Bruno,
Pressure treated lumber needs time to shrink if boards are "wet". 3-4 weeks should do it. When you fasten wet lumber as it dries , it shrinks. This will cause the wood to split,warp,bow,etc. For your next project if you don't have 3-4- weeks just make sure your lumber is dry. I would still wait a week or so, more than likely your lumber source does not keep thier lumber outside, so it will need time to "acclumate".
Gene
