Thanks Jim and Emmett for responding to my post.

6 mil poly - glad to hear that's what I should use for a vapor barrier, since I know where to find the stuff. That stuff comes in huge sheets, so fortunately there won't have to be many seams. Live with the nail holes... I hear you. What kind of tape do you suggest for the seams?

Air gap - yes I was wondering if leaving the gap above the insulation would be a good idea - thanks for confirming that. I will be sure to vent it at the eaves and ridge.

The slate I have ordered weighs 850lbs/square or 8.5lb/sq.ft. I used a snow load of 30 or 40 lb/sq.ft. when calculating/checking the timber sizes. (very very conservative for our area, but you never know what mother nature is going to dish out. Every 25 years or so we get a wet snow that collapses recently underbuilt barns and metal buildings). The timbers that support my roof (dead load) weigh something like 20lbs per sq.ft of roof area. So, if they support themselves without bowing, creaking, or moaning (which they have been doing nicely for several weeks!), they should support the slate and snow. I have done the calculations, but it's nice to see it standing upright without any visible deflection whatsoever. It looks like you could park a bull dozer on the roof. (Which is good, because I've ordered 40,000 lbs of slate! More weight than a bull dozer!)

I will try to make sure that any errant water will not get trapped within the roof system as you suggest. I am resigned to the fact that leaks are going to happen at some point, so the roof should be built to survive a leak that goes unoticed for a while. This is one reason that I do not plan to use plywood or OSB in the roof. It would be nice for the stiffening effects, but after reading a book on slate (and reflecting on several asphalt roofs that I know of that needed a sheet or two of plywood replaced when the 20yr. asphalt shingles were being replaced), I've come to my own conclusion that you can't beat solid oak boards under slate.

Thanks again guys. I'm looking for more suggestions if anyone has them... we set 6 valley purlins yesterday, and they fit great. (i.e. the measurements of the actual roof, confirmed that we should field cut the purlins to within 1/16th of their theoretical numbers!) Sighting up the valleys from the ground shows that all the purlins line up as they should. Hope to saw, notch and set 3 ridge beams today. Roofing is not far away!

-Thomas