Planning for Thermal Movement: An Essential Element of Copper Roofing Design

For centuries, copper has been used as a roofing material because of its ease of installation, adaptability to simple and unique designs, resistance to the elements and superior longevity. Copper’s warmth and beauty complements any style of building, from Gothic cathedrals to the most modern museums and private residences. Its naturally weathering surface, whether in a rich bronze tone or an elegant green patina, is a clear indication that the building owner will only accept the very best.

Unfortunately, long-term performance of even the best construction materials can be compromised if the system is not designed or installed properly. For architectural sheet-metal installations, movement that occurs with changes in temperature must be considered during the design process. All metals expand when heated and contract when cooled. While this process is well understood, far too many contractors ignore thermal movement during system design or installation. Ultimately, this can lead to failure of the roofing and flashing system, causing extreme damage to the building. The Copper in Architecture–Design Handbook, which is published by the Copper Development Association (CDA) and available online as a free download, provides examples of how to accommodate for thermal movement of copper systems.

Calculating for the potential thermal movement of sheet metal is easy. Simply multiply a metal’s coefficient of thermal expansion by the metal’s expected temperature change by the length of the piece. Remember: It’s not the air temperature we’re considering; it’s the temperature of the metal. Anyone who’s touched a metal roof or the top of their car in the summer knows it gets significantly hotter than the air!

An example based on a 10-footlong piece of copper:

• 10 feet (typical flashing piece length) x 0.0000098 per degree F (copper’s coefficient of thermal expansion) x 200 degrees F (possible metal temperature change from coldest winter night to hottest summer day) x 12 inches per foot = 0.24 inch. In this case, the calculated movement is a little less than 1/4 inch.

Remember, the coefficient of thermal expansion depends on the type of metal you are using. Aluminum expands and contracts more than copper, and most steels move less. Series 300 alloy stainless steels are very similar to copper in movement, or expansion/ contraction rate. Naturally, temperature change is dependent on building location and exposure to the elements. Many professionals feel comfortable calculating the design movement with a temperature change in the 175 to 200 degree F range, but it’s the project architect or engineer’s responsibility to determine if this is adequate.

Modern rollforming equipment allows contractors and manufacturers to make very long panels, so potential total movement is even more significant.

Let’s investigate one type of common flashing design—in this case, at the eave, which is relatively simple but can easily be installed incorrectly:

• Based on the previous formula, with roof panels that are 20-feet long and installed at a temperature between the hottest day and coldest night: 20 feet x 0.0000098 per degree F x 200 degrees F x 12 inches per foot = 0.47 inch.

Because we’re installing mid-way in the temperature range and 0.47 inch is so close to 1/2 inch, dimension “A” can be 1/4 inch (one half the total potential movement). Naturally, the hem of the roof panel’s “loose lock” must coordinate with the length of the eave flashing to ensure the two are still engaged when the roof panels are fully expanded. While most contractors form eave flashings properly, some ignore the thermal movement gap “A” during installation, forcing panels to move fully onto the flashing. This eliminates the gap. When temperatures drop, the panels can’t contract, adding stress to the roofing system.

Through the years, countless thermal cycles and resulting stresses caused by expansion and contraction can take their toll. In the long run, something will fail. In some cases, work hardening of the metal can occur, causing it to crack or tear. In other cases, fasteners, such as those used to attach cleats, work back and forth, ultimately pulling them out of the substrate.

It’s easy, however, to avoid these problems. To ensure maximum performance of the roofing system, just follow the recommended design principles; understand how the different pieces of the system interact; and don’t cut corners. With a time-proven quality material like copper, proper workmanship and attention to detail can create a beautiful roof that could last the life of the building.

Learn More
For more information about architectural copper and roofing systems, visit the Copper Development Association’s website.