and is protected against wind uplift on three sides, which decreased the depth of the inexperienced roof media that we would have liked to put in.”
The inexperienced roof media would assist to guard the constructing’s EPDM roof from daylight and climate. However the inexperienced roof and photo voltaic would additionally impede any wanted repairs or upkeep down the street. So Pitts and Miller determined to put in a leak detection system earlier than laying down the roofing.
“We thought it was prudent to put in a leak detection layer—it’s a metallic mesh that goes beneath the roof membrane,” says Pitts. “What that lets you do is that if you happen to ever get a leak, you possibly can determine the precise location of it.”
The skinny chrome steel mesh is available in rolls and is put in on to the roof deck earlier than safety board and roofing are put in. “It appears like hen wire,” says Pitts. “There are two leads that you could run up the aspect of your parapet wall and hang around on the prime. And you’ll take a look at even earlier than you set the inexperienced roof on. We had the folks come out and take a look at the membrane as quickly because it was in place. They moist down the roof and apply {an electrical} cost they usually search for locations the place you get a connection.”
Miller explains that the leak testing crew makes use of a detector that appears like a push broom with metallic bristles. “They discovered 5 – 6 little pinprick holes that you simply by no means would have observed, and we patched them,” he says. “All you need to have is a roofer possibly bends over and possibly one of many metallic brads on their denims pokes a gap within the roof, or they drop their utility knife, and you’d by no means know. The leak detection crew will circle this spot with a Sharpie, and also you’re like, ‘Wait a minute, that’s a gap?’ However this fashion you realize on day one which your roof is leak-free. And you probably have a future downside, then yow will discover it.”
Subsequent comes the layers of the inexperienced roof, that are built-in into the high-density plastic bases for the photo voltaic array racks. “You will have your plastic safety layer, which retains the roots from rising down into the precise roof itself,” Pitts says. “Then you’ve gotten a safety mat, which is form of like a felt materials. That’s for safeguarding the roof membrane from the roughness of the rising medium. Then you’ve gotten a geotextile mesh, and the mesh that we had been utilizing has giant holes in it—it’s a unfastened weave—and that goes on prime of the bases that the inexperienced roof ballasts down.”
The mesh wraps below and round metallic edging on the roof perimeter, and it runs over the PV rack bases. An L-shaped metallic bar is put in into every of these bases, after which the rack is accepted into these metallic L bars, in line with Pitts. Every grouping of panels can also be tied collectively. “We had 4 particular person techniques on prime of every of the 4 particular person properties,” Pitts says. “So every panel grouping turns into linked collectively, after which all of them grow to be linked collectively by the geotextile mesh.”
Subsequent it was time to position the rising medium. “It’s quite common to have drainage trays and issues of that nature to assist cut back the depth of inexperienced roofs, however we needed it to be thicker as a result of we’re utilizing the load of it to carry down the photo voltaic,” Pitts explains. “We had a inexperienced roof media firm (Mulch Options of Falls Church, Va.) come and blow the media up onto the roof with its blower truck.”
There have been two varieties of media: a bigger, heavier drainage medium as a base layer, then a layer of rising medium on prime of that, separated by a layer of cloth. Your complete placement with the blower truck took lower than a day. Moreover the effectivity, says Pitts, one other benefit to the blower truck is that it avoids creating level hundreds on the roof in the course of the software of the rising medium.
“After which the vegetation arrived,” says Pitts. “There are two several types of inexperienced roof vegetation. There are ones that develop in solar, and there are ones that develop in shade. So we needed to demarcate the areas behind and beneath the panels, and allocate the shade vegetation to these areas; and the solar vegetation went all over the place else. After which we received all of the PV tools, and the PV panels got here up final.”
The Perry Road Townhouses had been accomplished in spring 2017, and the vegetation on the roof are spreading properly over the rising floor. “Inexperienced roofs actually require a number of care and feeding of their first couple of years to get them to institution,” Miller says.
Flywheel Improvement is at present engaged on two extra inexpensive net-zero developments in Washington, D.C., Stack Eight and Cycle Home, slated for supply in 2020 and 2021, respectively.
Ted Cushman
