Thermal Movement on a 180 ft Standing-Seam Roof
A 24-gauge steel standing-seam roof is 180 ft long with 12 in. wide panels and no endlaps. It is installed at an ambient temperature of 70°F. The roof surface is expected to reach 160°F in summer and -20°F in winter. The specification allows 1/8 in. of movement per 10 ft of panel length. The installer plans to fasten both ends of each panel and use fixed clips at 12 in. on center. The ridge cap is a standard 6 in. cap with butyl sealant, and the eave has a 2 in. fascia. The general contractor is concerned about oil canning and wants the roof closed before a forecast rain.
1. Calculate the thermal movement and compare it with the specification allowance.
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The panel length is 180 ft, or 2160 in. The temperature range is from -20°F to 160°F, a total of 180°F. Using the approximate coefficient of thermal expansion for steel, 6.5 x 10^-6 in./in./°F, the total movement is 2160 x 6.5 x 10^-6 x 180, which equals about 2.53 in. If the installation temperature is 70°F, the expansion to 160°F is 2160 x 6.5 x 10^-6 x 90, or about 1.26 in., and the contraction to -20°F is another 1.26 in. The specification allowance of 1/8 in. per 10 ft gives 180/10 x 0.125, or 2.25 in., which is slightly less than the calculated 2.53 in. The design should therefore use the calculated movement, not the minimum allowance, and provide at least 1.3 in. of movement in each direction from the fixed point.
2. Evaluate the plan to fasten both ends and use fixed clips at 12 in. on center.
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Fastening both ends prevents the panels from expanding and contracting, so the movement will go into buckling, oil canning, or fastener distress. A standing-seam roof needs a fixed point, usually at one eave or at a mid-slope point, with sliding clips elsewhere so the panels can move. Fixed clips at 12 in. on center may be acceptable for uplift at some locations, but they must not restrain longitudinal movement unless they are the designated fixed clips. The 180 ft run with no endlaps is long enough that an expansion joint or a designed end detail is likely needed. The ridge and eave details must allow at least the calculated movement without tearing sealant or opening the cap.
3. Describe the ridge, eave, and clip details you would require before the roof is closed.
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The panels should be fixed at one end and allowed to float at the other, with sliding clips spaced per the manufacturer's uplift table. The ridge cap should be attached to the fixed structure, not to the panel ends, and should have a sliding closure or enough clearance to accommodate at least 1.3 in. of movement at each end. The eave should use a sliding cleat or a standoff that lets the panel move without loading the fascia. Sealant should be applied in a way that allows shear movement, not as a rigid bond across the joint. Before closing, the installer should mark the fixed point, verify clip type and spacing, and confirm that the ridge and eave details match the calculated movement. Closing the roof before rain is not a reason to omit these details.