Help: What goes in the chimney framing gap?

[mannyrock]

Dear Guys,
 
    I am having to reframe my roof around the chimney,  because believe it or not, when the redneck !@#$#@ built it 32 years ago, they short cut the tops of the rafters to create a rectangular opening for the chimney to come up through the roof, and then just left the top ends of the cut rafters leaning against the chimney itself!  No  horizontal header board was installed to fasten the rafter tops to.  Heck, the cut-off rafters were not even nailed or screwed in any manner.    Just free floating up and down against the side of the chimney, until the butt ends are rotted out from dampness and torn up by the brick and morter.
 
    I am installing a strong horizontal double header board, supported by the adjacent trusses, and the ends of short-cut rafters are being cut at the exact angle to fit flush against the header board.  All will be properly fastenedd using hangers and gussets and nails.
 
    My header board will be located two inches away from the brick of the chimney.    But here is my question. What is suppose to go into this gap between the header board and the chimney?     I think that when the new roof decking is put on, it will only go up onto the top edge of the header board,  and not all of the way to touch the chimney.  (Is this correct?)     And then I think that the gap between the edge of the new decking/header board and the chimney is just covered over on the top by metal flashing that is mortered into a brick seam of the chimney and slanted down at at curved angle, covering over the gap.  The flashing is then to be nailed on top of the new decking about four inches down, away from the gap.
 
   But does that gap in the framing  just sit there forever empty?    I cannot find any references in any carpentry books that speak to this, except one.  This book said that the gap is suppose to be filled with "rigid foam".  But, it didn't say what type, or whether it is suppposed to be fireproof or not, or whether it is open celled or closed celled, or anything else.
 
       Is there a brand of foam that is specifically approved for this?
 
    All advice about whether to fill this framing gap, and what to fill  it with, would be much appreaciated. 
 
Regards, Mannyrock
     

[SHOOTALL]

You can chink it with fiberglass insulation with out backing if you want an air stop. When the roof is installed the roofer should step flash the chimney and that will cover the gap if installed correctly of course that would be on a sloped roof . If flat then continuous flashing would work.