Metal Roof Thermal Movement and Sliding Clip Range

THE PANEL TEMPERATURE RANGE IS MUCH WIDER THAN THE AIR TEMPERATURE RANGE, and that is what makes the movement large. A dark panel in summer sun reaches well above ambient, and on a clear winter night it radiates to the sky and goes below it, so a design based on the local air temperature range understates the movement substantially -- a swing of 140 to 180 degrees Fahrenheit on the panel itself is ordinary across most of the country. Aluminium moves about twice as far as steel for the same swing, which is why the material belongs in the calculation rather than in a rule of thumb. THE FIXED POINT DETERMINES WHERE THE MOVEMENT GOES. Each panel is anchored at one location -- eave, ridge, or a point between -- and expands away from it, so the movement at the far end is the full expansion of the whole panel length. A panel fixed at its middle halves the movement at each end, which on very long panels is the only way to keep the clip travel within range, and the saving from moving the fixed point is reported here because it is often the cheapest fix available. THE FAILURE IS PROGRESSIVE RATHER THAN SUDDEN, which is why it gets blamed on workmanship. Clips at the ends of long panels reach the limit of their travel; then the movement goes into the clip itself, then into the fastener holes, which elongate; and the roof gradually loses its attachment and begins to oil-can and leak at the details. It looks like poor installation years after the fact, and it is a movement allowance that was never there. A single straight panel run, uniform temperature, free to move. It does not address the eave and ridge details themselves, the flashing and closures that have to accommodate the same movement, or the sealant joints that carry it at penetrations and transitions -- all of which fail the same way. It does not evaluate the clip's structural capacity, only its travel, and a clip adequate in travel can still be inadequate in uplift. It does not address panels restrained at both ends, which is a common detailing error that turns movement into stress, or the noise a moving roof makes. Panel temperature ranges vary by colour, slope, orientation, insulation, and climate. The panel manufacturer's expansion and clip data, SMACNA and the metal building manufacturers' guidance, and the roofing contractor govern.

movement = coefficient of thermal expansion x length x temperature swing, taken on the PANEL temperature range; the movement at an end is that end's share of the length from the fixed point; the longest panel a clip supports is its travel divided by the movement per unit length.

The thermal expansion relation by name, with steel about 6.5e-06 per degF, aluminium about 1.28e-05 and copper about 9.8e-06. The PANEL range is much wider than the air range: a 140 to 180 degF swing is ordinary. It checks clip TRAVEL, not clip capacity. The panel manufacturer's expansion and clip data, SMACNA and the metal building manufacturers' guidance, and the roofing contractor govern.

One multiplication on a coefficient and a temperature range; no manufacturer clip table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: panel_length_ft, alpha_per_f, temp_swing_f, fixed_point_fraction, clip_travel_in, total_movement_in, movement_up_slope_in, governing_movement_in, travel_margin_in, max_panel_length_ft, centre_fixed_movement_in

• Panel temperature range is entered it varies with colour, slope, orientation, insulation and climatethe panel manufacturer's data
• Travel, not capacity a clip adequate in travel can be inadequate in upliftthe panel manufacturer's tested assembly
• One free straight run eave and ridge details, flashing and sealant carry the same movementSMACNA and the metal building manufacturers' guidance