b'THE OFFICIAL NFBA MAGAZINEcontinued from page: 17Temperature Fluctuations framing because they directly impact temperature differences Temperaturesareconstantlychanging.Inadditionbetween panels and framing.First is the presence of thermal tobeingthedrivingforceforcyclesofmoistureinsulation between the panels and framing the more condensationandevaporation,italsoproducesinsulation, the greater the temperature difference.Second cyclesofthermalexpansionandcontraction. is panel surface finish. On a sunny 90 F day, the temperature on an unpainted metal roof can be upward to 145 F.While All common building materials expand and contractthis is less than many darker colored roofs, it is 30 to 40 F when heated and cooled, respectively. The magnitude ofmore than white roofs and roofs with paint coatings infused this change in length is calculated using the materialswith materials that selectively reflect infrared radiation.coefficient of linear thermal expansion (CLTE, or ).The thermal expansion of roof panels that takes place under RESEARCH & TECHNOLOGYSpecifically, the change in length of a material is equal tosudden sun exposure (breaking clouds) may be associated with its original length multiplied by its CLTE and the changeaudible popping sounds as components grow and slide past in temperature. When the Greek letter delta () is usedeach other. These sounds should be a reminder of the cyclic to indicate the difference or change, the equation is: movements that can loosen fasteners and slot roofing panels. Length = Length x CLTE xTemperature The longer a metal roof panel, the greater will be its The CLTE for steel and concrete are both approximatelyoverall change in length for a given temperature change. 12x10 -6(C) -1 . Wood has a parallel-to-grain CLTE rangingTo avoid problems with differential movements between from 3.1x10 -6to 4.5x10 -6(C) -1(Wood Handbook, USDAlong panels and underlying framing it is best to avoid Forest Service), glass has a CLTE around 8.5x10 -6(C) -1 , andthrough-fastening of the panels. Instead, switch to a most unreinforced plastics a CLTE between 50x10 -6andfloating roof systema system with metal panels that 150x10 -6(C) -1 . With a CLTE for steel of 12x10-6 (C) -1(=sare held down with clips that allow for the unrestricted 6.67x10 -6(F) -1 ), a 30 ft long sheet of steel when warmedexpansion and contraction of the panels. One drawback by 50F would increase in length by 0.01 ft or 1/8 inch.of floating roof systems is that they do not contribute to diaphragm action (i.e., the distribution lateral building forces 30 ft x 6.67x10 -6 (F) -1x 50F = 0.01 ft induced by wind and earthquake) to any significant degree.0.01 ft x 12 in/ft = 0.12 in Figure 9 shows an asphalt shingled roof 20 years after A building component that can freely expand and contractoriginal installation. This was a black roof and hence is generally no concern from a durability perspective.frequently subjected to extreme temperature swings. Conversely, attention must be paid to any component thatThe resulting expansion and contraction produced the is restricted from freely changing length as this typicallycracking shown. It is important to note that cracking means that when heated and/or cooled it will induceexposesasphaltbelowthegranularsurfacemaking forces in other components. Such is the case when there isitmuchmoresusceptibletophotodegradation.differential expansion and contraction between two or more mechanically connected components. Of greatest concern in this regard are the localized forces associated with the through-fastening of metal roof panels to underlying roof framing. Metal roofing is thin and thus has a low heat capacity per unit surface area.This means it will heat up and expand relatively quickly when exposed to direct sunlight. Underlying framing will neither expand as much nor as quickly as thin metal roofing because of its greater heat capacity, protection from direct sunlight, and in the case of wood roof framing, because of its lower CLTE.This results in differential thermal expansion between panels and framing and a build up of forces in and around through fasteners. Over time, the forces and differential movement will tend to loosen up through-fasteners or form slots in the metal panels; actions that increase the likelihood of roof leaks.FIGURE 9. BADLY DETERIORATED BLACK ASPHALT SHINGLE ROOF There are two factors that significantly impact differentialAFTER ONLY 20 YEARS OF SERVICE.thermal expansion between roof panels and underlying 18 / FRAME BUILDER - JAN2023'