Guidelines for designing and installing metal roofing
Sheet-metal wall and roof systems offer virtually unlimited cost-effective choices to meet job requirements. ... There are two general categories of metal systems: those that require continuously backed support and those that are structurally adequate with intermittent support.... There are many systems the sheet-metal contractor can provide.
-Whether expansion will be controlled by anchoring the roof at the ridge, at the midpoint of a rise, at the eave or in the transverse seams of pans that are limited to 8 or 10 feet (2.5 to 3 meters).
-Forty-five feet (13.7 meters) is a practical limit for continuous pan runs. Expansion joints should occur at a maximum of 32 feet (10.5 meters) in aluminum. A step should be provided in a run of 100 feet (30.5 meters) or more. Panels of 10 feet (3 meters) or more will normally require accommodation of expansion.
-Cleats should be at 12-inch (300 millimeters) maximum intervals. Two fasteners per cleat are used with cleat tabs folded over the fastener heads. Expansion cleats should be used for standing-seam pans of 30 feet (9.1 meters) or more continuous run. Fixed cleats can be placed midspan with 5- to 10-foot (1.5- to 3-meter) spacing, with expansion cleats elsewhere to allow movement in two directions.
-Where alignment of panels is critical at transitions and how much flexibility of movement is needed at transitions. Expense is an important variable for these conditions.
Waviness can also be caused by misaligned or contoured supports, shrinking substrate, warps in roll-forming, twist during handling, lack of expansion allowance at installation and over-driving of fasteners. Thin metals are more susceptible. Panel gauges and widths scheduled in this manual were selected to minimize oil canning with proper installation.
Use of nonreflective or textured finishes masks waviness. Use of slope restrictions to 1 percent or 2 percent as is done on curtain walls is not practical for metal roofing.
(The sixth edition of SMACNA's Architectural Sheet Metal Manual is now available. For information, write 4201 Lafayette Center Drive, Chantilly, VA 20151-1209; call (703) 803-2980; fax (703) 803-3732; see www.smacna.org on the Internet.)