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What To Know About Solar Panels On Flat Roofs


Most of the solar panel arrays we see on homes or small commercial buildings are of two varieties – either mounted to a pitched roof or on a ground-mounted metal frame or pole. But can you mount solar panels on a flat roof? The answer is yes, and although it is a different process than the more common pitched-roof installation, it is no more complicated or expensive, and it even has some key advantages.

Why Solar Panels Work Well On Flat Roofs

Of all the benefits to a flat roof-mounted solar panel system, perhaps none is more significant than that it can be installed without drilling holes in the roof itself. Solar panels on a pitched roof, by contrast, are mounted with lag bolts in a process that requires drilling dozens of holes in the roof surface in a basic multi-panel array. Drilling even a single hole into a roof carries the risk of it becoming a leak, and it requires the expertise of a solar system installation contractor to ensure that it is done properly.

Because flat-roof solar arrays do not run the risk of sliding off the roof, drilling is not necessary.  They are often installed with a solar ballast. The solar panels in a ballasted roof mount are first attached to a steel rack – typically at an ideal angle for maximum sun exposure in that location – which is then held firm to the roof with weights. This not only preserves the integrity of the roof surface, it also makes removing the panels easier if the roof requires resurfacing. Before hiring a contractor, however, make sure they have the experience and specialized tools and hardware required to build a ballasted array.

Another advantage to a flat-roof solar array is that the whole thing may not even be visible from the ground. Many homeowners do not like the way solar panels affect the curb appeal of their house, so much so that they believe it reduces the property value.

One minor retraction to flat-roof solar is maintenance. Panels will gather more dirt, grime and debris than those on a pitched roof, and they occasionally need to be cleaned. This can be done easily with simple household cleaning supplies. Depending on your age and/or agility, however, it might be a task you will need to outsource.

Installation Costs Are Similar For Flat-Roof Solar Panels And Sloped-Roof Arrays

Although they use different hardware and present different challenges, the costs of installing a flat roof solar system and those for a more traditional pitched roof are comparable.

On one hand, because installers are working on a flat surface and are not having to drill, their job is comparatively safer and easier, which could reduce costs. But on the other, the ballasted system that secures the panels does involve some extra construction. The steel racking that the panels mount onto has to be fabricated, particularly if it is designed to place the panels at an ideal pitch to maximize direct sun exposure. There are the weights that secure the system to account for, as well.

One way costs can be curtailed in the flat roof system is to forego the ballasted construction and stick with the basic drill/lag bolt install. This is possible but not recommended. Drilling always raises the possibility of a leak, especially on a flat-roof surface where rainwater can pool and collect. Historically, the primary reason for pitched roof construction in the first place was that it allows for more effective water runoff.

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What To Consider Before Installing Flat-Roof Solar Panels

As with any major and expensive home construction project, it is vitally important to have a solid plan for the design, installation and maintenance of your flat-roof solar system before the first panel is ordered. Here are some of the key factors to consider before beginning.

Roofing Material And Age

There are two common flat-roof materials. One is EPDM (ethylene propylene diene terpolymer), an extremely durable rubber membrane that is relatively affordable, inexpensive and easy to repair. The other is BUR, or built-up roofing, which is an older and more labor-intensive roofing that employs alternating layers of fabric and asphalt topped with stone or gravel. Your solar installer should be familiar with both materials and make a thorough inspection of the roof’s condition and age.

If you plan to install a flat-roof solar panel array, you should also know the age of the roof because you may have to replace the roof once the system has already been installed – a situation you would rather avoid. Typical flat roofs can last a long time. A BUR roof can last 20 to 30 years and a properly installed EPDM roof can last up to 50 years.

Particularly if you plan to drill into the roof, you might have to consider replacing the roof surface before installing solar panels. Some experts suggest that if the roof has 10 years or less life before replacement, you should replace the roof before installing solar. This is because the cost of removing an installed array and reinstalling after construction will be prohibitive.

Removing and reinstalling a ballasted system, on the other hand, is simpler and less expensive. But if the roof is old and in need of replacement soon, you still may want to install a new roof no matter the type of solar construction you choose.

Roof Size

Another crucial calculation that must be undergone before construction is how many solar panels you will need to install to meet your power generation goal. This is a complicated procedure that involves your power needs and the sun exposure where you live.

The typical solar panel is about 5.5 feet x 3.25 feet, so having an idea of the total square footage of your roof’s surface can give you a good idea of how many panels you can install. Remember that you’ll have to account for mounting hardware and proper spacing. If the panels are mounted at an angle, however, you may be able to fit more than if they were to lay flat. Your Rocket SolarSM Advisor can help determine how many panels you’ll need, as well as how many you can fit on your roof surface.

But even if your flat roof is not big enough to fit enough solar panels to meet all your power needs, you can still install a solar system to offset some of your power requirement. With a grid-tied home solar system, you can remain tied to your local power grid but reduce your electric bills by supplementing the grid with your solar system.

Consider All Required Components

Although they are the most visible, solar panels are not the only components required for your flat-roof solar system. We’ve mentioned racking, but other parts needed (depending on whether this is a standalone or grid-tied system) can include an inverter, system disconnect, battery, system power meter and backup power source (such as a gas-powered generator in case of system shutdown). A qualified solar installer will know which components are required for your configuration.

May Require More Maintenance

Flat surfaces are going to collect more dirt, rainwater, snow and other particulates. Because of this, flat solar panels may require additional cleaning and maintenance than panels mounted on pitched roofs. Pitched-roof panels do require some maintenance as well, but it may be less than panels on a flat roof.

Homes Covered By Historic Preservation Rules May Need Extra Expertise

Is it possible to install a flat-roof solar array on a house that is designated as historical? That depends on how the designation was established. There are many ways a home or building can receive an historic designation, whether from the federal, state or local government, or even from a particular neighborhood. All of these designations can have vastly different restrictions and regulations on how a home or building can be modified.

Unless the home or building is associated with an important national or statewide event or historical person (such as the birthplace of a U.S. president or a key stop on the Underground Railroad), most regulations on historic homes come from a local government or organization. These local historic designations are often interested in preserving the architectural integrity of individual buildings or even entire neighborhoods, such as Beacon Hill in Boston or Old City in Philadelphia.

While many organizations that wish to preserve historic buildings also support “green” energy installations like solar, they also have strict rules on how structures can be modified. In many instances solar installations are allowed as long as they can be concealed from street level. For an historic home with a flat roof, this could likely be accomplished. Before pursuing a solar project, however, be sure you have a full plan and that it gets approval from the governing body that oversees the property’s historic designation.

The Bottom Line: Solar Panels On Flat Roofs Can Be A Great Option

There are very few downfalls to installing a solar panel array on your flat roof. In fact, in many ways a flat-roof solar array has several advantages over a traditional pitched-roof system. If you go to the extra expense of having a ballasted panel racking system built, you can angle the panels for maximum sun exposure while not having to drill mounting hardware into the roof surface, which can lead to leaks. If you go with this option, take care to find an installation contractor with experience in designing and fabricating flat roof ballasted systems.

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