Zombies have the ability to Noclip walls,Floor. You name it, they can do it.
when you attract them by sound or vision They will follow you and they will walk and run through floor, Walls, Fences, Wells, Buildings or any terrain or building/materials/items
(they cant climb stairs, however) wich makes houses too safe and boring.
If you can have a player being blocked from moving through buildings, walls and floor. (it can happen but rarely) then why is it so hard to implement it on zombies ?