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Component Manufacturing dverti$er
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Don’t Forget! You Saw it in the
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Adverti$er
January 2019 #11234 Page #46
Addressing the Roof Truss Design Note: “Provide adequate drainage to prevent water ponding.”
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By Frank Woeste and Scott Coffman The purpose of this article to examine the significance and implementation of a typical note that appears on metal plate connected roof-truss drawings when a top-chord-pitch of ¼:12 or less is specified by the Construction Documents. Three questions will be addressed: 1. What is the mechanism of “water ponding”? 2. How does the building code (IBC) address the ponding subject whereby the project design team can establish that the installed trusses and roofing system will “provide adequate drainage to prevent water ponding”?; and 3. To whom is the instruction “Provide adequate drainage” directed?
1. Visualizing Roof Ponding Behavior—A Safety Issue The loading mechanism of trusses by “water ponding” creates a path for the possibility of a catastrophic structural failure. Unlike a uniform load, such as a 10 psf Dead Load (DL) that acts on the entire span and does not change based on the deflection of the span in-service, loading by water is a dynamic process whereby any additional deflection by the water produces more deflection. To help understand and visualize the process, a Florida carport that experienced a ponding failure is depicted in Figure 1. The carport in this example is designed with a slope for drainage to a gutter on the left side. The blue arrows show the flow of rain water that took place for decades due to a sloped roof. In contrast, the red arrow depicts the flow of water toward the center section of the span which produced substantial panel deflections due to the weight of the accumulated water.
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