T,
Some observations for your consideration.
Historical background: My home in the finger lakes in NY state is a classic timber frame with shipplank front, clapboard sides and a slate roof all circa 1804-1811.Standard center hall 4 over 4 with 11'ceilings and rooms roughtly 20 x 25.
the roof system is simplicity itself only complicated by the box gutters which need lotsa maintenance. The rafters are half rounds about 6-8" in diam spaced more or less at 18-20". The battens are randowm width pine and cedar planks just under 1" thick. the slate is laid directly on the wood.
In recent repairs we added ice damn on the lower 30" as we replaced slate and repaired the box gutters. At some point circa 1850 the center of the roof started to sag so a long purlin was added about mid rafter span on the underside and diagonally braced to the attic floor. The roof is still sound even with monstrous lake effect snow loads and weeks of sub freezing weather each winter. The house is unsulated by blowing in stuff between the ceiling joists and under the attic floor do a depth of about 10". Works like a champ over the lath and horsehair plaster ceilings below. Net effect is a cold roof over a slightly warmer attic space with no significant moisture problems.
What is your snow load and average winter temp ?
Keep in mind a vapor barrier is meant to separate a warm space from a cold space. In the case of a roof system the warm space is below the ceiling. You can stretch house wrap after you lay the t&g and before you set the 2x10's.
By insulating over the ceiling planking your only moisture in the roof is minor infiltration around the slate and mayhap on the eave ends if snow/ice buildup is routine.
The insulation needs to breathe via soffits and a ridge cap. Ice dam material on the lower 30" of the batten boards is climate driven and influenced by the roof pitch. The paper under the slate helps protect the slate from breaking under stress (including walking on it).
By all means check with your engineer for the monstrous weight of the slate.
good luck,
deralte
