I'm 90% done raising my frame and it is just about time to put a roof on it. Most of my roof is framed with principal rafters that accept 6x8 purlins (and a 8x8 ridge beam) roughly 4' on center.

Above this I plan to put
1. 7/8" oak tongue and groove, nailed to the top of the purlins.
2. a vapour barrier
3. 2x10's on edge, 24" apart, running from the ridge to the eaves. Two layers of 4" polyurethane (Firestone) insulation between the 2x10's.
4. 30lb felt paper
5. 1x4 oak battens running horizontally
6. slate

I have a pretty good handle on everything except the vapour barrier. Can anyone give me suggestions for what kind of material to use for the vapour barrier. I used some 6mil platic sheeting (commonly available at Lowes or Home Depot) under my concrete basement slab. I later tried to use this same stuff for covering my frame from the elements. (for that it failed miserably - the wind ripped it to shreds within two weeks - once torn, this stuff tends to rip easily). Is this the same material that everyone uses for the vapour barrier directly above the t&g ceiling or is there something more robust (e.g. rubber like?)? What kind of adhesive does one use to join the seams? Do you worry about the inevitable multiple nail punctures (vapour breaches) it will endure?

Also, does anyone have a suggestion for alternatives to the 30lb asphalt impregnated felt that I plan to use directly under the slate and battens? It seems like something with a little more vapour permeability would serve my roof better. Since the slate is not air tight, I'm thinking that the slate should be able to let the moist air that breaches my vapour barrier eventually escape to the outside... assuming the 30lb felt will let the vapour through. (is that a good or bad assumption?)

Comments and suggestions on the vapour barrier, felt, and the whole roof plan in general would be greatly appreciated.

-Thomas