Surface temperature change is a major consideration when installing standing seam metal roofing panels /
Metal roofing stands up to the forces of nature as well as or better than any type of roofing material and you should know how to sell that benefit to your customers. This is the fourth in a six-part series on how metal roofing holds up against Mother Nature. Each part will focus on a different phenomenon. For this issue it’s metal vs. temperature changes that cause thermal movement.
Mother Nature knows how to test your metal roofing installations. She can get up in your face with hurricanes, wildfires and hail. She also has a subtler side.
Changes in surface temperature can affect the integrity of a standing seam metal roofing system — if the system is installed without regard for thermal response (expansion and contraction). Metal expands and contracts when exposed to the ever-changing elements; this is what is known as thermal expansion. The longer the panels, the more thermal movement occurs.
Editor’s note: What follows is a segment from Part 6 in a multi-part series about metal roofing in today’s market, authored by Rob Haddock. The entire A-Z series is downloadable from the Metal Construction Association website.
Panels are either attached with exposed or concealed fastenings. Exposed fasteners are used for certain panel types, including ribbed and corrugated profiles. This “direct” method of attachment can provide increased wind resistance but does not provide for thermal movement of panels and has the obvious disadvantage of penetrating the weathering surface.
Such methods and systems must be utilized with some precaution for these reasons. When the structure to which they are attached consists of wood or steel purlins, thermal cycling is relieved to some extent by flexure or rotation of the purlins.
When the same systems are used over solid wood decks or bridged bar joists, however, fastening fatigue can result from repeated thermal cycling. It may, therefore, be a good idea to limit roof lengths for such applications because thermal movement is directly related to length. Hundreds of years ago, brake-formed shapes were all fix-cleated to the structure, but their end-to-end joining was done with loose locks, and pan lengths were short, so thermal movement was never accumulated. Now, with roll forming manufacturing methods, panels have gotten considerably longer and thus accumulate more movement.
Most (not all) “concealed fastening” provides for differential thermal movement of panels to structure by the interface of the clip with the panel. A simple clip design would have this interface be a frictional engagement wherein the clip is rigidly attached to the building structure but slip-connected to the male seam component of the panel. This (one-piece) clip method is quite popular with most steep-slope roof products and especially those that have “snap-together” seam types. The clip is stationary but allows for differential movement between panel and clip.
When sealants are used within a panel seam, oftentimes the one-piece clip design cannot work. This is because the differential movement between the panel and clip would abrade the sealant, jeopardizing weather integrity of the seam.
In such cases, a different clip design must be employed. The most popular designs for such a seam involve dual-component clips. The clip base is attached rigidly to the structure and the clip top folds into the panel seam. Differential movement then takes place within the clip itself, between its two components — base and top. It bears mentioning that the clip is an integral part of the assembly and unique in most cases to the panel profile with which it is used.
Using clip fastenings that allow the panel to cycle freely in response to thermal loads also makes it necessary to deliberately “pin” or “fix” the panel at some point along its length to prevent it from migrating out of its intended location. Gravity loads, or “drag loads” as they are sometimes called, will act in a direction parallel to the roof’s surface trying to pull the panel down the slope of the roof. These loads are primarily comprised of vertical loads (snow, wind, foot traffic, etc) on the roof’s surface. The only resistance to these loads (other than the panels designed point of fixity) is friction between panel and structure.
Panel fixity can be accomplished by using one or more “fixed clips,” or by some method of direct panel fastening at the desired location. Use of the fixed clip method depends upon the nature of the interface of clip to panel seam; with some designs it is not possible.
The location of choice for fixity of steeply sloped architectural systems is most often at the ridge where direct (or through) fastenings can be hidden beneath a ridge cover. The system will accumulate movement to its eave end.
Conversely, the popular point of fixity for low-slope systems is at the eave. The primary reason for this preference is that such systems are often hydrostatic by design, and it is much easier to waterproof a joint that is stationary than one that is moving. Exposed fastening is usually tolerable from an aesthetic standpoint on low slope systems, so it is a logical choice. Such a system will then accumulate thermal movement to the ridge where a “bellows” style ridge flashing can accommodate differential movement of the two opposing roof planes while maintaining a hydrostatic seal.
These statements are not meant to be exclusive; there are exceptions in both cases. It is also occasioned in design to see a panel fixed at its midpoint, dividing thermal movement in half by sending it in both directions rather than one.
Having chosen a point of fixity for the metal panel system, it then becomes critical to ensure that such a point is singular. In other words, the panel should not be pinned inadvertently at any other point along its length. To do so would likely produce a failure from the thermal loads. On occasion, the thermal movement integrity of a roof system is violated because some construction detail or roof accessory mounting did not preserve this characteristic. Design and as-built construction should be scrutinized in this regard. A fascia break detail, for example, fixes the panel at the point of the break; to fix it again at its opposite end would constitute dual pinning.
