We are re roofing my parents retirement home built in 1973 by family and friends, the original construction is 2×10 rafters at 16 inches, 3/8 sheathing 
a 30 year comp shingle roof now 48 years old.  Inside R19 foil faced fiberglass
10 mill poly vapor barrier lapped but not taped or sealed, and a beautiful redwood board ceiling we want to preserve.  There is wood paneling and no dry wall throughout so no mold issues even with plenty of leaks.  All work will be from above.
   4:12 pitch clerestory section with good soffit vents all around, good upper side vents on 2/3 of rafter bays with walls above and skylights obstructing the remaining 1/3 of rafter bays; we will add metal dome vents in those areas. 
    Our licensed professional roofing contractor will add new 1/2 inch plywood sheathing  over the existing, old time Grade D felt and professional quality flashings and edge metal and new composition shingles.  Options:  1  replace a small part of the old sheathing and insulation where we expect to find rot or matted insulation or,  2 remove all the 3/8 and R19 and replace with unfaced R30 fiberglass which will compromise the venting but save heat loss, or, 3  possibly R30 foam board cut to fit which would cost more but allow a better venting layer.  We don’t want to use spray foam, and want to keep a roof that breathes.  It’s on California coast, CEC climate zone 1  plenty of south facing insulated glass, until the last few yeas only stinking hot a few weeks a year.