Log in or register to create Field Kits and Sales Worksheets. Why register?

Roof Anchor System for Solar Panels

Scope

Install a mounting system for solar thermal or solar photovoltaic panels. Understand the type of roof anchor systems for solar thermal and photovoltaic (PV) panels or integrated roof modules that are available on the market and how they are mounted to the roof structure. Consider the roof type (material and slope), weatherproofing, installation convenience, and wind and snow loadings. Choose an appropriate racking and mounting system for the type of PV module, and install the system along with needed flashing and seals.

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

Description

Photovoltaics (PV) are being used more often today on the rooftops of homes to capture and convert solar energy into electricity, thereby reducing total energy costs for the homeowner. These may be large PV panels that are assembled into an array, or PV shingles that are used in place of standard roofing shingles. Solar thermal systems for heating water mount in similar ways.

When adding any type of PV (solar) array on the roof of a newly constructed house, it is important to consider the type of roof anchoring system for the PV modules. Choosing an appropriate and well-designed racking and mounting system for the type of PV module being installed is crucial for the most effective function of the module, as well as the integrity of the roof and house.

Several different types of roof racking and mounting systems are available on the market by various manufacturers. Although each manufacturer has its own uniquely designed mechanisms, the systems generally have the same basic components. For PV modules, the racking and mounting system typically consists of the following:  a two-railed track that is bolted into the rafters and onto which the panels are clamped, a flashing component for the weatherproofing of the penetrations, and all required hardware such as bolts and clamps. Some systems eliminate the need for the rails, thereby removing those additional connections. However, if integrated roof modules (e.g., PV shingles) are being installed on a house, it is common that no additional mounting system would be needed. Photovoltaic shingles, for example, typically can be installed as easily as standard roofing shingles by being nailed directly to the roof deck. This installation typically takes place during the normal roofing construction and would require only one roofing contractor that is experienced in installing PV shingles.

Another factor is whether the PV modules will be suitable for the roof slope and orientation or if additional slope needs to be added by the roof mount system. In the past, design strategies such as matching the tilt angle with the latitude of the home were recommended to optimize the annual power generation from the PV system. Given the lower price of PV modules today, the design of the PV system may no longer need to impose additional tilt on the PV array to make the installation economically feasible. This decision also can significantly affect the overall aesthetics of the PV installation.

A roof mounting system for a PV system
Figure 1. A roof mounting system for a PV system. (Image courtesy of IBACOS).

When choosing a racking and mounting system, factors such as the roof material, roof slope, weatherproofing, installation convenience, and wind and snow loads should be taken into account. Certain mounting systems may be specific to particular roofing materials or slopes or may be able to function with all roof types. Masonry roofs may require additional structural considerations because of the extra weight of the roof tiles.

Weatherproofing is an extremely important element of a PV anchoring system. Because many penetrations will be made in the roof to secure the mounting system, it is critical to ensure that leaks do not develop from those penetrations. In the past, sealants alone were used to waterproof the penetrations, but this became problematic because the sealant often would break down from the impacts of the ultraviolet rays from the sun, oxidation, expansion and contraction, and wind and other vibrations. Therefore, a better practice is to have additional weatherproofing in place, including flashing.

Another important consideration in the choice of a racking and mounting system is the ease of installation. A system that is designed to make installation easy and more efficient could result in cost savings from fewer labor hours and potentially less rework. Reference the manufacturer’s documentation for the exact system being used to better understand all of this information.

Roof Penetrations, Flashing, and Roof Attachments

With the exception of raised-seam metal roofs, most all roof types require penetrations for rack mounts. In the best installations, standoffs, lag screws, or bolts tie into structural members. All brackets should have butyl tape or a high-quality caulking such as polyurethane or polysulfide, to seal any bolt penetrations and under struts, brackets, or mounting feet.

If standoff mounts or other brackets can be installed before the roofers install the finished roof, roofers can more easily shingle or tile around the flashing, and may install the flashing for the mounts. This approach helps to ensure that the roof warranty is intact. Solar installers need to coordinate with roofers prior to the installation to ensure warranty issues are addressed, proper flashing materials are on hand, and installation procedures have been worked out.

