Part of an ongoing series of 5e Rules notes.

Rolling skills to get something done can be a tense moment. The whole campaign might depend on how well you can sneak, or spot someone sneaking, or open that lock, or disarm that trap.

And, since a D20 provides a linear distribution of results, it’s quite possible to fail that roll.

Then what?

How to Succeed at Skill Rolls while Trying A Whole Bunch

So, what is a Skill (Ability) Check? Well, per PHB 174:

To make an Ability Check, roll a d20 and add the relevant Ability modifier. As with other d20 rolls, apply bonuses and penalties, and compare the total to the DC. If the total equals or exceeds the DC, the ability check is a success — the creature overcomes the challenge at hand. Otherwise, it’s a failure, which means the character or monster makes no progress toward the objective or makes progress combined with a setback determined by the GM..

What happens when you fail a Skill / Ability roll? Can you try again? How many times?

Can you try, try again?

Interestingly enough, there’s no easy answer there. I’ve read DMs assert that they only let a single roll happen; if you fail, that shows it’s just not doable (by you, at least). I’ve read others say you can only retry if the circumstances or your approach explicitly changes.

(I’ve also seen guidance that rolls should only be asked for if the results of failure are significant or interesting. So there’s that, too.)

To my mind, a lot depends on what it is you are trying to do (duh). As much as D&D tries to make all skills identical in their structure and use, they really aren’t. Some skills, in their application or in the circumstances at hand, lend themselves more or less to retries.

• “I search the room.” Okay, you blew your Perception roll. Can you search it again, search it harder, search it in a way you didn’t before?  Sure. Tell me what you’re doing differently.
• “I try to convince the guard to let us pass.” Okay, you blew your Persuasion roll. Can you try again? Well, certainly not the same way or with the same line of argument. I mean, if she didn’t believe the Captain sent you when you said it once, she’s not going to believe it a second time.
• “I try to remember my History to see if I know of the dread Egnarts.” If you fail, chances are you’re not going to succeed in “remembering” again, without explaining a very different approach.

In some cases, letting an attempt be retried is just fine. In other cases, retrying at a Disadvantage seems to make sense (“Oh, did I say the Captain? I meant the Duke, my uncle …”).

What does “failure” mean?

We tend to think of “failure” as “What I asked for didn’t happen.”

• The lock didn’t pop open.
• The guard wasn’t convinced.
• The mule refuses to move.

But look at that definition of an Ability Check again, particularly on the “failure” part:

Otherwise, it’s a failure which means the character or monster makes no progress toward the objective …

Okay, that’s what we usually think of failure like.

… or makes progress combined with a setback determined by the DM.

Which is very modern “failing forward” game design for something like D&D, and, frankly, is something I never thought of for this system — and it’s something that makes sense, esp. if (a) the DM wants to move things along, and/or (b) you just barely missed your roll.

• You hear a couple of tumblers in the lock move, but it doesn’t open; your next attempt will be at Advantage … but that will take more time.
• You got the lock open … and the door opens wide, to reveal the room full of guards.
• You got the lock open, but broke your favorite lockpick, putting you at Disadvantage in picking locks until you can get it replaced.
• The guard grudgingly lets you pass, but sends a runner to check with the Captain, just in case. 
• The mule moves, but quite intentionally steps on your foot in doing so. 

Those are all legitimate things for me as the DM to do (or you as the player to suggest).

Can’t I just “Take 10” or “Take 20”?

So those are D&D 3.5 rules, but 5.0 kinda-sorta has them. Kinda-sorta.

Taking 10 in 3.5 usually just meant “Act like I rolled a 10” so as to avoid the chance of a low roll (when a high roll wouldn’t really be needed).

This is essentially the equivalent of using a Passive Skill in 5e.  Which is a little weird (“I’m searching the room … passively”).

Taking 20 was the interesting one in 3.5:

When you have plenty of time (generally 2 minutes for a skill that can normally be checked in 1 round, one full-round action, or one standard action), you are faced with no threats or distractions, and the skill being attempted carries no penalties for failure, you can take 20. In other words, eventually you will get a 20 on 1d20 if you roll enough times. Instead of rolling 1d20 for the skill check, just calculate your result as if you had rolled a 20.

Taking 20 means you are trying until you get it right, and it assumes that you fail many times before succeeding. Taking 20 takes twenty times as long as making a single check would take.

Since taking 20 assumes that the character will fail many times before succeeding, if you did attempt to take 20 on a skill that carries penalties for failure, your character would automatically incur those penalties before he or she could complete the task. Common “take 20” skills include Escape Artist, Open Lock, and Search.

5e doesn’t have this … precisely. But on DMG 237, “Multiple Ability Checks,” there’s a “Take 20”-like mechanism:

Sometimes a character fails an ability check and wants to try again. In some cases, a character is free to do so; the only real cost is the time it takes. With enough attempts and enough time, a character should eventually succeed at the task. To speed things up, assume that a character spending ten times the normal amount of time needed to complete a task automatically succeeds at that task. However, no amount of repeating the check allows a character to turn an impossible task into a successful one.

So in cases where failure doesn’t incur a penalty (except burning time), you can spend ten times the normal amount of time (ask your DM for a SWAG) and just assume a success if the task is possible (which I read to mean, if rolling a 20 on the skill would allow it to succeed). This is a bit looser and more cinematic than 3.5’s rule, but there you go.

It does mean that, if the party is willing to take the time, the DM can dispense with Perception rolls in each room and just say, “After about an hour, you find the hidden compartment under the book case. And, no, that doesn’t count as a Short Rest.”

Is that a good thing? That’s up to you to decide.