How does thermal movement occur?
As metal panels get hot, they expand, increasing their length dimension. When they get cold, they contract, reducing that dimension. This cyclical changing of dimension is called thermal movement.
This is a linear effect. In other words, it will accumulate in direct proportion to the panels’ “unbroken” length. If panels sections are joined end-to-end with mechanical fasteners through the lap, then the unbroken length is the total length of two or more panels, not just one.
Thermal movement does not accumulate across the width of the panels because the unbroken length in that axis is so small. The geometry of the panels and their joining method at side joints allows flexure at each joint so the thermal effects never accumulate. Small, unitized metal covering products like shingles in like fashion minimize unbroken length dimensions; hence thermal movement is rarely a consideration for such systems.
Total (or worst-case) thermal movement is calculated by extending the material’s coefficient of expansion over its length and the anticipated in-service temperature range throughout its service life. It is the surface temperature of the material and not ambient air that affects these extremes.
The maximum high-end temperature will be conditioned by the color of the panel and its solar absorption characteristics (lighter colors and high-gloss finishes will be cooler than dark colors and low-gloss finishes). A dark-colored panel with low gloss at right angles to the summer sun can approach temperatures of 200 degrees. Use of “cool” (or reflective) pigments reduces these temperatures significantly because they lower the solar absorption.
In cold winter nighttime scenarios, the low extremes of surface temperature can actually dip 25 or 30 degrees below ambient air. This is because of the principles of radiant energy. Skyward-facing objects radiate heat energy to the night sky. As this energy transfer occurs, the material loses BTUs (heat), reducing its temperature. It is this same effect that results in dew or frost on the ground, roof or windshield of your car when vertical surfaces do not. It is a combination of these factors that can result in ∆T (difference of hottest to coldest surface temperatures) figures of close to 250 degrees in cold northern climates.
Strictly speaking, it is the differential expansion between roof panels and structure that must be accommodated by the panels’ attachment clips. The clip (or its base in the case of two-piece clips) is mounted to the structure; panels move differentially to both. An open canopy structure may experience some temperature-induced change in dimension that would reduce the differential movement of its roof panels. Most often, however, the structure is a conditioned element inside a shaded or insulated building envelope. When this is the case, it experiences little or no change in temperature, and therefore no change in length. This means the differential movement between roof and structure is equal to the total movement of the roof with no offsetting or mitigating thermal cycling of the structure.
Unlike many other aspects of engineering and design, calculations involving anticipated thermal movement are not augmented by factors of safety. In fact, it is not unusual to see as low as 80 percent of this theoretical calculated thermal movement actually used in design. Panels distort a bit; structural mountings or members may be deflected and strained, but roofs don’t seem to fail.
On the other hand, if thermal-movement calculations are based upon ambient air (a frequent and novice mistake), they will often be only 50 percent of the correct extremes, and I have seen such roofs fail — repeatedly. A single metal panel exerts forces measured in tons when it tries to move thermally; hence, undue restriction of this anticipated movement can easily precipitate attachment fatigue and failure.
I have also seen professional engineers who try to prove the panel will undergo a “buckling” failure before the attachment will fail. In other words, it will hump up, oil-can or otherwise move out of plane to relieve the thermal forces. The trouble with this theory is that a member only buckles in compression (during an expansion cycle). Most attachment fatigue and failure occurs in tension (during cold-cycle contraction).
Manufactured two-piece clips usually include some mechanism to ensure they are centered at the time of installation. In theory, the roof panels are installed somewhere in the midrange of their in-service extremes. Although it may not be exactly at the halfway mark, common practice does not compensate for exact temperatures at installation. It is absurd to suppose that installers will move clips to some predetermined location contingent upon installation temperature. Even if they did, the temperature is likely to be different when the mechanical seaming is done, thus botching the whole theory. Most clips find their own “centering” within the range of thermal cycling of the roof within the first few months of service.
S-5! founder, Rob Haddock, serves as director of the Metal Roof Advisory Group Ltd. and is an adjunct faculty member at the University of Wisconsin School of Engineering. He has authored a number of training and educational curricula for various trade groups and studied metal roofing history and applications both domestically and abroad.
Earlier articles in this series can be viewed online. To read the first article Metal vs. Hurricanes, visit http://www.constructionmagnet.com/metal-roofing-magazine/metal-vs-mother-nature-hurricanes-and-high-winds; to read the second article Metal vs. Hail, visit www.constructionmagnet.com/metal-roofing-magazine/metal-vs-mother-nature-hail; to read the third article Metal vs. Fire, visit http://www.constructionmagnet.com/metal-roofing-magazine/metal-vs-mother-nature-fire.