Photovoltaic Systems & Roofing

The history of large-scale solar photovoltaic (PV) array installations on low slope commercial roofs dates back about a decade.  Solar installations only began to become more commonplace around 2005-2006 as the cost of solar panels began to drop and federal (PTC-1603) and various state solar incentive programs became available. 

The number of solar installations have increased dramatically over the past few years with the introduction of solar financial products such as power purchase agreements (PPA’s) and newer solar lease programs (SLP’s). 

Corporations have gone solar.  Companies such as Wal-Mart ^® and IKEA^® now embrace roof top photovoltaics to lower their energy cost, reduce their carbon footprint while improving their corporate environmental image.  In the near future, Solar PV will become a standard roof component on most low slope roofs as the cost of solar continues to fall and compete with utility generated electricity.

The roofing industry is just beginning to see the long-term effects of standard solar arrays with crystalline silicon solar panels and the newer flexible thin film solar panels on new and existing roof system installations. 

Solar integrators and EPC contractors with limited or no roofing experience installed many early roof top PV systems, especially on residential steep slope roofs.  This practice continues today.  Only in the past several years has become it more common for roofing contractors to install photovoltaic roof top systems and for roofing manufacturers to offer solar PV options for their roofing systems.

While most small solar installations on residential roofs are technically less challenging, requiring for the most part a basic PV racking system with limited roof top penetrations.  Larger solar roof installations on low slope commercial roofs with the many different solar rack system choices combined with different roof deck and roof system assemblies are technically more challenging, for not only the non-roofing solar contractor, but also professional roofing contractors.

Historically, the roofing industry is conservative and is slow to adopt new technologies.  Only within the past 6-7 years did major roofing manufacturers begin to offer solar roof top system options.  While non-roofing solar integrators continue to dominate the market, we are beginning to see a changeover to the roofing industry.  The National Roofing Contractor Association recently began to offer solar technical programs, solar certifications (RISE) and system recommendations for their members as more professional roofing contractors become involved with solar installations. 

Roof Top Solar Problems & Challenges

A roof’s primary function is to protect the underlying building from the elements and keep it dry.  Adding solar to a roof adds to the roof’s basic function.  The roof becomes a power-generating support platform for the solar array.  The roof must support and maintain the long-term power production of the solar array, while keeping the building weathertight.  Installing solar arrays on an existing roof creates a completely new set of technical challenges to the roof’s long-term performance and the roof warranty.

Short Roof and Long Solar Service Life Disparity

Most solar installations are over existing roofs and not on new construction projects or on reroofs.  The biggest challenge facing roofing and solar is the service life disparity between the two systems. 

Solar on roofs to make economic sense and to finance requires a power service life cycle of 25-30 years to justify the cost and be competitive with utility generated electricity.  Nothing shorter pencils out.  While roof system manufacturers do offer twenty-year standard full roof warranties and higher cost twenty-five and thirty year warranted roof systems. 

The reality is most existing roofs will not last as long as their warranties after solar array systems are installed and over the course of the solar service life, the roof will have to be replaced.

Installing solar on a roof only four or five years old can be financially challenging if the remaining roof service life is shorter than the power production life of the solar array.  Often when calculating the system cost per watt (CPW) and return on investment (ROI) and the levelized cost of electricity (LCOE) of the solar array, contractors, designers and solar integrators many times fail to factor in roof replacement cost and power down time at some future point in time during the 25-year solar performance life. 

Roof Service Life

The current estimated average performance life of a low slope commercial roof is 17.2-years before the roof needs replacement.  This means over half of all new roofs will fail and the roof system replaced before the end of the solar PV system useful production life. 

Of course, any existing roof will have an even more limited remaining service life. 

A number of factors contribute to a roof’s service life shortfall.

• Some roof materials have not perform as expected, for example earlier generation TPO and some PVC roof systems.
• Contractor workmanship can be a contributing factor.
• Poor roof design or the wrong roof system choices for the location.
• Limited roof budgets, the roofing industry knows how to build a 30-year roof, if the owner is willing to pay for the additional cost.
• Lack of maintenance or neglect by building owners, who assume a new 25-year warranted roof will last that long without any effort, out of sight, out of mind.  
• Some inexpensive roof systems may simply have a shorter service life, for example, a 2-ply basic asphalt cap and base sheet or 45-mil TPO.
• Roof top solar.
• All of the above.