Composition Shingle Roofs

Many efficiency programs recommend that on existing homes, roofs should have 10 or more years of useful life remaining.

Stanchions and hanger bolts can be used with flashing at the mounting points. In new homes, it is best if both the bolt and the stanchion are applied before the roofing is installed. The lag screw end of the hanger bolt is threaded directly into the structure. The stanchion is lag-screwed to the roof structure (into a truss or rafter). After these are placed, a flashing is applied over the bolt or stanchion. Hanger bolts are preferred due to their low cost and quick installation, but should not be used with solar thermal systems that use integrated collector storage for structural reasons. Stanchions may be flashed with roof boot type flashing.

Flat Roofs

Curb mounts work well for flat roofs. Elevated racks are often used on flat roofs to provide an appropriate angle for the collector or module. Racks may be mounted on curbs or standoff mounts. Another reason for using racks in colder climates is to place modules and collectors above snow levels. Curb systems are described in the NRCA manual.

Standing Seam Metal Roofs

Clips can attach to raised seam metal roofing that do not require a roof penetration and can be used to clamp racks to the seam (Christian 2006).

Structural Insulated Panel Roofs

As part of a study of affordable ZEH in Tennessee, a process was worked out for installing racks on roofs made with structural insulated panels (Christian 2006). A more recent article in Home Power describes an approach using a toggle bolt (Sughrue 2013). Follow SIP manufacturers instructions.

Low-Profile Mount

Many PV systems come with arrays, racks, and clips that are designed to mount together. One method of reducing the visual effect of a solar array is to make the mounting system as close to the roof, and as small, as possible. All major PV manufacturers produce PV modules that can be mounted in low-profile racks.

Thin Film

Thin film solar cells use layers of semiconductor materials only a few micrometers thick. Thin film technology has made it possible for solar cells to now double as these materials:

  • Rooftop or solar shingles
  • Roof tiles
  • Building facades
  • Glazing for skylights or atria.

Thin-film rooftop or solar shingles, made with various non-crystalline materials, are just now starting to enter the residential market. These systems replace typical roofing materials and do not require roof anchors or racks.

Some PV systems are designed to be integrated into roof designs. Perhaps the most popular integrated system in the U.S. is made up of PV modules that are sized and mounted to replace Since these products are replacing roofing material and are the first line of defense against the elements, they should be compliant with local and national roofing requirements.

How to Choose an Appropriate Roof Racking and Mounting System for the most common PV Modules

The following provides a general description of the steps that should be taken to choose the most appropriate roof racking and mounting system for the specific type of PV module being installed on the roof of a house. Standardized design packages with pre-engineered mounting systems and integrated components can substantially reduce installation costs (Ventre, et al., 2001). These systems tend to include documentation such as drawings and instructions that aid with permitting and inspections.

  1. Identify the roofing material and slope. The first step in choosing a roof-mounted PV anchoring system is to identify the type of roofing material that will be installed and the slope of the roof. These parameters will affect the type of anchoring system because the systems differ, based on these elements.
    Roofing material and slope
    Figure 2. Roofing material and slope. (Image courtesy of IBACOS).

  2. Identify the type of PV module being installed. The second step is to determine the exact type of PV module that will be installed. The choice of whether discrete PV modules will be connected as an array or as an integrated roof module (e.g., PV shingles) will greatly impact the type of rooftop anchoring system to be used. Some integrated PV modules do not require any additional mounting hardware, making the following steps irrelevant. Reference the manufacturer’s product documentation to better understand this.
    PV module installation
    Figure 3. PV module installation. (Image courtesy of IBACOS).

