I find that building in general gives way too much experience points compared to killing zombies. For instance, upgrading a cement block from a frame gives you 480 exp, as much as a zombie kill. My friend and I did a test just to proove the ridiculeness of this, and we found out that we we leveling way faster to only set tons of frames and upgrading them into cement without any specific purpose than going for missions and killing. In this test we only leveled from upgrading cement blocks; no need to kill zombies or do missions. Conclusion: it is killing the purpose of the zombie killing game if you exploit that thing in the game by being selective on what you do in terms of exp gains focused. Now could you please reduce the experience gain from upgrading stuff like that, split 70% at least of the ammount gained.