Rack Mounted Solar Arrays Roof Impact

Rack mounted solar arrays can affect the roof’s service life performance in a number of ways, depending on the roof system and deck assembly.

1. Rack mounted solar consist of many parts and pieces assembled on the roof.  This exposes the roof to heavy foot traffic and material stockpiling as the roof becomes the work surface during the solar array construction.
2. Rack mounted solar arrays are either mechanically attached to the deck or ballasted and each system affects the roof differently.
3. Mechanically attached racks systems increase the number of roof top penetrations.  On built up and modified bitumen roofs, this means more pitch pockets.  On single-ply roofs, more boot flashings.  In either case, the increased number of roof penetrations increases the possibility of future roof leaks if the penetrations are not properly installed and maintained.
4. Both mechanical attached and ballasted solar arrays heavier weight loading points can increase deck defection between structural supports, creating new roof ponding areas or expand existing roof ponding areas and adding stress to the roof system.
5. Ballasted solar arrays with their paver ballast can alter or block roof water drainage flow patterns to drains and scuppers.
6. Ballasted solar rack arrays can contribute to increased membrane surface wear.  The ballasted array PV systems can be abrasive to the membrane surface from shifting and movement caused by different environmental forces such as wind and thermal cycling.  At a bare minimum, a membrane slip-sheet should be placed under any paver or block ballast and array frames in contact with the roof surface.
7. Solar array’s bright metal frame surfaces of aluminum and steel and the reflective glare from solar glass panels can concentrate, focus and increase UV and heat exposure on the roof membrane.  Higher UV and heat exposure accelerates the membrane aging process, creating the need for more frequent repairs and premature roof replacement.
8. Solar arrays greatly increase foot traffic over the course the array service life, as the solar panels require periodic cleaning to maintain optimal power production.  The array itself will need frequent maintenance inspections to check the structural integrity of the array, service roof penetrations, check and maintain the solar inverters.
9. The solar array installation work and increased maintenance traffic can accelerate granule erosion on granule surface modified bitumen membranes and standard cap sheet system.

Figure 1: Ballasted solar rack over existing roof system

Thin Film- Building Applied Photovoltaics (BAPV) Roof Impact

Newer flexible thin film solar modules pose a different set of roof top challenges compared to conventional rack mounted crystalline solar panels. 

1. Like rack mounted solar, flexible BAPV systems consist of many parts and pieces (panels, wiring, and inverters) assembled on the roof.  This exposes the roof to increased foot traffic and material stockpiling as the roof becomes a work surface during the array construction.
2. Flexible thin film PV panels when bonded with an adhesive to the roof membrane surface can heat the roof surface to over 180-degrees in the summer sun.  Higher heat loading and wider thermal cycles on the roof can cause accelerated heat ageing in many roof membranes, drastically reducing the roof service life.
   
   PVC and TPO membrane manufactures are creating new formulations and requiring thicker membranes to resist the higher thermal loads from the solar panels at a higher cost.  The standard solution has been placing a secondary sacrificial membrane and solar panels attached over the primary roof system.
   
   More roof manufactures require a sacrificial membrane sheet with the thin film PV panels bonded to the sacrificial membrane sheet to reduce heat or adhesive damage to the primary roof membrane.  The sacrificial membrane is mechanical attached, adhesive bonded or welded to the primary roof membrane, depending on the roof system type.
3. Flexible PV thin film modules bond directly onto the roof membrane surface.  The roof had better last as long as the projected 25-year PV panel service life.  The bonded solar panels are nearly impossible to de-bond from the roof membrane surface without damaging the solar panels.
4. Certain PV panel bonding adhesives are incompatible with specific roof membrane polymers.  A good example is PVC roof membranes.  PV panels bonded with the wrong adhesive to PVC will cause plasticizer loss to the PVC roof membrane, making the PVC roof brittle and eventually fail.
5. Flexible thin film PV bonded directly to granule surface on existing bitumen roof systems may cause the underlying granules to debond over time along with the solar panels. Liquid and self-adhesives can be affected by the lighter volatile processing oils in asphalt and de-bond.
6. Solar array installation work along with increased maintenance foot traffic can accelerate granule erosion on mineral surfaced cap sheets.

