how to vent cathedral type roof in an addition

[lex1125]

howdy folks.

I'm a bit stuck on how best to vent the roof on a new addition I'm building. 

I have my existing cabin at 16x16 foot print with a 12/12 roof. the addition is a shed roof type and I'm wondering firstly if it is really necessary to vent as it is attached to the main building and if so (which I'm pretty sure I do need to) what is the best way to vent the top. 

the roof is a shed type 16degree metal with 1/2 plywood sheating and is made up of 2x8 rafters that attach to the main cabin at a band.

The bottom of the roof, at the eave, it's fairly easy to add a long vent across BUt I'm stuck at how to vent the top becuase of the cathedral type ceiling.  I'll use shoots to create an inch of air space between the insulation and the plywood roof deck but what to do at the top?

I'm def. a novice and learning as I go so any help is v. much appreciated.