Can you double up plywood roof sheathing without inviting disaster?

2000 sf of 1/2" CDX ply went on the roof before I had a chance to take a look at it and it is really sorry stuff.  It got nailed over 2 X 12 joists @24" o.c. on a 2:12 pitch roof.  If I had known the product was going to be so flimsy, I would have sent it back for 5/8" at the least.  We only had to rip about 5 sheets off for serious voids, but the rest of it isn't much stiffer.  When I was (a lot) younger, 1/2" plywood was actually pretty thick.  I'm sure it will support the roofing just fine, but I am concerned about higher-than-normal foot traffic to service the rooftop mounted PV and solar panels, heat pump units, etc.  I'm 235 lbs and when I walk on it, some of the roofing clips pop off from edge deflection as it is.

We are racing the monsoon season and right now, I am leaning towards nailing down a second layer of the same stuff perpendicular to the first before putting on the peel and stick underlayment.  Ripping it all off and starting again might take too much time and it has already been wet for a week.......

So, in order of top to bottom, it would be standing seam metal roofing, Certainteed Metalayment, (2) thicknesses of 1/2" CDX 4/5 ply, 2" spray foam from below and 9-1/2" blown-in-blanket above the gypsum wallboard.

Aren't two sheets of ply, sandwiched right next to each other, going to be essentially like a single piece of ply twice as thick for vapor and drying purposes or will this put something over the top in Western Washington?

Maybe the added support and rigidity from the spray foam will solve the problem?

I concur that plywood keeps getting weaker and made from crappier wood. But, even 3/8” plywood can easily span to 32” on a flat roof and still meet the minimum deflection requirements with an engineer’s stamp in most areas of the U.S. The “walking” on the roof will likely be the largest point load on the roof, and it will not permanently deflect the plywood, but it could damage the roofing felt with enough deflection at the joints or pull spray foam off the underside, so your concern is not unwarranted. If it were my roof, I’d leave it as is, but if you do decide to put another layer of ½”on, put it perpendicular to the rafters not the longways. As you are probably aware, the orientation of the outer veneer layers spanning the short direction will give the plywood its maximum strength – both layers in the same orientation, but do a two directional offset to isolate the joints.

As far as vapor and drying, your putting foam underneath, so its essentially moot, even if it's open cell foam.

What you haven’t mentioned is your location or snow load, however, a more important load would be where drifting can occur against a wall. Also, isn’t the plywood stamped for 24” span?

About the foam underneath the roof sheathing: I assume you are filling the entire attic space with blown-in up to the underside of the spray foam? You don't want to leave a void unless it's ventalated - that would defeat merits of the spray foam shell layer. There is plenty of discussion on this forum and others if you want more on the physics behind not having cavities in your insulation system.

but if you do decide to put another layer of ½”on, put it perpendicular to the rafters not the longways

The first layer is, of course, perpendicular to the rafters. Shouldn't the next layer go the other way?

What you haven’t mentioned is your location or snow load

Western Washington

Snow load is 25

I assume you are filling the entire attic space with blown-in up to the underside of the spray foam?

There is no attic space. It's all unvented "cathedral" ceiling. The blown-in will fill the entire rafter or joist space below the spray foam layer.

I can see the reasoning behind running the plywood the other way, if you were gluing them together to make a thicker plywood.  However, each sheet can slip past the other, so each should be optimized independently because their cummulative effect will never match that of a glued laminated sheet of the same thickness. Whichever way you orient the sheets, if you stagger the joints, it will ameliorate your greatest potential problem – deflection at the joints. 

Perhaps you’ve been on a roof where someone put in a filler piece of plywood oriented in the wrong fiber direction and when you stepped on it, you immediately realized viscerally how weak this piece is compared to the others.  That piece likely would sag over time.    

Thanks for your comments.

Unfortunately, with the imminent arrival of the rainy season, I had to leave it the way it was.

Maybe some super-rigid foam below to make it stiffer.......

I don't like the idea of a rottable substance (plywood) next to an impermeable exterior membrane with warm air behind it.

It's unsettling, but everyone says it can dry inward.

So, does that mean interior paint with no vapor barrier?