Learning Objectives

This course, EDC0912ROOF, has been approved by AIA for 1.0 LU/HSW/SD. It has been approved by GBCI for 1 CE Hour as course number 0090008737.After reading the article, you should be able to:

1. Describe the basics of photovoltaic arrays and roofing systems.

2. Review the key issues that should be considered with rooftop mounting of photovoltaic (PV) systems.

3. Detail essential guidelines for roof system selection in conjunction with typical PV system designs.

4. Define “solar ready” roofing and explain the service life considerations for a roof system versus a PV system.

Roofing has become more than just a lid on a building. It has become an active asset — even generating payback. The rooftop is now a place that provides “value-added” benefits like energy savings, environmental enhancement and even energy generation. For a roof system to offer optimum performance, it should offer superior sustainability. Sustainability in roofing involves avoiding or minimizing the impacts of a building on its occupants and the environment, both local and global, from design to construction, through maintenance, rehabilitation and eventual demolition with an emphasis throughout its life cycle on using natural resources efficiently.

Sustainability can take many forms on the rooftop. For example, highly reflective white roofs, known as “cool roofs,” can lower the roof’s temperature and make the building more energy efficient, reducing cooling requirements and mitigating the urban heat island effect. Rooftop gardens can provide aesthetic enhancements and convert rooftop areas into usable space, but they offer other benefits as well. Vegetative roof systems help reduce storm water runoff and keep the roof surface cool, reducing utility bills and protecting the membrane.

But high-performance roof systems can do more than just lower energy bills. They can actually serve as a platform for generating renewable energy from the sun’s rays.

Emerging new technologies with photovoltaic (PV) systems are increasing efficiencies, improving paybacks, and making PVs a more viable electricity generating source. PV systems offer a clean source for energy and, with net metering arrangements, building owners have the opportunity to sell excess electricity to the power grid, further enhancing system payback.

The growth of rooftop photovoltaics has exploded over the past few years, helped by incentive programs, technological improvements, and the need for alternative energy sources because of rising energy costs, tight supplies, and environmental concerns. Building owners also have an increased awareness of roofs as an active asset. Rooftops can be an excellent place to install PV because it is usually unused and unobstructed space. But utilizing a rooftop to locate PV is not without risks – one of which is the potential to compromise the integrity of a critical component of the building envelope: the waterproof barrier that is the roofing system.

PV and roofing systems that have been effectively integrated will work in harmony to provide clean energy, savings in energy costs, and protect the building and its contents for many years. This article will look at the elements to consider when selecting a sustainable roofing system to go under a PV system on a low-slope roof.

A poorly designed and improperly matched roof system is a candidate for failure, leading to potential damage to a building and its contents and possible premature removal of a PV system in order to repair or replace the roof.

The best roofing systems are those that are “solar ready” — meaning that the roof system is able to integrate with virtually any type of PV system and mounting method and provide years of leak-free performance while the PV system is generating power.

Why Photovoltaics? Why Now?

There are many reasons photovoltaic (PV) systems make sense now and for the future. Energy costs will continue to escalate and supply will continue to be chased by demand. Expanding the use of renewable energy sources such as PV can help meet some of the demand and relieve some of the cost pressures on electricity. Increased use of PV can also help reduce some dependence on foreign sources of fuel, leading to less potential for hardship due to supply disruptions.

Rooftops are a good place to locate PV systems because they are typically little used and are free from obstructions that can hamper the performance of PV systems. Utilizing rooftops can also reduce land use, making the land available for other uses or simply as green space.

PV is a clean, unobtrusive energy source, meaning that it does not pollute while it produces energy, eliminating the environmental issues associated with many other forms of electricity generation. The question arises as to whether the net life-cycle benefits of power generation from PV outweigh the monetary and environmental costs associated with production, installation and disposal of a PV system. That analysis is crucial, but it is not the focus of this article. But remember that regardless of what design actions are taken or what products are used, profit is a key criterion for sustainability – if an act is not profitable it is not sustainable.

