When creating their characters, all players come up with a number to open doors, based on their Strength. Must the characters make checks to see if they can open inn doors, the doors to their rooms, or a carriage door? Of course not. Under most circumstances, don't worry about the chance to open a door. Sometimes, however, there are doors the characters aren't meant to open. That's when the check becomes important.
Doors can generally be divided into different groups. First are regular normal doors. These open when pushed or pulled because that's what they are supposed to do. The DM who requires a check every time the characters try to enter a tavern is misinterpreting the rules.
The next group are those heavy, old, musty, swollen and rusted doors found in dungeons and ancient ruins. These don't open with an easy pull. The hinges may be frozen or the wood swollen in the frame. To open these the characters must make a check, yanking on the handle or giving the door a good shove.
Finally, there are locked, barred, and ensorcelled doors, ones that are closed and sealed on purpose. These take a bit of doing to open.
Every character has a chance to force open a door, but it is up to the DM to determine when it is appropriate to use this ability. The DM can legitimately allow the characters to force open a door held shut by a flimsy lock or rotted bar. An extremely heavy dungeon door, swollen in its frame may be unforceable. The characters throw their shoulders against it and just bounce. If picking a lock is particularly important to the adventure, then that might be the only way to open the door (short of stealing a key).
One important note to remember is that if a monster opened a door and fled through it, the characters should be able to open the door with equal ease. The key here is "equal ease." What is easy for a troll or hill giant may be quite a bit more than a gnome or halfling can manage! Frequent opening and closing will also affect the ease with which a door can be used.
If a door fails to open on the first attempt, a character can try again—there is no limit to the number of attempts, but each subsequent attempt will reduce the character's chance of success by one, as he grows more and more tired of yanking or banging on the door.
Another common tactic players use to deal with uncooperative doors is to put multiple characters on it. Up to two people can attempt to force open a door at the same time (more than this and the characters tend to trip over themselves). The chance of opening the door is increased by half the lesser character's chance (with fractions rounded up). Thus, if Rupert opens doors on a 1, 2, 3, or 4 (on 1d20) and Delsenora on a 1, 2, or 3, together they can open a door on a 1, 2, 3, 4, 5, or 6 (Rupert's 1-4 plus half of Delsenora's 1-3, rounded up to a +2 bonus).
Resourceful characters sometimes go after doors in a big way, improvising battering rams to bash them in. The characters need a suitable ram (a stout log will do) and some running room to gain the full advantage of this method. Such a ram will enable the characters to total their chances to open the door. Even without the running room, the characters can swing the ram into the door. This allows more than two characters to apply their muscle at one time.
Each character on the ram contributes one-half his normal chance of opening doors to the overall effort. Thus, Rupert (1-4), Delsenora (1-3), Tarus (opens doors 1-6) and Joinville (opens doors 1-2) would have a (2 + 2 + 3 + 1 =) 1-8 chance of bashing down the door swinging a ram into it. Their chance would be (4 + 3 + 6 + 2 =) 1-15 if they were able to charge the door full tilt with their ram.
Of course, bashing down doors does have its disadvantages. First, the door is ruined and can't be closed behind the group. The characters will leave a clear path, one any pursuers can follow, and they won't be able to block their rear. Unless the site has regular maintenance, the DM should note on his key what doors have been destroyed for future references.
Forcing doors open also tends to be noisy. Unless the door bursts open on the first try, creatures on the other side cannot be surprised. Even if there isn't anything behind the door, those nearby will be alerted (and if intelligent, may take action). Finally, the noise may attract unwanted visitors. The DM should immediately make a wandering monster check (if any exist in the area) each time a door is smashed down. Silently picking locks can have its advantages.
Concealed and Secret Doors[]
In addition to all other types of doors, the arcane architects of most fantasy buildings like to include a few secret and concealed doors. These can range from simple priest-holes to pivoting bookcases opening into hidden crypts. The only limit is your imagination.
Secret doors operate differently from normal doors. First and foremost, they must be found. This isn't something that happens without effort (if it did, the door wouldn't be very secret!). With the exception of elves, characters must search for secret doors to find them.
Searching a 20-foot section of wall takes about 10 minutes, during which the characters tap, thump, twist, and poke, looking for secret catches, sliding panels, hidden levers, and the like. The exact amount of time can vary according to the amount of detail on the wall. A relatively barren wall section will go fairly quickly, while one loaded with shelves, ornamentation, sconces, and other fixtures will require more time. A character can search a given wall area only once, although several characters can search the same area.
Normally, when a character discovers a secret door, he has found the means to open it. Therefore, no roll must be made to open the door. In very rare cases, the character may discover that the secret door exists (by finding its outline, for example) but not know how to open it. In this case, a separate check must be made to open the door.
Secret doors cannot be forced open by normal means although they can be bashed down with rams (at half the normal chance of success). Indeed, it is even possible for characters to see the secret door in operation and not know how it is operated. ("You burst in just in time to see Duke Marask, the vampire, disappear from sight as the sliding bookcase swings back into position.") In such cases, knowledge that the door exists will increase the chance of finding its opening mechanism by 1.
It is a good idea to note how each particular secret door works and how it is concealed. While such notes have no effect on the mechanics of the game, they will add a lot of flavor and mystery at the expense of a little effort. Which is more exciting—to say, "You find a secret door in the north wall," or "You twist the lion-headed ornament over the mantle and suddenly the flames in the fireplace die down and a panel in the back slides up?"
Furthermore, colorful descriptions of secret doors allow you to place the burden of remembering how a given door works on the player characters—"What, you forgot what to do to make that secret door open? Well, I suppose you'll have to search again." If used in moderation, this will help keep them involved in your game, encouraging them to make maps filled with all manner of interesting notes.
A concealed door is a normal door that is purposely hidden from view. There may be a door to the throne room behind that curtain or a trap door under the rug. The door isn't disguised in any way or opened by secret catches; it is just not immediately obvious.
Any search for concealed doors will reveal them and once found they can be opened normally. Elves can sometimes sense concealed doors (if they make their die roll) without having to stop and search. No one knows how this is accomplished, although some theorize elves notice subtle temperature gradients when they pass near these doors.