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Let's say a rogue tries to investigate a trap, then fails. What would happen to them if they then tried to disable that trap? He doesn't understand it completely since he failed his investigation check, right?

Here's how I understand the process for dealing with a trap: Perception check (To see the trap) + Investigation check (To understand how it works) + thief tools check (To disarm the trap). That is the pattern this question is built upon.

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    \$\begingroup\$ Hi Anthony, welcome to RPG Stack Exchange. Check out our tour to see how we work here. Titles are meant to represent the body of the question, not ask a separate question entirely to the body, so I've given this a revision to unite the two. Also, when you reach 20 rep, you're welcome to join us in Role-playing Games Chat. \$\endgroup\$ – doppelgreener Feb 15 '18 at 15:08
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    \$\begingroup\$ Was investigating used to find a trap, or to disable a trap? Question is unclear, since disable is usually a dexterity based check, and investigation is usually an intelligence based check. See "using ability scores" in Chapter 7 of the PHB and / or Basic Rules. Once you review those you may want to revise you question? \$\endgroup\$ – KorvinStarmast Feb 15 '18 at 15:08
  • \$\begingroup\$ @KorvinStarmast Correct me if I'm wrong, but I thought that the pattern for traps was: Perception check (To see the trap) + Investigation check (To understand how it works) + thief tools check (To disarm the trap). So neither, the check was to understand the mechanism. \$\endgroup\$ – Runescape Feb 15 '18 at 15:10
  • \$\begingroup\$ I suppose that's one way to do it. I edited that understanding into your question, let's see what responses you get. When you put that detail into the question, your problem to solve becomes more clear. :) \$\endgroup\$ – KorvinStarmast Feb 15 '18 at 15:13
  • \$\begingroup\$ Thank you, this is my first question so I'm sorry for anything that's unclear. \$\endgroup\$ – Runescape Feb 15 '18 at 15:17
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Strictly speaking, you typically use either Wisdom (Perception) or Intelligence (Investigation) to locate the trap, then a Dexterity check (with thieves' tools) to disable the trap.

Asking the player for a further Investigation check to understand a trap that they've already seen is a valid use of the skill, but it's not typical. Normally you simply use Perception to spot it, then Dexterity to disable it.

In the event that the DM asks the player for a Investigation check to understand the trap before attempting to disarm it (something which would be more likely to apply to especially complex traps), it's entirely up to the DM what happens.

From the Player's Handbook:

Finding a hidden object

When your character searches for a hidden object such as a secret door or trap, the DM typically asks you to make a Wisdom (Perception) check. Such a check can be used to find hidden details or other information and clues that you might otherwise overlook.

Perception:

Your Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something.

However, Investigation can also cover the skill of finding a trap:

When you look around for clues and make deductions based on those clues, you make an Intelligence (Investigation) check. You might deduce the location of a hidden object...

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    \$\begingroup\$ This seems to cover it. In instances where the extra check is made that is usually to grant advantage or disadvantage on the actual roll. So if you spot and understand the trap you might make the check with advantage, or it might cancel out a natural disadvantage from it being a complex trap. \$\endgroup\$ – SeriousBri Feb 15 '18 at 15:28
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    \$\begingroup\$ I might have excuses to make to my rogue player then, he's failed a lot of investigation checks and I ruled it to give disadvantage to his disable trap checks. Thank you! \$\endgroup\$ – Runescape Feb 15 '18 at 16:22
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    \$\begingroup\$ @SeriousBri - That's about the best use of the extra check I can think of. Realize the more checks you add for them to pass, the more likely it will be for the poor rogue to fail. \$\endgroup\$ – T.E.D. Feb 15 '18 at 18:36
  • \$\begingroup\$ To be a complete answer, I think you should add what typically happens when the first "detection" skill fails, which is of course that the trap is unnoticed and then triggered. If the player is alerted to the trap by some other means, like a map or warning from a cryptic ghost, then the first "detection" check can be skipped. I think this is the source of OPs confusion. \$\endgroup\$ – Segfault Feb 15 '18 at 20:41
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    \$\begingroup\$ For the purposes of having a complete answer, I would add that in the 5e SRD and Player's Guide, as well as in the helpful online Compendium from Roll20 that contains this sourced information, the DM can call for a roll for Investigation if they so wish. "You might call for an Intelligence (Investigation) check for a character to deduce what needs to be done, followed by a Dexterity check using thieves’ tools to perform the necessary sabotage." \$\endgroup\$ – SeraphsWrath Feb 15 '18 at 21:26
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If a trap was so complex that I as a DM would make the Rogue roll (not offer them the chance to roll, but require the roll as part of the Disarming process) Investigation before they attempted to Disarm it, then there would definitely be consequences for them if they failed.

I would like to clarify that I'm primarily referring to mechanical traps, as magical traps would require an Arcana check to know exactly what the sigils on that trap do.

The way disarming traps typically works is:

  1. A Perception or Investigation check to spot the trap [Use Perception if the party is moving or if the Rogue is making a cursory glance, and Investigation if the Rogue is actively searching for traps, et cetera]
  2. A Dexterity check to actually disarm the trap (using Thieves Tools).

