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Shining a Light on Solar Panels

By Keith M. Bouchard, P.E.

No matter what you think about global warming, the benefits of using clean and renewable energy sources are undeniable.  As technology stands today, the easiest way for municipalities, businesses, and homeowners alike to tap into clean energy is by installing solar panels on the roofs of their existing buildings.  These arrays of photovoltaic panels are typically installed using one of two methods: ballasted arrays on flat roofs and attached arrays on sloped roofs.  Both methods come with a number of factors for a building owner to consider when performing a cost-benefit analysis of a solar installation.  Some of the most important items to consider are as follows:

Ballasted Arrays

  • Roof Condition:  It does not make sense to invest in a large solar array if your existing roofing is near the end of its service life and will need to be replaced in the next five years.  It’s much better to couple a solar installation with a re-roofing project or to install an array over a recently installed roof.  If the roof was recently installed, the supplier should be contacted to determine what measures will need to be taken during the panel installation to prevent voiding the roof warranties.
  • Additional Weight:  Per its name, a ballasted system is not positively attached to the roof and is weighed down to resist wind loads.  Depending on a number of factors, the weight of the system can vary anywhere between 5 to 15 pounds per square foot (psf).  The existing roof structure will need to be investigated by a structural engineer to determine if it can support the additional weight.  It should be noted that wind uplift forces taken directly from the code will result in ballast designs that are too heavy to be feasible on most buildings.  As a result, it is critical that the manufacturer of the panel racking system has performed thorough wind tunnel testing of their product to determine accurate uplift forces and ballast designs.
  • Roof Geometry:  In the Northeast, solar panels will be tilted to the South to maximize sun exposure through all seasons.  With this in mind, the roof layout needs to be evaluated to determine how much open space is available for the panels.  Any partial shading of a panel caused by roof projections or nearby trees will greatly reduce the efficiency of the entire string of panels attached to the shaded one.  Also, roof edge zones have greater wind uplift forces than interior zones and wind tunnel testing may not account for this effect.  As a result, it’s best to avoid the edge zones.

Attached Arrays

  • Roof Condition and Geometry:  The same concerns about the service life of the roof and panel shading apply for attached arrays.  Roofs sloped to the South are ideal for panel installations as long as nearby trees do not shade the array. 
  • Concentrated Loads: The weight of an attached system will typically be between 3 to 4 psf and the roof structure will need to be investigated to support the additional weight.  A greater concern is that an attached system transmits concentrated snow and wind loads to the roof structure through penetrations instead of as uniform loads over the whole roof area.  Concentrated loads may lead to individual framing members being overstressed.  Design of an attached system is typically an optimization process to minimize roof penetrations while limiting the concentrated loads to values the structure can support.
  • Roof Penetrations:  Attached arrays are positively fastened to the roof structure to resist wind loads.  This can lead to numerous penetrations for large arrays and each penetration is a potential leak.  Proper flashing details and certified installers are critical to maintain roof warranties and minimize the potential for water intrusion.

These are just a few of the factors to consider along with the reduced energy bills, tax incentives, and that warm fuzzy feeling that comes with “going green.”