  3. Choose the most appropriate anchoring system for the project. Once the roof type (material and slope) and the type of PV module have been identified, choose the mounting system. When choosing a system, make sure that it includes the necessary components (as discussed above). Also, consider the efficiency of the installation process and the potential additional loads on the system from wind and snow. Reference the manufacturer’s documentation for the exact product to ensure that the anchoring system will integrate properly with the chosen PV module.
    Anchor system installation
    Figure 4. Anchor system installation. (Image courtesy of IBACOS).

  4. Verify that installation of the PV modules and anchoring system was completed correctly, according to best practices. When the first row of PV modules has been installed on the anchoring system, check the installation to verify that it was completed to a high standard of quality. This may help to prevent future costs related to weather damage or rework.
    Completion of the first row of PV panels and quality assurance checks
    Figure 5. Completion of the first row of PV panels and quality assurance checks. (Image courtesy of IBACOS).

 

Ensuring Success

Early in the design process, ensure that the correct type of mounting system will be used to integrate the PV modules on the roof, and verify that the chosen mounting and racking system will be compatible with the PV panels. This can be done by considering the roof material, roof slope, weatherproofing, ease of installation, and wind and snow loadings. Closely examine the manufacturer’s product documentation for all of the systems involved, both early in the design process and throughout the installation.

Early in the project, confirm with local code officials that the mounting and racking system will meet all code requirements. Most jurisdictions require that PV installations are properly flashed and waterproofed according to roofing codes.

There are two structural concerns, dead load, the weight of the systems bearing on the roof, and live loads, the intermittent loads created by wind, snow, and maintenance people. Collectors should never be supported by roof sheathing between structural members. The project structural engineer or truss designer should be involved early in the design process. Even if not needed to accommodate the load, additional framing may be added as a convenience for easier solar installations even if structural loads are within limits.

Dead loads are typically minimal in PV arrays, no more than 5-10 lbs/ft2. However, the loads are often transferred to the rooftop through mounting devices that concentrate the array dead loads onto small surface areas of the roof or individual load bearing members. These conditions can significantly add to the loading conditions of a single truss, rafter, joist, decking or other roof component. Live loads can be large in magnitude, but are intermittent, and attributed to wind, snow, and maintenance personnel. Most PV modules are rated for dead loading of 50-55 lbs/ft2, or equivalent to the pres­sure of constant 110-120 mph winds (Barkasi and Dunlop, 2001).

Designing mounting systems for wind uplift is more critical in areas subject to hurricanes and excessive wind speeds. Manufacturers of collectors, modules, and mounting systems typically have their mounting systems pre-engineered for worst-case wind loads (DEG 2005). In parts of the world that are vulnerable to hurricanes or extreme wind storms, the collector and its mounting structure need to be able to withstand intermittent wind loads up to 146 miles per hour. This corresponds to a pressure of about 75 pounds per square foot (FSEC 2006). Rudin and Becerra (2006) describe approaches for analyzing severe wind loads.

Wind loads may be greater near the roof ridge. Mounting collectors or arrays near the ridge may increase wind loads on the equipment (FSEC 2006). Similarly, locating collectors and arrays in from eaves may reduce wind loads (Rudin and Becerra 2006).

Good engineering cannot make up for poor instal­lation. It is easy to miss structural members when fastening mounting systems to the roof. Care must be taken to insure that fasteners are correctly positioned. Failure of a bolt to torque down is a clear indication the structural member has been missed. Also, bolts shorter than those recommended by the manufacturer should not be used. Mounting systems can be secured to structural blocking placed between rafters/trusses if the layout of rafters/trusses do not align with the desired collector location. All bolts securing collectors or modules to racks or brackets must be securely tightened.

And overview of mounting systems and manufacturers can be found in an article by Rebekah Hren in Home Power (http://www.homepower.com/articles/solar-electricity/equipment-products/m...).

Check the quality of installation after the mounting system and PV modules have been installed to ensure that future damage will not occur that could have been prevented.

