We just finished a trex deck this week, and with the 1 1/2 x 5 1/2 decking you can go with joists 24" on center. I was a little surprised also that the deck's plans called for 24" centers, but the decking felt fine. Scott
We use the same details to hang our varge rafters here in california. I guess if you have snow loads then you might need something stronger, but it is standard where I live.