Incentives from federal, state and local governments and from utility companies can ease the financial burden on investment in PV and encourage its introduction and development of more cost competitive PV technologies. A good source of information on incentives is the Database of State Incentives for Renewables and Efficiency at www.dsireuse.org.

While improving the energy efficiency in buildings (by way of reflective coatings, better insulation, high efficiency windows, daylighting, etc.) is essential, it cannot be the only component in pursuing Zero Net Energy Building design. Sources of renewable energy are necessary to supplement conventional energy usage of buildings to achieve a net zero state.

Finally, as the cost of conventional sources of energy goes up, and availability goes down, the cost to convert solar energy to electricity is approaching parity with most conventional sources.

Types of PV and Some History

Crystalline silicon panels encapsulated in glass are the most common type of solar module utilized today. They make up about 95 percent of all PV systems installed. Monocrystalline cells invented by Bell Labs in 1954 were cut as wafers from specially grown cylindrical silicon crystals.[1] They are still the most efficient of the PV systems but they have poor tolerance for low light. They are also fragile, expensive and they require very heavy frames for rooftop mounting.

Polycrystalline cells are made from multiple sources and are not as dependent on perfect crystal growth. They are less expensive than the monocrystallines, but they are also fragile and they are less efficient at converting sunlight to electricity.

While some crystalline manufacturers claim higher levels, typical silicone-based PVs have power production between 12 and 18 watts per square foot and operate with 14 percent to 20 percent efficiency. High temperature and shading negatively impact the output of crystalline silicon panels.

The initial cost of these systems can be high and vary widely, with estimates ranging from $3.80 to $4.50 per watt. For comparison, the table in Figure 1 lists the capital costs for some other generating systems.

During 2010 global PV production increased more than 140 percent, and a similar increase is expected for 2011. With the increase in production volumes and efficiencies, the average price for PV modules through the first three quarters of 2011 dropped by 35 percent. Some crystalline modules are now selling at or below $1 per watt.

Thin-film PV systems don’t require the use of crystalline silicon. They utilize very thin layers of materials such as amorphous silicon, a mixture of Copper Indium Gallium, and diSelenide (CIGS), or Cadmium Telluride. They can be flexible or rigid and can be adhered to a roof covering or a rigid material.

The first generation thin-films are mounted on a glass substrate, are relatively inexpensive to produce, but they are about 50 percent less efficient than monocrystallines. A heavy support frame is required and there have been issues with longevity and durability.

Second generation thin-films are mounted on a flexible substrate. They also do not require crystalline silicon to produce and they are easier to manufacture than the first generation thin-films at the same cost. There is no requirement for special framing or support structures because they are much lighter than other PV systems. These thin-films are rugged, and a special benefit is that they are easily integrated with modern roofing membranes after they are installed.

Because thin-films are typically surface-mounted, heat gain is an issue and these systems can compromise the benefits desired from reflective roof systems. Thin-film systems have power production of 5 to 10 watts per square foot and operate with 6 percent to 12 percent efficiency.[2] Compared to crystalline silicon systems, thin-films are more effective in low light situations and are less affected by high temperatures.

While this paper specifically addresses photovoltaic arrays, it is appropriate to give mention to another type of rooftop power generation that produces electric energy from the sun. Concentrated solar power, or CSP, uses lenses or mirrors to concentrate solar thermal energy onto a small area such as a tower. Typically, the mirrors are positioned on the roof to reflect sunlight to the tower mounted on the ground at one end of the building. The tower houses an engine, usually a steam engine, to drive an electric generator.

CSP requires very clear skies to work most effectively, and steam generation requires a fair amount of water, which can be an issue in hot dry climates. Compared to PV, more maintenance may be necessary due to the moving parts in trackers and generators.[3] But focusing the sun in this way also has several advantages.

