WWTEng:
Ask the Arch. if his profession licence expects that he follow the applicable building codes; and/or use skilled judgement which can be supported (defended) using established principals, when something is not specifically covered by the codes. Ask him if he expects you to follow the Structural building codes, or will he send you a letter showing that he assumes full responsibility, and holds you harmless, where he wants you to deviate. Then send him a letter showing your objections and an explanation of the code deviations he is asking for. He hired you for your engineering expertise and judgement, and should take it; or else explain to your satisfaction how his detail complies with today’s code.
He may well need the top layer of plywd. to attach his metal deck to, but that better be thick enough for the type of fastening system the decking requires. And, that plywd. support layer has to support all roof loadings, in particular uplift loads in many areas of the roof. That uplift does mean nailing or screwing down into the 2x6 structural deck with sufficient cap’y., but that’s different than any shear transfer. Woodman’s table 4.2D is about what I expected to see, but mine would have been an older version. We don’t know what shear cap’y. you need from this diaphragm, but you have to explain to the Arch. what the code allows you to do, or get that letter, or get him to use that other (magician) engineer. Obviously, you have to attach the 2x6 decking to the primary structural members sufficiently to transfer all the various roof loads, then maybe btwn. planks, and then maybe plywd. atop the decking for additional stiffening and strength. What the Arch. thinks he saw work/saw done 25 years ago may not meet today’s codes, we have much higher lateral load and wind load requirements with the latest rounds of code changes. This should mean that the newer buildings are being designed to a higher required standard, not that the old building is obviously at risk of failure, or that the earlier engineer didn’t do his job.