Getting Ahead of the Curve: Simple Tips for Curved Metal Roof Designs

by Jonathan McGaha | October 25, 2010 12:00 am

Curved metal roofing and decking are popular for all types of buildings from school gyms to shopping centers. This article will provide design pointers about metal roof and deck systems that are formed using “crimp-curved” panels, the prevalent process today for prefabricating radiused panels. By using these tips, you can take maximum advantage of curved designs that are both eye-catching and economical while avoiding common design pitfalls.

 

Tips of the Trade

Think outside the box. Sometimes, curves can be used very creatively and symbolically. In airport terminals and aviation museums, for example, roof structures have been curved to mimic the look of an aircraft fuselage or wing. More often though, curved roofs are used quite simply to eliminate the boxy look associated with conventional building designs-creating a softer, more pleasing aesthetic.

There are many ways to achieve this goal, but one method is to design bow trusses on top of a conventional building, topped with a radiused roof that extends out and curves back toward the building wall to form a mansard or canopy. Another method is to literally go “outside the box” using arched entrance ways or window awnings to break up the visual monotony of big-box retail or industrial building forms.

 

Lighten up. One of the most common errors we see is over specification. Designers often assume they should use the same thickness gauge that would be required for straight panel applications. But crimp curving (unlike rollforming or stretch forming) greatly increases the load factor of the panels, often making it possible to go with a lighter-weight profile.

For structural decks, 18-gauge panels are often unnecessary when 20- or 22-gauge can do the job more economically. For roofs, 24- gauge is the most frequent and often the best choice because it combines good strength and longevity with the ability to be curved down to a tight radii. Sometimes you can go even lighter: 26-gauge panels are suitable for curving, but the yield should be less than or equal to 50 ksi or panels are likely to be brittle and subject to cracking. Twenty-six-gauge panels are best suited to larger-radius applications and reskinning Quonset huts, warehouses, sound studios and similar structures.

 

Eliminate extra layers. In underestimating the strength achieved by crimp curving, designers consequently tend to over design curved roof structures. Here are three common ways to eliminate an extra, unnecessary layer of material:

1. For roofing applications, eliminate a solid deck and apply the curved roof panels directly to purlins. Both exposed and concealed fastener roofs can be constructed in this manner though there are some restrictions on the types of standing seam that may be used.

2. For acoustical applications such as gyms, use acoustical perforated decking to create a “two-in-one” ceiling and structural roof deck. The curved perforated deck, left exposed on the underside, can eliminate the need for a separate acoustical ceiling.

3. For open roofed structures such as canopies or walkway covers, you may choose a roofing or decking profile- there is no need to use both. The span characteristics of the single-layer cover will be excellent and require minimal substructure support.

As an interesting footnote, the structural framing for curved walkway covers, carports, shelters, etc., can be built from cold-rolled framing and purlins, which are less costly and easier to ship and handle than hot-rolled components. This is an often-overlooked approach that can bring significant additional savings.

 

Design metal roof decks to go “against the grain.” When crimp-curving is used, metal roof decking panels can be installed “against the grain” or across the top of the structure as opposed to curved decks formed from straight panels laid parallel to the length of the building. The resulting deck is stronger and more weathertight and can bring all the economic benefits previously mentioned (lighter-gauge panels, reduced substructure). This approach also frequently allows the use of a B-deck profile, which is more economical and provides better installation coverage than an N-deck.

Consider curved flashings to complement your roof. Segmented curved flashings can be produced at a lower cost than stretch forming or other methods typically used to produce these components. Curved flashings can be of any size and can improve the appearance and watertightness of a crimp-curved, rollformed or curved membrane roof. There are just a couple limitations: Segmented flashings are produced in single-curve radii only, and they are not suitable where specs require flashings made to standards of the Sheet Metal and Air Conditioning Contractors’ National Association, Chantilly, Va.

Know the curving parameters. For successful designs, you need to be aware of guidelines regarding minimum radii, substrates suitable for curving, maximum panel widths and lengths, etc. Instead of starting with a mindset of “I’ve always used 18 guage, so that’s what I will do here,” ask yourself, “What is the best way to achieve my design goal using these parameters?” You might be surprised at the depth and breadth of possibilities. Many panels can be curved in concave or convex directions to form single, double or multiple-radius curves. Nestable B-deck panels can even be curved into S-shapes to make bold design statements. Curving limitations vary according to panel substrate, gauge and depth, so it is best to consult your curving fabricator in the early design stages.

 

Terry Holman is president of Met-Tile Inc., Ontario, Calif. For further information, visit www.met-tile.com.

http://www.met-tile.com [1]

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