First, less space is required for the collection units so less land or rooftop space is required. The cost per watt for CSP is currently par with PV but has the potential to be half that of PV. CSP steam generators produce AC power so they can integrate directly into existing infrastructure without power inverters. With natural gas backup or molten salt heat storage systems, CSP has the potential to operate continuously in the event of extended periods of cloudiness or shading of part of the array. CSP is projected to be a larger share of solar power generation within 10 to 15 years due to its efficiencies and potential reduction in cost (and by extension – the ability to generate power more cost effectively) due to technological advancements.[4]

Roofing Basics

There are many types of roofing systems that meet the needs of building owners depending on several factors. Building type, building use, geographic location, building codes, and whether it is a new construction project or a re-roofing project are just a few of the factors that can influence roofing system selection.

Many who are not directly involved in the industry assume that roofing is either tar and gravel (for flat roofs) or shingles (for sloped roofs). When it comes to low-slope commercial buildings, many think of traditional built-up roofs (BUR), in which layers of reinforcing felt are sandwiched between layers of bitumen asphalt or coal tar. But there are many types of other systems including metal, modified bitumen, shingles, tile, slate, rubber, Hypalon, and plastics designated with any number of acronyms such as PVC, TPO, CPA, EIP, and CPE. Thatch is even still in use today.

So when it comes to using PV on a flat or low-sloped roof — which encompasses most of the roof types used for larger PV arrays on commercial buildings — what are the most effective roofing systems?

To start with, some systems are just not compatible with flat or low-sloped applications. Shingles, tile, slate, and some metal systems that require overlapping are excellent for steep-sloped applications, but they really are not effective for flat and low-sloped applications. Roof systems that require regular maintenance (such as built-up systems), have historically short life-spans, or have weak seaming methods are probably not good candidates for roofing underneath PV systems.

PV systems can be expected to last 20 years or more, so roof systems that don’t require much routine maintenance and have similar lifetimes to PV are probably the most effective. Some single-ply roofing systems are virtually maintenance free, offer proven longevity, and are very effective for flat or low-sloped applications.

According to the Single-Ply Roofing Institute, there are three main categories of single-plies: modified bitumen, thermosets and thermoplastics. See Figure 2.

Modified bitumens are similar to BURs in that they are asphalt-based. These systems can require regular maintenance, especially at points where there are penetrations through the roof or changes of plane from horizontal to vertical. Adhering a PV system to a modified system is, at best, problematic. And rack-mounted PV systems can require many penetrations to attach the racking to the roof structure. Modified bitumen systems may have a lower initial cost to install, but ongoing maintenance costs for taking care of the multiple penetrations may be prohibitive.

Thermosets are single-ply membranes that require adhesives to attach the seams and flashings. While the membrane can be very dependable with these products, the adhesive can be the weak link requiring regular monitoring and maintenance to retain system integrity.

The third category of single-plies is known as thermoplastics, which utilize heat-welded seaming and attachment of flashings. Heat-welding is the most reliable seaming method because the seams fuse so completely that routine maintenance is minimized and ponding water will not compromise the seams. The expected life of many thermoplastics exceeds 20 years with some systems showing proven service for 30 years or more.

The two most widely used thermoplastics, TPOs and PVCs, are also two of the most widely preferred systems for use with PV installations.

Cool Roofs and Sustainability

Single-ply roofs with highly reflective surfaces are also known as “cool roofs.” “Cool” performance is a big part of what makes many low-slope roofing systems so sustainable. High reflectivity is a key characteristic that helps keep buildings cooler and reduce energy usage. Reflectivity, or albedo, is the ability to reflect energy from the sun. Stated in percentage terms, the higher the percentage, the more energy is reflected. As much as 40 percent less cooling energy may be needed for buildings that have highly reflective roofs.

Cool roofing delivers other benefits that are not as tangible as dollars saved, but are valuable nonetheless:

• Insulation can be 25 percent to 50 percent more effective. Extremely high temperatures reduce the effective R-value of the most widely used types of insulation. Cooler surfaces help preserve and keep rooftop insulation materials cooler.

• HVAC equipment can operate more efficiently. Inlet air temperature can be 5 degrees to 15 degrees cooler 30 inches above a cool roofing surface compared to a black surface. Most HVAC units are designed with efficiency ratings evaluated at 95 degrees Fahrenheit; rooftop temperatures on a black surface can reach 160 degrees F or higher.

