Hi all,

Toying around with an assembly for a fiberglass roof deck in Philadelphia.  Rehabbing a nearly condemned property and there are certain budgetary restraints, which is why we’re doing a fiberglass roof deck.  Many of them fail due to condensation from the interior – standard practice here is to spray 2″ of closed cell insulation, and either top off with open cell or flash and batt with fiberglass.

I want to do exterior insulation under the roof deck – thinking of the following assembly:
5/8″ zip system with taped seams
(2) layers of 2.5″ polyiso
3/4″ plywood
fiberglass roofdeck

There’s a lot I don’t like about this assembly – in a perfect world, I’d do a tpo warm roof with a floating deck.  It’s a rental house and we can’t afford that.  My fiberglass installer is very talented and am confident in his work.  

My question is, should I be gluing the polyiso, screwing through the 3/4″ plywood and into the joists, both?  Is this destined for failure?

I don’t like the idea of potentially trapping water between 2 membranes; in this case the zip and the fiberglass.  We’re doing everything we can to ensure success.  All the original roof joists have been sistered on both sides with 2x12s and through bolted, our slope is .5″/foot, and we will have a continuous plane (only one small cricket). 

There’s not much history for residential warm flat roofs in philly, and even less on fiberglass warm roofs.  Any insight would be appreciated!

Sasha