Thickness

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If you are adding to your existing 5/8, I didn't thijnk you had to go that much. The rules I always heard (and used) were 3/4 min for plywood, and that's plywood alone. Years ago before they had cement board, that's what they used and it worked well for decades. I've put tiles down on 3/4 plywood with no problems. Is using cement board better, of course? But it's not like it's going to fall apart without it. Look in some old houses and you'll find tile down for decades with perfectly good grout and there's no cement board under it. If you're on 16" center, adding 3/8 would more than get you there under the old rules. If you want to go over, thicker is better, and cement is better. I'd definitely try and hit the joists, and a couple inbetween wouldn't hurt. All IMHO.