T-111 is a finish siding that usually has a Fir finished face and comes in different grades and patterns. It looks a lot better in a finish application than sheathing plywood, but can serve the same function. T-111 is typically used in non-structural applications in a 5/16″ thickness, but may be 5/8″ when used for shear walls.
T-111 is often used on the eaves of buildings with the finished face down, where the rafters are not enclosed on overhangs. It is rare to see the entire roof sheathed in T-111 because its more expensive. T-111 can also be used as finished ceilings on porches, enclosed patios or soffits. Skylights are an optional item, you may want to introduce more natural light. Also consider using reinforced light-boxes that can eventually accommodate ceiling fans if desired in place of ceiling light fixtures.
What is the over-all style and architecture of the building? There are lots of possible treatments for the ceiling, posts, rails, etc.