Metal roofing beneath roof deck

Paul Arquette
1. I have designed a 2nd-story residential deck. It is to be built partially above a 1st-story deck, and partially above a finished space. 2. The deck joists must be 2x12 because they span a long distance. 3. A ceiling must be applied to the bottom of the joists. Due to space limitations, the ceiling cannot be lower than the bottom of the joists. 4. Each joist space is to serve as a trough to drain rain water. Each trough is to have a sloping layer of plywood. 5. The plywood troughs and joists are to be covered with metal roofing. 6. Composite decking (Trex or similar) is to be screwed to the tops of the joists. Because the metal roofing wraps around the joists, each screw will pierce the metal decking. QUESTIONS: A. Do I need to be concerned about water leaking through the screw holes? B. Do I need to be concerned about the metal expanding and contracting with temperature fluctuations, thus making the screw holes bigger? C. What type of metal should I use for the metal roofing and the deck screws? I want the metals to be the same to prevent corrosion. Obviously, the screws will need to be strong enough to hold the decking in place, and the metal will have to be a type that's appropriate for roofing. Attached are three details per this project. Any advice or constructive criticism anyone can give would be appreciated. Thank you! Best regards, Paul A., AIA Architect
Todd Miller
Isaiah Industries, Inc.
Paul, thanks for posting. While what you plan may be feasible (though there are some complicating factors), there are systems designed specifically for this purpose. They are called "Underdeck" systems ... have you looked into any of those? Frankly, those systems are engineered and designed for this purpose so I would feel much better about your using one of these. One manufacturer I like is Quality Edge and they have an "Inside-Out Underdeck" system with many beautiful options. Thanks and good luck.
Eric Novotny
An informed customer is the Best Customer!
Paul, I appreciate your inventiveness but this thing looks like a recipe for disaster if you as me. The answer to all of your questions are yes. You would want to use stainless to prevent galvanic corrosion but I don't know what you have specified for the metal, underdeck, drain system. You are cladding the wood in metal and the first bit of moisture will not be able to dry out to either side. This will foster rot and make every detail so critical that the likelihood for failure is high. How about posting up a picture of the home so that we can see what you are up against.

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