• Substrate deterioration may be slowed by as much as 75 percent. Ultraviolet and infrared radiation and moisture penetration accelerate substrate deterioration. A cool roofing system will reflect this radiation and help protect the substrate.

• Ambient interior temperatures can be up to 20 degrees F cooler than outside. Studies of worker performance with machine operation and high physical activity reveal that productivity drops 10 percent at 84 degrees F and 38 percent at 95 degrees F.

Considerations With Rooftop-Mounted Photovoltaics

Mounting PV systems on a rooftop requires careful consideration of more than just the efficiency and effectiveness of the system for electricity generation. It also requires consideration of the underlying roof system and maintaining its integrity. Electricity generation mixed with roof leaks is not a good combination. There are a number of issues to consider when marrying the PV system to the roof system.

One of the obvious considerations is preventing and/or repairing leaks. This roofing issue is significantly magnified when a PV system is placed on top of the roof. Depending on the PV mounting method, limited access to the roof system can make it difficult to track and/or repair leaks. While an adhered PV system will create fewer penetrations that have to be flashed, it also restricts access to much of the roof. A rack-mounted system can allow access to the roof, but very effective flashing methods must be used to avoid compromising roof integrity. Ballasted rack-mounted systems can allow roof access and have a limited number of openings that must be flashed, but wind loads, seismic loads, and weight may be issues.

Location and the amount of foot traffic should be considered when selecting a roof-mounted PV system. Often rooftops also have HVAC equipment, water towers, antennas, satellite dishes and other such equipment on them. This equipment may not only affect layout of the PV system, but it often creates more foot traffic for installation, repair and maintenance. Walkways should be planned to allow access to equipment, and to help control the location of traffic and provide protection for the roof surface. In some areas local codes stipulate specific requirements for roof access for firefighters in the event of an emergency.

Heat is an important consideration when selecting a mounting method as well as in selecting the roof system. While PV systems offer the opportunity to generate electricity from a renewable source, a surface-mounted PV system may negate any energy saving benefits of a highly-reflective cool roofing system on the affected surface areas. Some types of membranes have been shown to experience premature aging due to elevated temperatures from heat emitting equipment or reflective sources such as reflective wall flashings or curtain walls. This could influence the decision on which type of membrane is most suitable for a PV application.[5] Some products have been designed specifically for use in these types of applications.

Structure weight allowances, snow loads and seismic loads can also play into the design and installation considerations of a PV system. Existing structures may require additional reinforcement prior to the addition of a PV system. For new structures, if a PV installation is planned, or if there is a possibility one will be added in the future, the design of the structure should be modified to satisfy the additional requirements. And while snow loads and seismic loads are more specific to geographic location, it is important that local codes be consulted and that load calculations include the addition of the PV system.

Fire ratings may be impacted by the installation of a PV system. During design for new construction guidance should be obtained from local code authorities especially those directly involved in writing and enforcing fire codes. For installation on an existing structure not only should local codes be considered, but the building owner’s insurance provider should be consulted to assure compliance with their guidelines. Caution should be taken to avoid misrepresenting fire ratings of PV assemblies because most are not Class A compliant.

In some geographic areas, wind and hail resistance are critical issues. Not only should the roof system meet the requirements for wind uplift and resistance to hail damage, but the PV system should be able to resist damage as well. PV manufacturers should be able to provide wind and hail test data on their systems.

Maintaining proper drainage is very important with any roofing system. Care must be taken when planning and installing a PV system to not compromise drainage. Drains should be easily accessible to allow for inspection and cleaning. Water and snow can add significant weight to a roof (e.g., one inch of water equates to 5.2 pounds per square foot of roof area), resulting in structural deflection and increasing the risk of catastrophic failure if the roof’s structural capacity is exceeded. Some types of roof drains can also be a potential source of leaks into the building, so accessibility is important to allow repair in the event of leaks around the drain.

