It is not uncommon for players to encounter a door that resists being opened – the door may be stuck, barred, locked, barricaded, etc. A locked door could be unlocked by finding a key or passing a Difficulty Check to pick the lock, or it could be broken down.
In the D&D 5e Dungeon Master’s Guide page 237, the examples for when a strength ability check might be used are: “Smash down a door, move a boulder, use a spike to wedge a door shut”. Which seems fine – set a DC and see if the player can beat it with Strength check. This seems to be confirmed by the Player’s Handbook page 176, again in reference to when a Strength check may be used: “Force open a stuck, locked, or barred door”.
However, page 246 of the DMG also details object Armour Class and Hit Points. One could argue that a door is considered to be an object (at least, as much as a wall is considered to be an object by the book). Lets say the door is made of wood, its a medium sized object and it is resilient, so it has an AC of 15 and 18 Hit Points. When the door reaches 0 HP, it opens.
When would it be appropriate to use the AC and hit points of a door to determine if it opens versus setting a DC Strength check?
For the purposes of this question, assume there is always a chance for success and a risk of failure, there are no automatic successes or failures. Also assume that the players are determined to break the door, they are not interested in looking for ways around it – the door will be broken and open eventually, it is more a case of what method should be used to determine when the door opens.
Note that although this question is specifically asking about breaking doors, it would apply to any situation where it might be appropriate to use either an AC or a DC to break something. I have simply used doors as my example as they are one of the most likely and most common things a player may try to break.
Best Answer
In my interpretation, the AC VS DC thing depends on how the PC would like to open the door.
DC would mean the character wants to keep the door whole, but just get rid of whatever would stop the door from opening (like the small metal part of a lock sliding into the side of a wall or a wooden bar behind it). This might leave the door damaged, but still usable (depending on the lock method).
AC would mean a player attacks the door, so they would just physically destroy it, which I would just describe as a player trying to return a wooden door back to some planks with nails sticking out or something similar.