Cupping is due mainly to 2 factors.
1. The way the board has been cut from the log, tangentially or flat sawn (99% of decking)Â is where you will see the most movement and 1/4 cut has almost no movement. The cost of 1/4 creates lots of waste and is very, very expensive.
2. The equilibrium of moisture on the upper side of the board should be the same as the underside. If you have a wet underneath and a dry top as the moisture transpirates out of the timber it will cause movement in that side, so if the outer side has no movement then the board will cup.