If your joists are 16" o/c you may not want to use 5/4 PT wood. The span is most likely to much. Butting the boards you might start with 4 ft long deck board and increase in 4 feet increments to have a staggered look and have the ends land on joists ( 3x16"=48") So use 4'8'12'16' lengths staggered to get your patterns. If dividing it up as shown above yes you need to double or triple some joists to get nailing ability for the perpendicular trim board.

Thanks for the reply, Kahuna

I was not aware 16" spacing was too wide for 5/4. I assumed it was pretty standard. Hmm. I'll need to do some more research.

Looks like I'll probably be adding a few extra joists.

I'm also trying to avoid the typical screwing of two butt-end boards into the same joist. It seems that's always a weak point, with the screws so close to the ends.

I'm probably over-designing it, and should just do the standard method of putting the ends on random joists and doubling them up. I really want this thing to last, though.

David

The other option, depending on where exactly your perpendicular board falls, is to put some short cross-braces in, so it looks like a ladder between two of the joists. They should be 16" OC as well, all the way down. Then your perpendicular board can lay anywhere in that 16" wide span that it needs to, to be centered properly.

I'm replacing 2 x 6 with 5/4 x 6 also, and on a very long deck. I'm going to do exactly what was done originally: place the boards on the diagonal. I've already overthought this so much, that I decided to simply 'refresh' what I already had with new wood! Good Luck!