Figure 2: Thin film PV over existing cap sheet roof

Roof Warranties & Solar Array Installation Conflicts

How the solar array affects the roof warranty on an existing roof is another problem area.  As a consultant, I have encountered solar integrators and non-roofing solar installers who failed to understand that most commercial roofs are under a manufacturer’s warranty.  Any new roof top modifications, changes in use or additions to the roof without prior notification and approval from the roof manufacturer or work performed by a non-manufacturer certified roofing contractor could void the building owner’s roof warranty.

In some cases, the existing roof system may not be a suitable candidate for a solar array and the roof manufacturer may walk away from the roof warranty on a new solar installation.  Other conditions:

• The roof may be too old or not in good overall condition.

• The roof membrane might be an older roof technology with an inherited short service life.
• The roof membrane might be an inexpensive and less durable roof system with a known limited service life.
• The roof may have been leaking, have wet insulation or blisters in the case of modified bitumen or BUR roof systems.
• The roof system design might not be suitable for solar, for example a mechanical attached single-ply roof or gravel surfaced asphalt roof.

A whole range of roof top environment factors can affect future performance outcomes when installing a solar array on an existing roof.  The roof warranty is an important protection tool for the owner.

Roof Replacement for Roof Top Solar

Roof replacement cost on a solar array roof is significantly higher compared to a standard re-roof.  During re-roofing solar power production is shut down resulting in revenue losses for PPA and lease financed projects and increased energy cost for the building owner.

• On rack-mounted solar, one must factor the cost to disassemble the solar array, create an inventory list with a re-assembly plan, remove the solar array components and solar panels from the roof, and stockpile in a secure area.
• Roofs with BAPV pose a different set of re-roof problems.  Fully adhered solar panels are nearly impossible to remove from the roof surface without damaging the solar panel. 
• ·BAPV Systems using a sacrificial sheet removed from the existing roof may need to replace the sacrificial membrane sheet without damaging the solar panels.
• Many single-ply roof manufacturers are starting to require at a minimum fully adhered roof system with a hard surface underlayment over the insulation and a thicker (60-80 mils), more expensive roof membrane, more walkways and higher warranty fees to increase the roof service life and reduce their long-term liability.
• Modified bitumen system manufacturers may require more inter-plies and thicker dual reinforced cap sheets and walkways.
• Once the building is re-roofed, the solar array is loaded back onto the roof and re-assembled according to plan, hopefully without damaging the solar panels or losing parts in the process.  

Solar technology is constantly changing.  In a space of five or six years, the current solar panels may no longer be available.  Lost or missing rack parts may no longer be available.  We already see lower watt rated panels disappearing from the market as newer solar panels become more efficient and larger.

The solar industry is undergoing a global industry wide consolidation created by excess manufacturing capacity and rapidly falling pricing.  Over the next few years, over 200 manufacturers of solar modules and racking systems are expected to shut down and disappear or consolidate.

Replacing broken or missing solar panels affects the long-term financials on the solar array, increasing the cost per watt of the power generated.

Figure 3: Re-roofing – no easy task

The Solution – Elastomeric Roof Coatings

Elastomeric roof coatings are a simple and economic solution to increase an existing roof service life.  Modern roof coatings create a new durable and weatherproof roof surface and provide an economical solution to the challenges and problems outlined above.

There are certain instants where applying an elastomeric coating can provide additional benefits to a new roof system.

Improving Roof and Solar Service Life with Elastomeric Roof Coatings

Installing a new solar array over any existing low slope roof system means in nearly every case, the current roof system will wear out long before the solar array stops producing energy. 

• If the existing roof is less than 10-years old and good shape and not leaking.  An owner may find it difficult to justify the high upfront cost of a new roof prior to installing the new solar array.  One must factor in the higher future cost of a new roof and include this cost when calculating the CPW and ROI on the solar system.  This additional cost may make the cost of solar unattractive to the owner or the solar financing source.
• If the roof is more than 10-years old and in fair shape.  The cost of an installing complete new roof system, especially if a tear-off is involved, may make the cost of the solar array unattractive.  Installing a solar array over an older roof makes it harder to finance the solar array or to maintain the existing roof warranty.

