by jokeroz
Funnily enough I was talking about this on the SW forums. When I first got my game of Zombicide the boards warped and I placed them under books.I noticed again the other day while having the boards out for several hours (I play a lot of solo games and leave the game set up) that the boards were warping again. As I was mid game I left them and the next day they had settled.
So in summary try the heavy books but also give them a little time to self adjust if the warping is minimal.