I´m pretty sure that many people reports this bug, that zombies can walk through everything. Many of us give a feedback in the steam forums for example http://steamcommunity.com/games/221100/announcements/detail/1405428094197860387 when a new update released and all what we see is new content and other bug fixes.

But this bug is the biggest fun killer in this game, because they are to fast.You can´t escape from them, if you go in a house at the one side and leave it from the other side or stay in a house to be safe. The Zombies like ghosts nothing stops them like walls, railings and gateways. If i go to a city to find something to eat or a weapon i always think that i give up the live and commit suicide in the game. So please fix this bug, many of us report this several times but nothing happens. {F30732}