Prior to installing a new solar array on an existing roof.  Applying a new high performance elastomeric roof coating system can extend the performance life of the existing roof for another 10-to–20 years.  In many cases, the roof coating system can be re-coated again once the first roof coating warranty period is up.  The elastomeric roof coating synchronizes the roof service life with the solar power production life, without re-roofing.  Roof coatings can be applied to existing roofs with an existing solar array.

Figure 4: Concrete Plant 20-year Hypalon Roof Re-Coated with Urethane

Roof Warranties & Elastomeric Coatings

Commercial limited roof warranties are an important investment feature for a building owner.  To the roof manufacturer and the roof contractor, roof warranties reduce their liability if something goes wrong.  Warranties are important marketing tools.  Roof warranties are limited, the big print tells the owner what is covered and the small print tells the owner what is not covered.

Solar arrays can affect the long-term performance of roofs in a number of ways.  A new elastomeric roof coating on the roof creates a new warrantable roof system while eliminating some of the problems described above. 

• The building owner gets a new and longer-term extended roof warranty.  The new roof coating warranty may provide more or less protection than the original roof system warranty depending on the coating manufacturer.
• For the manufacturer of the original roof system, the new roof coating warranty replaces the original warranty, reducing or eliminating the original manufacturer’s warranty liability, especially if different manufacturer provides the roof coating and coating warranty.
• If the original roof system manufacturer provides a roof coating system.  The manufacturer might provide a new hybrid warranty covering both the original roof system and new coating system tailored to account for the additional roof top liability. 
• The original manufacturer might cancel the original roof warranty and issue a new coating warranty, specifically tailored for a roof with a solar array.  The new roof coating warranty reduces their liability while extending the roof service life.

Solving Technical Roof & Solar Performance Issues with Elastomeric Coatings

Applying an elastomeric or liquid applied roof coating systems on roofs prior to or even after the solar installation can solve a number of technical roof and solar array problems.

• By increasing the roof membrane thickness, elastomeric roof coatings can improve the roof surface durability to withstand the increased foot traffic on the roof caused by servicing the solar array.  Elastomeric roof coatings such as polyurethanes with their stronger coating membranes are a good choice for high traffic roofs and roofs with ponding areas. 
  
  Adding contrast colored coated walkways with silica sand provides a durable and slip resistant work surface around the solar rack and roof top inverters and improves roof top safety.  Roof coatings chosen must be compatible with the roof membrane.
• On older and thinner 35-45 mil EPDM, TPO and PVC single-ply roof systems.  The new elastomeric coating increases the overall membrane system thickness, reducing the risk of premature roof system failure.  The coating protects the original roof membranes from higher UV exposure from the reflective light and heat of the solar panels and frame. The roof coating may improve the roof and solar system fire rating.
• Coatings, by sealing the existing roof membrane surface from the external environment and reducing the roof top surface temperatures can help reduce membrane UV and fire rating package failures that were common in early generation TPO formulations and plasticizer loss in older PVC roof systems.
• Roofs with modified bitumen and standard asphalt mineral surfaced cap sheets over time can suffer from granule erosion, which can greatly reduce the roof service life.  Elastomeric roof coatings bonds and seals the granules in place.
• On new or existing modified bitumen roofs, applying elastomeric coatings prior to bonding the BAPV flexible self-adhesive solar panels to the roof surface extends the roof service life and reduces the heat aging impact of the dark solar panels on the roof membrane by sealing the roof membrane from external environmental roof top conditions.  The roof coating provides a smooth surface to bond the solar panels without risking granule bonding failure.(¹)
• On new or existing single-ply roofs, applying elastomeric coatings prior to bonding the BAPV flexible self-adhesive solar panels to the roof extends the roof service life and reduces the heat aging impact of the dark solar panels on the roof membrane by sealing the roof membrane from external environmental conditions.(¹) 
• The original roof coatings can be re-coated again in the future continuing to protect the roof and to renew the roof coating warranty.  The roof coatings will not adhere to the low-surface energy tension top panel surface of the flexible solar panel making it easier to re-coat the roof around the PV panels.(¹) 
• When using an adhesive to bond the flexible BAPV solar panels to asphalt based roof systems.  The elastomeric roof coating forms a protective barrier between the roof and flexible panel, preventing volatile oils in the asphalt from causing adhesive failure and the solar panel de-bonding.  (¹)
• On roof systems with extensive field seams and pitch pockets for roof penetrations such as found on EPDM and modified bitumen and cap sheet type roofs.  Elastomeric roof coatings seals the seams, creating a monolithic edge-to-edge roof surface, reducing roof leaks and repairs from seam failure.
• Elastomeric coatings provide a layer of protective surfacing to protect against roof surface abrasions caused by roof top solar array concrete ballast pavers to the roof membrane.  Coatings resistance can be enhanced by adding fabric inter-plies to the coatings or adding a silica sand surfacing to the coatings.
• On single-ply roof systems with adhesive or hot air welded seams.  Elastomeric roof coating form a protective seal over the seams, reducing seam failure and repairs under the solar array racks, which may be difficult to reach and repair after the solar rack installation.
• With standard crystalline silicon solar panels, higher roof surface and solar panel temperatures lowers the power production output of the solar panels.  An elastomeric coating applied to a dark surface roof such as EPDM or smooth or granule surfaced cap sheets will reduce roof surface temperatures and improve the solar power output. 
• The increase indirect reflective light from the bright white coated roof surface, curbs and parapets can increase the electrically power production of the solar panels compared to a non-reflective roof surface.  This helps to reduce both the coating and solar array cost over time.

