Hi Don , and welcome to this here forum!!  
firstly , are you talking about a MANSARD or a GAMBREL roof as they are sometimes refered to!!?? when you say a "cathedral roof!"

now to answer your questions/quandries as best as poss without seeing any other info or photos/details!!

particle board(green moisture/waterproof variety, IS allowed in place of O.S.B sheet material, which is now favoured by councils over ply!

Unless an architect has come up with this design to over come some design /strength issue the the reason behind the extra T & G along with the 2 layers of Particle board is beyond me as to the reason for it!!, i have searched for over an hour in my detail design spec books,  the only reasons i can see for the 2 layer particle board, are strength, and also as an insulation "membrane" of some discription!!  
you ask about putting an air pocket in the roof!, do you mean a void within the depth of the roof!! structure itself!! because in the type of roof you describe, this is not possible to do unless you basically add another layer of purlins laterally along the structure, but this will add weight and cause more complications of void ventilation,
there is already the issue of condensation, which has obviously been addressed by the ceiling fan!!

if you could give a bit more detail in regards to this structure it may be of assistance in making a better, more informed call on these aspects, width of the building  etc would be good !!
hope this info is a start to helping you

regards matt!

No problems, glad to be able to help!!
believe it or not al the latest tech books refer to that as a "vaulted" ceiling""

if you do replace this roof, and you use waterproof(green coloured ) particle board!, then you could prior to the shingles being replaced, put on a layer of the "one way" breathable fabric, lets moisture evaporate, yet stops water ingress, (i must admit, i have forgotten its name at the mo!!-sorry!)
and as for shingles, i do not know, as you did not say, if they are the timber variety, as they do now do a nice line in asphalt ones, in a range of colour as well, i used red on my workshop front last year whe raising the roof!! quite effective!!a lot more water tight too!!
any way, glad i could help, .. you know where WE ALL are now!!!
regards matt

Your not being nosy, just intrested.  A nursing home called my company because they had some shingles blow off the roof in a high wind. When I climbed up to look at it I realised that everything between the shingles and the two layers of planking was rotting away.                                                                                   So we're going to strip everything down to the planks, sheath it with 1/2 inch plywood, paper and shingle it, and install a gable vent on each end to assist a fan that is mounted in the center of the roof.
                             

Is this all from the other side of the pond?

[quote author=WOLF  link=1136459621/0#8 date=1136751756]GIVE THAT MAN A GOLD STAR DUDE!! (LOL)
well it will look better than that "slug" you had on over xmas dear chap!!!

yep, once i realised that the newbie Don, was from across the pond, it all fell into place, the codes of practice/permits/zoning laws etc  dictate things to be done in differing ways, and some of the material over there, to here may be listed/called the same names. but are totally differing stuff!!! [/quote]

Well hiy'all y'all!  Dagnabbit!  Dang ma breeches!

It was the mention of shingles, cathedral roof, big fractions (rather than metric), ranch style etc that made me think.........