I have 2 options for a new floor:
9mm plywood on joists, with tongue and groove floorboards on top 19mm thick
28mm tongue and groove floorboards on joists (no plywood subfloor)
Which is the better option?
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I've been on a lot of decks that are made from 1.25" (31 mm) softwood planks, not grooved, and they aren't bouncy. I suspect that using 28mm T&G boards (stress is spread from one board to the next one) would be sufficient by itself.
The 9 + 19 may work if you glued the finish floor to the subfloor in addition to fastening at the joists.
Is the difference in price between 19 mm and 28mm enough to compensate for the cost of the plywood and the aggravation of putting down another layer?