(¹) Covered by US Patents: US 2010 - 7,666,466 & US 2013 – 8,372,226

Financial Benefits – Elastomeric Coatings & Solar

• Elastomeric roof coatings provide an economical benefit by increasing the existing roof service life and synchronizing the roof asset life with the solar-array power production life.
• Roof coating systems are less expensive compared to replacing the roof with a new roof system.  Coating the roof prior to installing the solar array may increase the whole solar package cost  by $ 0.10/watt to $ 0.50/watt depending on the roof type, roof coating type and percentage of solar roof top coverage.  Some federal and states incentives may offset the coating cost.
• Applying a roof coating to an existing roof with an existing rack mounted solar array is less disruptive and may not require a system shut down to the power production of the solar array.  Reducing revenue loss for a PPA or leased solar roof.
• The cost of the elastomeric roof system under some cases may be expensed year one, rather than depreciated a re-roof over a 39-year period.  Allowing the owner to write off the coating cost in one-year.
• A white reflective roof coating cost and solar might qualify for the PTC 1603 tax credit.  The PTC is a tax credit worth 30 percent of the cost of the roof coating and the solar cost, in addition to the state-level credits that may be available.  The roof owner is eligible to take the entire tax deduction for the depreciation of the solar power system over five years, half of it in the first year — normally, roof depreciation is deducted over the course of 39.5 years, during which time its value declines because of inflation.  This tax credit option is based on the IRS private letter.  A roof coating manufacturer or owner would need to secure a private letter from IRS.
• Many utility companies offer “Cool Roof” rebates when applying a certified cool roof coating to a dark surface roof, such as EPDM, granule surface cap-sheets and asphalt roofs.
• Some states and cities offer energy efficiency rebates in addition to solar array rebates. 

Roof Coating Systems for PV Enabled Roofs

Today there are an excellent number of elastomeric liquid applied roof coating systems for creating sustainable and protective weatherproof surfaces over existing roof systems. 

• Acrylic Coatings: Typically white in color but can be colored or tinted to any color and made from acrylic latex or other acrylic resin polymers.  Acrylic coatings can be applied to built-up roofs, modified bitumen roofs, single-ply roofs (PVC, Hypalon, EPDM, TPO, Co-polymer Alloys, and Neoprene), metal roofs and polyurethane foam roofs.  Acrylic coatings should be formulated to meet ASTM D-6083 at a minimum.
• Polyurethane or Urethane Coatings: Typically one-component aromatic and aliphatic moisture cured urethanes polymers, sometimes two-component catalysis cured urethanes or newer high solid-low solvent one-component urethanes can be applied in one or two coating applications (base & finish coat) and are manufactured in a wide range of colors.
  
