Best Practices for Solar Mounting on Standing Seam Metal Roofs

What are the best design and installation practices when using seam clamps to structurally attach PV systems to standing seam metal roofs?

By Rob Haddock

Standing seam metal roofing (SSMR) is the most convenient of any roof type for mounting PV modules. The seams of SSMR are beam-like elements that provide convenient attachment points for the PV. This can be done with seam clamps. The clamps grab the roof seam by pinching it with non-penetrative, round point set-screws. When proper clamps are used, this establishes a secure mechanical interlock.

The SSMR industry has used this technology since it was invented by S-5! in 1992, and SSMR manufacturers recommend this method when making attachments to their roofs. Of concern to building owners, roof manufacturers, PV integrators, and code authorities is that the integrity of the roof system is maintained, and attendant warranties are not jeopardized.

SSMR is subjected to the most demanding wind tests in the construction industry (ASTM E1592, UL 580, and FM4471) but tributary loads must be uniformly distributed from PV to the SSMR. The result is a marriage that is maintenance-free for the service lives of both PV system and roof, while saving about 15 percent of total system cost when compared to other roof types and mounting methods.

Metallurgy is important when materials will be in electrolytic contact. SSMR is most often carbon steel, aluminum, zinc, and very rarely copper or stainless. Steel (painted or not) will have a protective coating of pure zinc (galvanized) or aluminum/zinc alloy (Galvalume or Zincalume) for corrosion protection. Aluminum seam clamps are compatible with all these metals, coatings, and their oxide layers, except copper.

A compatibility issue involves the exact profile of the SSMR seam itself. Many different seam geometries along with specific seam dimensional variances exist. Some clamp vendors promote a "no-brainer, one size fits all" philosophy. But "fit" is not really the challenge. Achieving acceptable holding strength while "fitting" is the challenge. Utilizing less than the best clamp (dictated by holding strength) can lead to system failure and damage to the standing seam roof.

There are no harmonized standards for testing of these aspects so there are no "compliance police". The number of untested products on the market is, well, shameful. So in most cases, the buyer must also be the policeman. Products that are not appropriately tested should not be used. Following are some tips for qualifying any product:

• The ultimate and allowable loads employing an appropriate factor of safety should be published on the clamp manufacturer's website.
• Holding strengths vary widely with the material, gauge, and profile of the SSMR. To determine the appropriate clamp and allowable load, these variables must be identified. Match the SSMR manufacturer and panel profile name and gauge to the listings on the clamp provider's website.
• Look at the test protocol and reporting. Is it appropriate and in the correct load orientation(s)?
• Scrutinize the laboratory that conducted the tests. It should be ISO/IEC 17025 accredited.

When relying upon tested holding strengths, recognize that the seam clamp manufacturer can test clamp-to-seam strength but not other elements of the roof assembly. The clamp connection in many instances exceeds the strength of other elements in a completed PV-to-SSMR assembly. The clamp to the seam may be a stronger link in the load chain than others—like the flexural strength of the roof panel, the roof's attachment to the substructure, and the buckling strength of the substructure itself. Do not rely solely on the strength of the clamp as the determining factor for attachment frequency and be sure that ultimate loads are reduced by adequate factors of safety.

The frequency of attachment of PV system to the roof is critical and often misunderstood. There are no standards that spell out these requirements, and rail producers often govern by the beam strength of their rail which may point load the SSMR. A competent professional should recognize other links in the chain and design and specify the clamp population so that undue concentrated loads are not introduced into the roof or substructure. PV modules are subject to UL 1703 so they must resist 30 pounds per square foot plus a 50 percent factor of safety. Racking components are subject to UL 2703 requiring testing of PV modules and attachment hardware in three different load orientations.

The above guidelines are valid whether attaching mounting rails or using direct attach (rail-less) systems. Slightly better space utilization is the advantage of rails. Economy and lower added dead loads are advantages of direct mounting. Mounting hardware all-in costs range from $0.04/Wp on large systems to $0.09/Wp on small ones.

For further information, please go to www.metalconstruction.org and select Resources: Technical Resources: Roofing (or Other): select the following:

• Technical Bulletin titled Dissimilar Metal Corrosion in Roofing Installations
• Technical Bulletin titled Fastener Compatibility with Profiled Metal Roof and Wall Panels
• WHITE PAPER Part 2: Metal Roofing from A (Aluminum) to Z (Zinc).

Rob Haddock is founder and CEO of Metal Roof Innovations Ltd., manufacturers of the industry-leading product line S-5! (www.s-5.com).