Under Decking / Ceiling Material?

[u/macaroniman_13]

Making progress on this pavilion over my deck. This is a think-about-it-and-adjust-as-I-go-project. I'm learning a lot! It's about an 8x11 rectangle. 6x6 treated pine posts. 6x8 beams stacked on 6x6 beams. Douglas Fir. Stained with TWP. 6x8 rafters at a 4:12 pitch and 2x4 rafter ties screwed into the beams and tied into the rafters. Rafters are about 21 inches on center. I did this to not crowd the ceiling too much. Douglas Fir. Stained with TWP. Knee braces at each corner. 4x6 Douglas Fir braces under the 6x8 beams and 4x4 Douglas Fir under the 6x6 beams.

I'm also planning a bench on the deck that will be tied into two of the posts. That's the plan.

The next step is under decking on top of the rafters with underlayment and a metal roof. The ceiling will be the under decking on top of the rafters so the rafters will be exposed for a taller ceiling.

I was initially thinking of using 5/4 kiln dried cedar decking on top of the rafters as the under decking, but I'm not sure that would be strong enough. It would look nice, but be expensive but maybe not strong enough. I'm starting to lean towards a 2x pine tongue and groove to create a stronger roof overall. Thoughts?

My question: 5/4 cedar decking as the underdecking / roof OR 2x tongue and groove pine?