Layout and positioning of panels should allow for access to rooftop equipment, rooftop traffic flow, and the optimal collection of solar energy. Trees and other sources of shading should be avoided since the generating capability of most PV systems can be seriously compromised by shade. Appropriate traffic areas should be included in the layout to avoid personal injury and prevent damage to PV panels.

Another consideration that should not be overlooked is liability. If the roof leaks, a fire occurs, or the PV system fails who will be responsible for repairs, removal of components, replacement? Responsibilities and limitations should be clearly outlined in writing prior to system selection and installation.

High membrane reflectivity, desirable for those areas that are not covered by the PV system, may also be an integral component of the PV system. Highly reflective roofing will help keep those surfaces uncovered by the PV system cool in summer, reducing the energy required to operate air conditioning equipment. Plus certain PV systems exist that actually utilize the solar energy reflected from the roof surface to increase system efficiency.

Mounting Methods

Methods for mounting rooftop PV systems fall into three general categories: rack-mounted, ballasted, and adhered. The method selected depends mainly on the type of PV system to be installed but can also be influenced by roof structure and geographic characteristics.

Rack-mounted systems should be anchored to the roof support structure, making it necessary to penetrate the roofing membrane. With adequate access between the roof and PV system, it may be possible to install the support structure prior to membrane installation. Penetrations must be properly flashed to avoid leaks. A major advantage of rack-mounting is easier access to the roof for maintenance, repair or replacement.

Ballasted PV systems are placed on top of the roof system and held in place with weight. Few penetrations are required with ballasted systems (basically only for electrical connections into the building), so the membrane is only minimally compromised. However, the National Roofing Contractors Association (NRCA) does not recommend this method. Wind uplift and seismic activity may cause ballasted systems to move excessively potentially damaging the roof or injuring bystanders.[6] If a ballasted system is used, it is recommended that a sacrificial piece of material compatible with the membrane be placed under the ballast points to protect the roof when the PV system moves.

Adhered systems are sometimes referred to as building integrated photovoltaics (BIPVs). BIPV can include other building components, but rooftop BIPV systems can be applied with adhesive directly to the membrane or to a sacrificial sheet of membrane which is then welded to the waterproofing membrane. The NRCA does not recommend adhering to mechanically-attached systems because potential billowing may damage the PV film or the system components. As mentioned earlier, the direct heat generated on the roof surface by the PV system may compromise cool roofing benefits or even degrade some types of membranes.

Roofs That Are Solar Ready

Because there are many different mounting methods and products on the market, there is really no standard method for integrating roof systems with PV systems. Regardless of the specific PV technology being considered, a roof system should be engineered to accommodate all PV systems available on the market today. It is important to utilize a roof system that is said to be “solar ready.” What does being solar ready really mean?

A solar ready system may utilize custom-fabricated stack, curb and other flashings that can virtually eliminate the potential for failures at the most critical roof areas: transitions, changes of plane, and the roof-penetrating support structures. As discussed, these penetrations are common for many PV installations.

A roof system that is long-lasting, watertight and requires minimal maintenance is best for going under a PV system that can be expected to be functional for more than 20 years. Even if PV will not be installed until a few years after the new roof is installed, the membrane formulation should facilitate the future PV installation by maintaining its flexibility and weldability.

Service Life: Roof System vs. PV System

During a new or re-roof installation it is good to know if PV is planned to be installed in the next three or four years. If so, a high-strength substrate and a thicker membrane should be installed. NRCA suggests that it is not a good idea to install a new PV system on a roof that is more than three years old.

Crystalline silicon-based PV systems have an average expected service life ranging from 25 to 50 years. Thin film systems average 20 to 25 years. Some single-ply roof systems can be expected to last 15 to 25 years, depending on several factors. However, according to the NRCA, the average life of a roof system is 17 years. This means that it is more than likely that a roof replacement will be required prior to the end of the PV system’s service life. PV systems placed over a single-ply roof will likely affect the rate of aging of the membrane — probably extending its life due to shading. However, some membrane formulations may actually experience premature aging under adhered thin film PVs due to the high heat generated.

