New Pressure Treated boards are wider than existing ones
That is because the new boards are still &quot;wet&quot; with the pressure treatment and are new. Once they dry out, in a few months, they will be the same size or close to it. Lay out the deck with the boards but don't screw them down yet. Let them dry out, flipping them over often. I let mine dry out for 12 weeks. Don't seal them yet either.