EddieWalker
Epic Contributor
Your joists are just really shims to level and support your decking, and since they do not span anything, you can use the smallest lumber you need. Spacing is more important. 24 inches apart is standard for 3/4 ply or bigger. I prefer 1 1/4 for decking, but it's more money and super heavy. You can also go closer together if you are going to have some heavy loads. 19.2 or 16 inches will only add a few boards to your job, but give you a lot stiffer floor.