Most traps are primitive, like falling-object or bear traps, so once spotted, it would be easy to see how they worked and likely that the Rogue was already familiar with them.

  • For example, the Rogue is attempting to disarm a primitive Deadfall trap suspended by a branch which is rigged to a tripwire. The Rogue would be familiar with these kinds of traps and would simply snip the tripwire. Then the Fighter could prop-up the Deadfall with a large, stout pole.

For moderately-sophisticated traps, like blowdart traps, I would allow the Rogue to make an Investigation check following detection to Gain Advantage on the Dexterity Roll to disarm it, or give the Rogue Advantage on their Saving Throw to avoid damage.

  • With the Blowgun Trap, if a Rogue has investigated the trap, they will have looked at the trajectory and targeting of the darts fired by the trap when tripped, and so would already know where not to stand and where is safe. Thus, I would give the Rogue Advantage on their Saving Throw because they know where is a safe place to dive towards.

If I were to incorporate an Investigation check into the disarming process, then the trap would need to be much more sophisticated, like a pressure-sensitive tripwire connected to explosives which would activate either when "tripped" or when cut in an attempt to disarm it. Another trap that could merit an Investigation roll would be a trap with most of its mechanism hidden from sight, a trap which in-turn activated other traps, or a trap which seemed to have no obvious mechanism or purpose.

  • For example, the Rogue is infiltrating the stores of a wealthy priest, and has successfully snuck-past the guards. She goes to take an expensive-looking gold artifact that rests on a pedestal on an altar, but notices right before she lifts it that the pedestal seems to rest on an unusual section of the altar. She carefully takes-apart the Altar to investigate and finds a strange device: The Pedestal is sitting on-top of an unusual balloon of some sorts which is connected by tubes to some pipe-things at the ends, but doesn't seem to have any purpose. The Rogue would then roll an Investigation check. If she passed, she would notice that the balloon was only partially inflated and seemed to have some sort of spring on the inside, and conclude that the trap probably activated when the balloon was inflated, and thus I as the GM would reduce the DC of the Disarming check. If she passed by a good margin, she would realize this and that the little fans were designed to spin really fast when the balloon inflated and sucked air through the tubes, making this trap a noisemaker, and thus I would not only reduce the DC of the Disarming check but also allow them to add half their Proficiency Bonus to their roll as an additional modifier. If she failed the check, she wouldn't get any additional information and would have to make the check at the standard DC. If she crit-failed the test, then she would become confused by the trap's mechanism and make the Disarming check at standard DC and with Disadvantage.

Consequences for failure might range from Disadvantage on the Dexterity check, increasing the DC of the check, forcing the roll to be performed in-secret by the DM, or, if a trap is sophisticated enough, they only can disable part of the trap (because the trap has multiple mechanisms and they only know of one).

  • The Rogue encounters what looks like a simple tripwire across a narrow passage, except it seems unusually thick. It has no obvious mechanism, but it still is a tripwire. The Rogue fails the Investigation check, and so I make the Disarming Roll in-secret (but with no other penalty). She rolls high-enough to disarm the tripwire, but not high enough to prevent the secondary trap from activating. I declare that the Rogue has successfully cut the tripwire. She strides proudly into the next chamber after avoiding certain doom from the explosives placed under the floor which were attached to that tripwire, but then the hidden portcullis slams down behind her, separating her from her comrades.
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If investigation fails, then the rogue doesn't get information about how the trap works. If they want to still try to disarm it you should ask them what precisely they are doing and then try to adjudicate what would happen if someone did that with the trap in question.

If what they want to try would help, then let them roll it or let it succeed, if what they want to try would not help or would make it worse, then auto-fail or disadvantage the roll.

This AngryGM Article goes over Investigation and Traps and could have more information in this vein for you. (Warning, bleeped out language)

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It honestly depends on the dungeon master's preference:

  • some DMs will rule that the rogue failed to notice the trap in it's entirety

  • other DMs will raise the difficulty for the check to disarm it, or rule that the roll is made with disadvantage, because the character either did not understand the exact workings of the trap, or simply overlooked a part of it.

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    \$\begingroup\$ The original poster is a new GM looking for advice. \$\endgroup\$ – T.J.L. Feb 15 '18 at 18:17
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    \$\begingroup\$ This answer doesn't really answer the question that was being asked. The asker is a new DM who wants to know what they as DM should do when the Rogue fails the Investigation part of the roll series (not technically required by RAW, but the SRD does offer this choice as an option if the DM wishes to do so.) Thus, it doesn't depend on "DM's preference" because this person is the DM. \$\endgroup\$ – SeraphsWrath Feb 15 '18 at 21:35
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    \$\begingroup\$ But it does offer an answer to the question. Even if the questioner is the DM, it still depends on his preference, and giving him examples of a couple of the more common preferences that DMs frequently select can help him to decide what his preference is to be. \$\endgroup\$ – Phil Boncer Feb 15 '18 at 22:05

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