Use a high-quality contractor with experience in PV installation to help ensure that the installation is done correctly. Insist that PV installers meet with other trades to work out equipment compatibility, supply, and installations issues. For example:

  • Roofers need to know what types of flashing to use, who will be installing it, and when. Roofers also need to know if BIPV will be used that will replace sections of the roof.
  • Trades requiring roof penetrations need to know what areas are off limits for vents or other elements that may shade the PV. This coordination issue was the number one area of difficulty identified by PV installers interviewed for this document.
  • PV and solar thermal installers need to know what parts of the roof to use for their systems. Typically the truest southern exposure should go to the PV installation.
  • For long-term performance, landscape designers and installers need to keep the southern exposure unobstructed from trees and outbuildings.
  • Painters and stucco crews need to take all steps necessary to avoid overspray.

Be sure that the PV panel system is properly grounded.

Climate

The U.S. Department of Energy “DOE Zero Energy Ready Home Consolidated Renewable Energy Ready Checklist” requires the following related to climate:

“Location, based on zip code has at least 5 kWh/m2/day average daily solar radiation based on annual solar insolation using PVWatts online tool.”

Average daily PV radiation per month and amount of kWh/m2 per day
The spread of average daily PV radiation per month over the United States, with the required and recommended amount of kWh/m2 per day.

 

Training

Right and Wrong Images

None Available

Presentations

None Available

Videos

None Available

CAD Images

None Available

Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Codes

  • International Building Code. Chapter 15 (Roof Assemblies and Rooftop Structures) discusses waterproofing. See Sections 1503, 1506, and 1507 for details.
  • International Residential Code. Parts of Chapter 9 (Roof Assemblies) and Chapter 23 (Solar Energy Systems) discuss the installation of PV panels and the associated details, including waterproofing.
  • National Electric Code. Although the code does not impose specific regulations for roof mounting of PV systems, grounding and other electrical requirements are outlined. Each roof mounting system must be properly grounded.

Standards

Several associations are involved in standards development and best practices for roofing, including roofing systems for PV modules:

Program Criteria

ENERGY STAR Version 3 (Rev. 08)

Complete contract documentation as described in this guide is not required by ENERGY STAR HOMES® Version 3.

U.S. Department of Energy Zero Energy Ready Home

Complete contract documentation as described in this guide is not required for Zero Energy Ready Home certification.

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Case Studies

None Available

References and Resources*

  1. Author(s): Baechler, Gilbride, Ruiz, Stewart, Love
    Organization(s): Pacific Northwest National Laboratory, Oak Ridge National Laboratory
    Publication Date: June, 2007

    Report providing an introduction to current photovoltaic and solar thermal building practices.

  2. Author(s): Aldrich
    Organization(s): CARB, Steven Winter Associates, SWA
    Publication Date: March, 2013

    Brochure on specifications for PV systems.

  3. Author(s): Baechler, Gilbride, Ruiz, Steward, Love
    Organization(s): Pacific Northwest National Laboratory, Oak Ridge National Laboratory
    Publication Date: June, 2007

    Best Practices document providing an introduction to current photovoltaic and solar thermal building practices.

  4. Author(s): U.S. Environmental Protection Agency
    Organization(s): EPA
    Publication Date: January, 2011

    The RERH specifications and checklists take a builder and a project design team through the steps of assessing a home’s solar resource potential and defining the minimum structural and system components needed to support a solar energy system.

  5. Author(s): Sughrue
    Organization(s): Home Power
    Publication Date: September, 2013
    A structural insulated panel (SIP) roof has no embedded lumber in the structure, and therefore nothing substantial for attaching PV array and solar hot water mounts. Single or double lumber splines could be put into the panels, or I-joists at 4-foot intervals could be added.
  6. Author(s): Department of Energy
    Organization(s): DOE
    Publication Date: May, 2012

    Website describing how solar water heaters -- also called solar domestic hot water systems -- can be a cost-effective way to generate hot water for your home.

  7. Author(s): National Roofing Contractors Association
    Organization(s): National Roofing Contractors Association
    Publication Date: December, 2006

    NRCA's premier technical publication gives you the most current and useful technical information in the roofing industry.

Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Last Updated: 12/30/2015