PV system warranties generally run 18 months to five years for materials and workmanship with pro-rated coverage for extended periods. Power production warranties are typically based on a certain proportion of loss per year of service, usually over 20 years.

Guidelines for Roof Replacement

An investment in a PV system is a significant financial commitment. While costs continue to decline with improvements in technology, crystalline silicon based PV systems still average $2.40 to $3.00 per watt while thin film systems average $2.10 to $2.90 per watt. Replacing the current roof system at the same time as the PV system installation should be strongly considered because the cost would be a fraction of the total investment. The NRCA strongly recommends this approach.[7]

If replacement of the roof system is required after installation of the PV system, another significant expense for removal and reinstallation of the PV system could be incurred. If it is decided that a new PV system will be installed later on an existing roof, it would be best if the roof design would facilitate the future PV installation. This includes planning for issues such as additional penetrations, additional rooftop traffic, design loads and drainage. Other enhancements might include high-compression strength insulation, a thermal barrier board under the membrane, and a membrane with higher mil thickness.

Another consideration for replacement is that some roof system warranties may be voided if the use of the roof changes. Roof system installers and/or manufacturers should be consulted for guidance with this decision.

The NRCA offers additional recommendations specifically for adhered PV panels, including adhering roof membranes and using thicker mils made with high grade UV stabilizers. They also recommend the use of insulation that is resistant to deterioration from high temperatures.

Getting Started

Designing a low-slope roofing system in conjunction with a PV system starts with collecting the same information that is required for designing any typical roofing system. In a re-roof situation some additional information is necessary to ensure effective integration between the roof and PV system. Following is some of the information to be considered:

• Type of existing roof

• Age and condition of roof

• Number of roofs in place

• Insulation type

• Structural load capacity

• Deck type

• Perimeter details and dimensions

• Core cuts

• Photos

• Moisture survey, if necessary

PV array design is a complex process that should be conducted by someone with adequate knowledge of PV system layout, installation and operation. To size the system the designer must consider the building’s orientation, solar shadowing and insolation (incoming solar radiation) for the geographic area.

Building orientation should be compared to the angle and direction of the sun in the specific geographic area in order to establish the optimal position on the roof.

Obstacles on or near the roof may cause shading at various times of the day or during the year. Shading of the panels reduces the amount of solar radiation that they can collect and as a result reduces the efficiency of the system. Obstacles that create shade should be removed or relocated if possible. If they cannot be removed, then the panels will need to be located away from the obstacle’s shade path for any time of the day or year.

The PV system designer should also understand electrical wiring and codes. Knowledge of local utility pricing schedules is also important to help in estimating system payback.

The PV system designer should work with the roofing contractor to develop the best plan for integrating the PV system with the roofing system so that neither the function of the protective membrane nor the PV system is compromised. Proactive communication by all parties involved with the project will not only help the process moved more smoothly, but will help avoid issues that may compromise any warranties.

Tap Into the Power

Generating electricity through the use of photovoltaic systems is a significant opportunity that is coming of age. Rooftops can be an excellent place to locate PV arrays, especially in locations where open ground may be at a premium. With good planning and design and the use of high quality products with proven installation methods, the PV system can function in concert with the roof system providing years of service and benefit from both.

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Notes

1. “Integrating Photovoltaics Onto Building Envelope Surfaces”; Gumm, Michael; Interface, December 2008.

2. “Solar Roofing Systems”; presented by Curtiss, Jacob, Applegate Johnston, 2009.

3. “Rooftop Concentrating Solar”; Energy Priorities: Smart Energy Ideas for Business Since 2004, DuBois, Denis, April 19, 2007

4 “Large Scale PV Versus CSP: Is There A Winner Yet?” presented at Renewable Energy World Conference 2011, Hauff, Jochen and Stenger, Jan

5. “Advisory on TPO”; Midwest Roofing Contractors Association™, February 10, 2010.

6. “Guidelines for Roof-Mounted Photovoltaic System Installations”; National Roofing Contractors Association, 2009

7. “The Convergence of Solar Technologies with Conventional Roof Systems”; Gumm, Michael, presented at the International Roofing Expo, 2009.