We are installing a deck over a cleanroom that will be constructed of 1-1/2" corrugated steel decking with 1-1/2" polysio insulation board and then 3/4" ResinDex unfinished board. Typically we would screw the sandwich together using screws that would penetrate the corrugated metal deck. I am thinking of glueing the polysio board down to the deck and then glueing the ResinDek to that. This is a Class A fire barrier so am wondering if this is permisable in the code.

Not sure it will work because you have layers that expand and contract at different rates. Maybe the corrugated steel will absorb that but not sure. Guessing the deck is a roof and not a walking area? Being I have never seen this in the code it defaults to UL listings, so if it is not in the code then is the product listed to work in that situation, likely no....You could try to get a PE to sign off on it but not likely seeing that either.....

Or you could do it and see if inspector catches it and put screws in later? is this your own place? or for a costumer.....for costumer I wouldn't do it.