Our recommendation for roofing your frame with tongue and groove ceiling
There are many options for roofing your timber frame. If you want to use tongue and groove so it looks amazing from the inside, here is what we recommend:
You can use 3/4" T&G then put 2x4's length-wise as a spacer, then put metal roofing straight on the 2x4s and screw them off. If using shingles you will need to add a layer of sheathing to give you the nail base on the 2x4's. This will also give you space to run any electrical wires.
Alternatively, you can use 2x T&G and put the roofing straight onto that.