Are you True to your Curves?
by Jonathan McGaha | April 30, 2014 12:00 am
Not all curved metal roofs are created equal. Understanding the variables an architect must take into consideration is critical, but the designs that can be accomplished with curved roofs can be stunning and functional.
For an architect, curved metal roofs, or radius roofs, are ideal architectural elements with an organic look and a functional design for large projects such as gymnasiums, airplane hangars and ice rinks. They can also provide striking design elements for smaller entrance canopies, dormers and walkway covers. Curved metal roofing makes a statement with beautiful, contemporary designs on churches, homes and much more.
When incorporating a curved metal roof into a building design, there are several important details that should be considered.
The Foundation: Building Structure and Substrate Construction
The composition of the underlying deck and framing assembly has a significant impact on the success of a curved metal roofing project. As in any roof assembly, having a smooth substrate is critical. Any inconsistencies will telegraph through the assembly and can interfere with the roof performance and aesthetics. Available substrates vary between smaller curved roofs and larger curved roof designs.
As the roof radius becomes smaller, corrugated steel deck running perpendicular to the roof slope may conform better as opposed to plywood or other materials. Layers of staggered rigid insulation can be added to the roof assembly along with the appropriate underlayments and vapor barriers. Large barrel-shaped roofs can create expansive low-slope regions near the apex of the roof, but the proper panel, substrate selection and installation techniques can reduce any water shedding and infiltration concerns.
Selecting the Correct Panel
Metal panels available for curved applications generally fall into four categories: single- and double-lock seam, snap-lock seam, snap-on cap and batten seam, and corrugated panels. For the most demanding applications, single- and double-lock seam is the best choice. They have taller seam heights to resist standing water and mechanically crimped seams that withstand the harshest conditions. These panels are specified down to slopes as low as 1/4:12 and generally work well for use at the apex of barrel roofs where the slope goes to zero.
These panel systems are water resistant and some have been tested to the stringent ASTM E2140 6-inch standing water test. Because of the taller seam heights, these panels may exhibit slight waviness in the vertical panel leg due to the material stretching that occurs during the curving process. Snap-lock seam panels are primarily only available as spring curved, and, due to their reduced weathertightness, are generally limited to slopes of around 3:12 and greater.
Snap-on cap and batten seam panels are the most versatile, with the ability to be curved to a very small radius-as little as a few feet. In curved applications where slopes dip below the 3:12 minimum, snap-lock seam, snap-on cap and batten seam panels require a waterproof underlayment.
In applications where weathertightness may not be a concern, single- and double-lock seam panels and/or expensive underlayments can be avoided. Corrugated products follow the same restriction and underlayment guidelines as snap-lock seam, snap-on cap and batten seam panels.
Curving Choices
Panels are usually either curved at the roofing manufacturer’s location or at the job site. Factory-curved panels present crating and shipping limitations and associated expenses. Curved panels do not nest, so fewer panels are available per crate. Crating becomes more complicated as it is built to fully enclose the curved shape of the non-nestable finished panels. Shipping is also a consideration as trucks can only carry panels of a particular rise (height of curve) and length.
Job-site curved panels eliminate the extensive crating needs, but require space for curving (and on-site rollforming equipment, if applicable) operations and on-site storage of product. All types of panels can be spring curved
(walked down) onto large radius roofs, but this capability varies greatly on panel type. Single- and double-lock seam and snap-lock seam panels with a radius of around 200 to 300 feet can be spring curved, as well as snap-on cap and batten seam panels with a radius as low as 50 feet.
Corrugated panels can be spring curved down to about 50 feet, although limitations are heavily dependent upon panel shape, gauge and material grade. Lastly, most corrugated products are also available as crimp-curved, which is a secondary process that takes flat panels and crimps them at regular intervals to produce the desired curve. A select few corrugated products may also be available as smooth curved.
Installation Considerations
The curved roof size, shape and selected panel type often determines the ideal fixed points to properly allow the panels to thermally expand and contract, as well as to resist drag loads. Panel fixing can be achieved by more than one method, but that too may be dictated by panel lengths and clip thermal movement capacities. Fixed clips, through-fastened end laps and eave fixing are common methods to hold the panels in place and resist drag loads. Panel points of fixity should always occur directly over a solid surface, so additional blocking may be required to achieve a suitable fix point.
Large curved roofs often require long length panels. Panel end laps may be required to facilitate handling and staging of panels, as well as potential handling and storage of panels that are subject to field curving. If the project requires lapped panels, the lapping should occur on the downslope side, not at the apex, to reduce the potential for water intrusion.
Installation of a metal curved roof differs little from the installation of a non-curved metal roof. Details must be configured to allow for adequate thermal movement, and the designer must make a choice between curved or segmented rake flashings. Segmented flashings have greater flexibility and fewer limitations when compared to curved flashings and are more cost effective, but provide a different finished look.
Curved metal roofs generally require more time and care to install, but, as a design element, curved metal roofs can break up what would otherwise be a massive roof spanning the entire project and can also create gentle spaces in the interior of a project, adding an interesting architectural element.
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Brian Gough is the senior product development engineer at AEP Span, West Sacramento, Calif., a division of ASC Profiles. To learn more, visit www.aepspan.com.
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