I have an 8'x14'(outside dimensions) mudroom attached to the back of my house with 2"x6"x14' ceiling joists/rafter ties @16" centers. I have 1"x6" collar ties installed and installed a center support (L-shaped) made up of 2"x4"'s running perpendicular to the ceiling joists which I tied one end to the gable end of the outer (14') wall of the mudroom and the other end to the existing house roof sheathing. I installed a 2"x4" vertical support in two spots which tie to the ridge board on one end and to the 2"x4" L-shaped support on the other end which is tied to the top of each 2"x6" ceiling joist. I have installed 1/2"x4'x8' drywall to the ceiling joists and would like to know if it would be safe to now install over the 1/2" drywall 3/4"x6" T&G wood planking.
See here for direct application of planks to ceiling drywall.