ever since I started playing the 1.11.2 version it seems zombies rarely ever broke doors. happened once and it was from the inside out of a house. seems zombies cant break down doors that are facing towards them (I.E: most villager doors) . so it seems doors that are faced towards the "outside" will make zombies unable to break down doors.

in attachment: been sitting around watching these doors for 15 minutes now.
(villagers inside forcing zombies to try to break in)
Edit: read the wiki, turns out only "certain" zombies are able to break down doors.. fair enough.