  Urethane coating are high performance coatings with high tensile strength, resistant to ponding water, most roof top chemicals and very durable.  Urethane coatings once cured form a strong membrane film for a long lasting weatherproof and waterproof surface over built-up roofs, modified bitumen roofs, single-ply roofs (PVC, Hypalon, EPDM, TPO, Co-polymer Alloys, and Neoprene), metal roofs and polyurethane foam roofs.  Polyurethane coatings that can be re-coated in the future to extend the service life of the roof again.
• Silicone coating: Silicone coatings offer exceptionally good weather resistance and long service life.  These coatings are typically offered in a gray color, as silicone coatings pick up dirt (if a white silicone is installed, it will soon become gray).  Silicone may present some problem with adhesives bonding to silicone low energy surfaces and may not be re-coated.
• Polyurea Coatings:  Polyurea are two component sprayed coatings that set up within minutes into a waterproof coating.  Polyurea coatings are seamless with excellent bonding strength to properly prepped substrates and good wind uplift performance.  Polyurea are relatively new to roofing and set up nearly within a few seconds upon spraying.
• Hypalon & Neoprene Coatings: Hypalon coatings generally are used to restore surfaces of single-ply membranes.  They offer good waterproofing properties and excellent resistance to chemicals, ultraviolet radiation and fire.  Neoprene coatings are composed of synthetic rubber polymers, are black and offer excellent elongation and recovery properties.  Neoprene coatings are used as a base coat beneath Hypalon coatings in some multi-coat applications.
• PMMA Coatings: Relatively new to roofing, PMMA is a two-part methyl methacrylate based acrylic resin, first developed in Germany in 1928 as a transparent thermoplastic.  PMMA primers and resins are very fast curing coatings and a short pot life.  PMMA coatings will adhere to almost any properly prepared substrate.  PMMA systems are also fully functional in ponded water conditions.  PMMA systems are monolithic, seamless, and normally reinforced.  One of the unique primary features of PMMA systems are their self-terminating flashing characteristics.  PMMA coating are one of the few coatings that can bond to flexible thin film modules, so PMMA’s should be applied with great care.
• Aluminum Coatings: Both water based and solvent-based asphaltic aluminum coating can be used with rack mount systems.  Their low temperature softening point makes this type of coating unsuitable for under and around BAPV thin film PV.  The coating should be Energy Star Rated.

Figure 5: Existing PVC Roof & Urethane Coatings - US pat.  8,372,226

Conclusion

Installing a white reflective “Cool Roof” coating is an inexpensive solution to extend the existing roof service life and synchronize the roof with the solar array service life.  Coating the roof with elastomeric roof coatings can reduce future revenue losses from shutting down the solar array, removing the array and having to re-roofing at a higher cost and re-installed the solar array. 

“Cool Roof” coatings reduce a building’s energy demand and electrical consumption.  The reflective roof coating may increase the electrical output of the solar array and reduce the overall electrical production cost of the solar power system.  Roof coatings can solve a number of technical issues that can crop up with solar installed on existing roofs without having to re-roof.

About the Author: Michael Gumm

Michael has been actively involved in the roofing industry for the past 30 years.  Michael is the founder of Corporate Roof Consultants, involved in both commercial roofing design and solar array design.  One goal of CRC is to help bridge the technology gap between the roofing and solar industries

Michael is the founder of SolarPower Restoration Systems Inc. and creator of SolarSeal Technologies™ focusing on developing new patented (¹) Building Integrated PV (BIPV) installation methods for low slope commercial roofing systems and the building envelope.  He is a co-founder of SunPods, the world’s first factory built ground mount solar array.

Michael is a member of ASTM’s D-08 and E-44 technical committees and involved with the ASTM task force writing the new standards for solar and roofing.  He is involved with Solar ABC "Solar American Board for Codes and Standards", Solar Tech Org, Solar 3.0, NRCA and the Roof Consultant Institute.  Michael is a published author, educator and speaker on solar and roofing technologies.

The content & opinions in this article are the author’s and do not necessarily represent the views of AltEnergyMag

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