plastic sheet on roof?

 
  This is for those in the roofing business.  What are your thoughts on having plastic sheets down on top of toung & groove notty pine (Exterior) then covering with ridgid foam insulation panels,decking, then asphalt shingles?
  My ceiling is finished knotty pine inside and old ridgid foam insulation is disintegrating into foam sand and falling through the T&G and rains into living room.
  The make of my roof is;  2x6 knotty pine, 1.5" ridgid foam insulation panels,then 3\8" plywood  deck, roofing felt (Paper) then asphalt shingles.
  I've been toying with an idea of laying some plastic sheet down on top of T&G (Exterior) then putting the ridgig from panels down.
  My concern is condensation forming between the plastic and the T&G.
The plastic would stop crap from falling through the T&G .
  Any thoughts?

Maybe modern building paper like Tyvek would be better than plastic. The paper is very rugged and you shouldn't have condensation problems with it.

Look at the 15 or 30 lb roofing felt used for tar and gravel roofs. Has little holes that would alloy things to breath not sweet as plastic would

What's the slope of the roof?
Ice Shield would seal any little hole that might let sand and grit come through, but it's pricey.

 
  Roof slope is approximately 22deg.  Didn't think about tyvek,it might be a little more than the plastic rolls I was thinking about using. The roof area I was
Thinking about laying the plastic is 504sq ft.

You are correct, if you apply a layer of plastic over the wood deck and than the insulation the moisture from the house will not be able to escape and will condensate. When this happens the moisture will be trapped and will rot out the top two to 4 ft of ceiling and create dry rot and mold. The way to install it is with a nailable deck which is the insulation, a 3/4 inch air gap and the plywood and at the ridge install a ridgevent to allow air flow.

 
  My ceiling is the bottom of the roof, its not a typical peaked roof, but open, and high to the back. I thought,by using plastic under the foam panels might cause issues, so was hesitant to proceed.
Thanks.
  I think I will lay some roofing felt down on all the new T&G above my chimney (Where most of the rotted decking was) and 3 new water leaks developed back in the March rains)