The Sun is Not Always Friendly to Solar Installations: The Hidden Dangers of Thermal Expansion


What do you think about when designing a solar system—energy consumption and production, wind loads, snow loads, seismic zones, or roof height? How about thermal expansion and contraction? Thermal expansion is a real, omnipresent effect, and overlooking temperature differences can have major consequences on a solar installation over time. It should be a major concern to the solar installer and homeowner. Fortunately, selecting products designed for the effects of thermal expansion can ensure a worry-free installation. Read on to learn more

What is Thermal Expansion?

It’s not just expansion. It is the natural behavior of materials to expand and contract with variations in temperature. Everything in our daily environment does this—from buildings to roads, sidewalks to bridges.

Although thermal expansion and contraction is a complex problem, it has a simple root cause. All materials are made up of atoms. When heated, the energy in those atoms rises, causing the atoms to move faster. The faster the atoms move, the more space they take up, and the material expands. When it gets cold, the opposite happens. This is an immutable law of nature. Why does all this matter when designing a solar system? Because the effects can be catastrophic to the system, the roof, and—ultimately—your business.

Thermal Expansion in Solar

Most residential solar is mounted on aluminum rails, and some manufacturers advertise the ability run 100-feet of continuous rail. Although you may be wondering how much an aluminum rail can really expand when it heats up, you’d be surprised once you do the math: a 100-foot length of aluminum rail will expand as much as 1.5 inches over a 100-degree temperature delta. This level of temperature difference is common all over the country from Phoenix to Denver to New Jersey. The internal forces and stress on the rail and attachments is equally dramatic—for a fully constrained system, the force in an aluminum rail with the same temperature difference can be as large as 2,500 pounds.

Of course, real life is never as simple as just doing the math. To fully analyze a complete solar system on a roof, all the different components of the system and how they are affected by these forces must be taken into consideration. Identifying the difference between the hottest and coldest days the system will experience and how it will perform under pressure is the central question to answer.

What are the potential consequences?

Expansion, contraction, and the exertion of those forces happens cyclically. A solar designer must learn to properly deal with these effects, not focus on getting rid of them. The real danger of thermal expansion is that the effects can either build up over time or happen all at once and affect not just the rail but roof attachments, module clamps, and rail splices.

Aluminum rails are constantly pushing and pulling on the roof attachments. This can result in loosening lag bolts, deforming shingles, or damage to the roof seal above and below the foot. That means leaks. In more severe cases, splices can fail, lags can shear, and module clamps can pop off the panel. Then, new problems arise—product damage, warranty claims, and—worst of all—threats to life and safety.

Some solar roof attachments have very small contact points to the roof (above or below the flashing). Some are even shaped like apple slicers at the roof contact point. Think about what this means for the shingle below that flashing as it heats up and cools down repeatedly every day. Once you understand all of the different failures these variables can cause, it is easy to envision unfavorable outcomes.

Rail splices are an even bigger concern. “Structural splices” are frequently advertised in the solar industry. For a splice to be truly structural, it would have to perform exactly as if it were a continuous length of rail. That means it cannot move or separate under 1,500-2,500 pounds of force. Of course, tek-screwed, t-bolted, click-together, or tool-less structural splices cannot withstand that kind of force. Take a second to consider what would happen if a rail splice separated even a little bit—a standard module clamp has a clamp area less than a quarter-inch wide, which means the modules near that splice are in danger of being released.

The Right Way to Design a Solar System

Unirac recognized the problems posted by thermal expansion when it introduced its first rail system more than twenty years ago. Our guidance was to space thermal expansion joints every 40 feet. We recently improved upon that guidance‑through extensive internal testing, third party engineering studies, and research with Kansas State University. We analyzed expansion rates for roofs and rails, all the varied components of the system from the attachments to the modules, and components that include spring constants, rail cross-sectional areas, clamp structural capacities, lag shear, and pullout strengths. We have come to understand the topic at its deepest levels.

What we found is that to adequately accommodate thermal expansion and simultaneously optimize system design, one must consider the actual, local temperature differences. When products are designed appropriately and local temperature differences are taken under consideration, continuous lengths of rail up to 60-80 feet are attainable. Here are some product guidelines:

  • Roof Attachments—For attachments, a wide base designed to absorb some movement and a water seal that protects the penetration above, below, and around the lag bolt (either through rubber seals or chemical sealants) is best. Attachments that only have a rubber seal above the L-Foot or below the L-Foot are inadequate.
  • Flashings—Flashings should be thick enough to prevent curling up at the corners when the lag is secured. This ensures the flashing has enough structure to not tear through after years of the L-Foot rocking back and forth.
  • Rail Splices—Splices should also be designed to withstand the extreme forces they will experience. Click-together splices may not be suitable and should not be considered structural unless these capacities can be demonstrated. Even t-bolted splices may not have the strength to withstand slippage under large temperature differences.
  • Module Clamps—Clamps should be designed to not easily disengage from the module. Any clamp that can be installed by hand probably doesn’t meet this requirement.

How Unirac Can Help

Unirac is here to help and can address all potential issues through our product design, installation guidance, and detailed product documentation. We have recently updated our SolarMount Installation Manual to address local temperature differences. Please review the manual to see how your specific thermal expansion guidelines have improved. You can also reach out to us at (505)248-2701  for further assistance. As part of our mission to spread better solar for all, we believe in building better solar systems from the ground up. Lean on us to help make your installations smoother and more efficient. We’ll be happy to help.