(The following is taken from the Sheet Metal and Air Conditioning Contractors' National Association's Architectural Sheet Metal Manual, Fifth Edition.)

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.

  • Long-length sections should be considered for reasons of least cost, visual effect and greater potential for water tightness.

  • Prefinished metals should not be required to have soldered joints because of expense and possible warranty violations by removing coating system at the joint. Other joint designs and high-quality sealants, gaskets and tapes are available and are recommended for prefinished metals.

  • Prefinished systems should have minimum exposure of sealants and touch-up paint because of differential coloring or fading relative to the main surface.

  • Provide appropriate temperature-resistant waterproof membrane as recommended under wall and roof systems.

  • Wind, live and dead structural loads, fastener/clip size, and spacing conditions must be reviewed for local conditions and code requirements.

  • Factors to consider in designing eave-to-ride runs of standing seam and batten-seam roofing are:

    -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.

  • Evaluate the potential for condensation occurrence within the roof and walls. Consult the American Society for Testing and Materials Standard E 241, Standard Recommended Practices for Increasing Durability of Building Constructions Against Water-Induced Damage, and American Society of Heating, Refrigerating and Air-Conditioning Engineers handbooks.

  • Design penetration locations to avoid roof-panel-joint locations. Limit penetrations.

  • Evaluate patterns of roof seams and ribs to see prospects of nonalignment at hips and valleys, lower planes of roofs with higher planes, roof vs. wall panels, mullions, etc. Criteria for acceptance with acknowledgement of practical limitations should be indicated. Wedge-shaped panels may be more expensive than those with parallel edges.

  • Require a uniform, true, warp-and-deflection-controlled noncorrosive substrate of continuous- or grid-contact support nature for metal roofing. The waterproofing and visual effect of metal systems can be compromised by irregular support systems.

  • "Oil canning," a perceived waviness in flat areas of metal panels, occurs naturally in the manufacture of metal sheets and coils. ASTM standards should be studies to discover the commercial-standard tolerances. Ordering tension-leveled and re-squared metal improves flatness.

    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.)