If You Are Thinking About a Rooftop Photovoltaic System, Read This!

I recently blogged about RoofPoint, the Center for Environmental Innovation’s rating system for environmentally appropriate, long-lasting roof systems. You can use RoofPoint as a roof rating system, and you can use RoofPoint as a comprehensive guide when designing roof systems. I’m going to get a little more focused in this blog; this is about rooftop photovoltaic systems, aka solar energy, which is just one of the 23 categories within RoofPoint.

Rooftop PV is an excellent renewable-energy source and definitely one worth considering. Locating PV on rooftops means electricity is generated where it’s used. And, because your building is already tied to the grid, additional transmission lines aren’t necessary. Your roof is not a public space, so it’s protected, and it likely gets a lot of sunlight, so it’s an ideal place for harnessing the sun’s energy.

However, before you add rooftop solar, it’s best to have an energy-efficient building (well insulated, minimal air leakage, efficient lighting, etc.) because energy efficiency trumps energy generation—always. Update your building so it uses the least amount of energy before you delve into energy generation.

So let’s assume your building is energy efficient and you’re looking to create some of your own electricity. Where do you start? Initial considerations include: the price of electricity, the amount of insolation (aka, sunlight) your roof receives, the amount of available rooftop space and the incentives available to help offset costs (unless cash flow isn’t a big concern).

Let’s call a spade a spade. If electricity is cheap (roughly in the 5 to 12 cents per kWh range, but there are no absolutes) and there are minimal local incentives and utility rebates, the return on investment is going to be difficult for most CFOs to swallow, unless you have an environmentally focused, long-term outlook. And I hope everyone does, of course, but I’m also a realist. If buying isn’t an option, power purchase agreements and leasing can be great options.

The roof is a big asset, and installing anything on a roof means there’s a concern about damage, leaks and loss of service life. Rooftop energy generation is secondary to the roof’s primary function—keeping Mother Nature out of your building.

The PV industry—justifiably—doesn’t want to penetrate the roof system. But it’s logical that anything on your rooftop needs to be secure, and secure for a long time. So it makes sense that at least a few penetrating attachments, to ensure the PV array doesn’t get moved around during a high-wind event, are a really good idea. Involving your roofing contractor to ensure those penetrations are watertight and talking to the roof system manufacturer to make sure your warranty remains viable are important first steps, and they should be part of every rooftop PV project.

And there’s still a lot more to think about when you’re considering a rooftop photovoltaic system. There are external forces that will act on the PV system. How does the PV system integrate with the roofing system and accommodate the external forces? The roof must continue to drain properly; the PV system shouldn’t prevent or block drainage. Workers need to be safe during roof maintenance, as do the workers who maintain the PV system. Fire and wind resistance are big issues, as is the added load of the PV system. All of these issues need to be addressed on every rooftop PV project.

Where can you learn more? And where can you find solutions to these issues? The Center developed high-level thought guidelines for installation of PV systems on a rooftop. These are free to anyone; the Center’s goal is the long-term success of all rooftop PV installations. I hope you’ll take a look and use them for all of your rooftop PV projects.

About the Author

James R. Kirby
James R. Kirby, AIA, is vice president of Sustainability for the Center for Environmental Innovation in Roofing. The center is a 503(c)6 non-profit, membership-based organization based in Washington, D.C., that provides industry-vetted information about environmentally appropriate, long-lasting roof systems.

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