I haven't been using traps much on account of my own unfamiliarity with them, but it's a good part of the game and I need to be competent for my PCs' sake!

In an upcoming scenario, the PCs will very likely search a storage room. So far they just say "I want to do an investigation to see if there's anything worth taking," and if they roll high enough I reward them with some kind of loot. This time around, I want to have a trapped container that springs a net on them.

1) Should I have it so finding the trap is responsive to just that initial investigation role?

2) How would one of the PCs find the trap? Does it need to be with a spell or action like "Find Traps" or something more like the PC saying "I want to check for traps/check this container for traps" – which might require me to clue them that a trap is around?

3) We have one rogue who has thieve's tools. To disable a trap it's like a high DC roll usually - let's say 25. Do thieves tools give the rogue advantage in this?

Thanks for the guidance, folks who know better!

1. Yes

2. High roll, or kindness

3. No. It makes there be no penalty

The way I see traps most commonly run is that the PC has to say something to the effect of "I'm going to check for traps". Then you do an investigation roll to see if they find the trap.

For a non-magical trap like the one your describing, a rogue can use their thieves tools to disarm it. They don't get advantage, they get to add their proficiency bonus from the thieves tools and their DEX modifier. The thieves tools aren't just lock picks, but also include things for disarming traps. But they wouldn't have advantage unless something else gave them advantage.

It depends on how you plan to have the net deploy. Is it hanging from the ceiling in clear view? If this is the case, a high passive perception might notice the net or the trigger (if it's a trip wire or some such). If the net is in some kind of container, it would take an active search.

If somebody says "I want to look for traps" let them roll even if there are no traps. That way it doesn't give anything away. I chuckle inside when they roll low and nothing is there.

As said above, you can use improvised tools to disarm traps but there is a penalty.

The trap should have some kind of addon. The net by itself is like nothing, but if an alarm goes off or if the party  is painted with glow in the dark ink/dust, then the tension level goes up.

Helpful all, thanks!

Every DM is going to handle this differently. For the most part. For me I use passive perception to see general stuff. The layout,. The desk the library for example. 

All of my traps are placed by a enemy, thing, person.. whatever it may be.  They all have stats and I roll on how well they place those traps. That gives me the dc for perception. 

Players looking for traps roll against that DC. If they see a trap then then can tell the others about it. From there a person good with traps. Rogue for example can further investigate how to disarm it or to try and disarm it and go from there. 

It's silly but players like to roll play those moments. 

Quote from Avohei >>

 

It's silly but players like to roll play those moments. 

 Nah, everybody wants to be Indiana Jones =)

I set a DC for the trap, if it is lower then the passive perception then the players just spot it, then I also have a DC to disarm. Otherwise I ask my players to specify that they search for traps. Sometimes I will set 2 DCs one to spot the trap, then a higher one to actually determine what it is and how it triggers exactly. For instance a statue holding a gem a DC of 15 to identify there is a trap there, 20 to determine it is counterweighted by the gem and when the gem is shifts it will trigger. 

I will say If you have not been running traps at all start gently, low or no damage traps to get them in the habit of looking, or a trap with a DC below the players passive score so you can explain they spot a loose looking floor tile, on closer look you see it is a pressure plate of some description. Investigation check to find out more. 

It is not enough for my players to say, “I search for traps” or “I make a perception/investigation check.” My players need to describe exactly what they’re doing. Are they feeling something with their hands. Are they picking the container up? Are they poking it with a pole? Etc. THEN I ask got a roll. Every trap of mine is different and has good or not-so-good ways of detecting it. 

Quote from drakeborn >>

It is not enough for my players to say, “I search for traps” or “I make a perception/investigation check.” My players need to describe exactly what they’re doing. Are they feeling something with their hands. Are they picking the container up? Are they poking it with a pole? Etc. THEN I ask got a roll. Every trap of mine is different and has good or not-so-good ways of detecting it. 

 That hardly seems fair when a lot of traps are triggered when an object is picked up.

You can split the difference, or even make it threefold, since dealing with traps really comes down to three steps: knowing/suspecting that a trap is there, determining the nature of the trap and how it works, trying to disable it based on what you can figure out about the trap.

Detection (optional): If I wanted to get super down in the weeds, I might allow a quite high passive Perception check for the Character to notice something off, without them actively searching for anything yet: the color of the floor panel is different; they notice there's a slot in the wall and ceiling encircling the corridor; there's a tiny glint of light off the trip wire, etc. This would essentially give them a very high-level, and high DC, clue that they should probably be looking for traps. This is an optional step that I'd more often than not leave out, since a competent trap builder isn't going to leave many of these clues lying around.

Detection & Nature: An Investigation check - if successful -  would allow them to detect that a trap exists ( if they hadn't passively noticed something already ), give them some further information about the nature of the trap, and - very importantly - give them some clues as to how to disarm it: the floor panel is slightly raised, they see faint hints of light reflecting in dust motes from the web of light beams traversing the corridor from one side of the slot to the other; there's a row of dime-sized holes leading up the wall directly above the trip wire ( which disappears into the wall ). Here is where the how they are detecting comes into play, as that would affect the DC. If they're blowing talcum powder into the corridor, that's great for finding the laser trip wire, not so great for finding the pit trap, while the opposite is true for tapping on the floor with a 10' pole. A higher roll/better success might give them more or better clues as to how to disable the trap.

Disarming/Avoiding: Disarming the trap would require that the Player intelligently apply the clues from their investigation in order to come up with a strategy based on the clues found in the detection phase: the Rogue taps in wedges around the edge of the panel to avoid it from depressing; the Party uses their hand mirrors to carefully open a hole in the light-beam web so they can sneak through without breaking any of them.  Avoiding is simple in just finding a way around it: the Party all carefully steps over the trip-wire. If you're worried that you're now having to rely on the abilities of the Player - who presumably has less experience with traps than the Character - don't forget that you can "prompt" the Player by making reference to the Character's past experience: something about this is tickling your instincts ...  something here is reminding you of that weighted trigger that was under that golden statue in that second story job you pulled in Baldur's Gate ... 

 And side notes for designing good traps: 

1. Every trap is a security device that is there for a reason, and has has one or more purposes. They're not just "GM screw jobs". The might be a security gate which the right people need to pass through; a security barrier which denies anyone passage; an alarm which doesn't bar passage but alerts authorities, etc.
2. Every trap has an enforcement mechanism: renders miscreants unconscious; kills intruders; traps intruders; sprays them with blue paint so that security can pick them out of the crowd; etc.
3. Every good trap is a system, which means that if you can figure out how to interfere with that system, you can defeat it. Design the trap system & you know how it can be defeated, and you know what clue to feed the Rogue when they get that high roll on their Investigation. Security gate style traps have a built-in bypass, which is handy. Alarms might have a built-in bypass if someone needs to occasionally legitimately access the area. Barriers are armored against being bypassed since there's no legitimate need to do so.
4. Security gate style traps work by using a test to determine if someone should be let pass, or whether the trap should be sprung on them. This can be a test of kind ( the trap lets lighter Gnomes pass because they're too light to trigger the floor panel ), a test of knowledge ( the guards know that pressing that one tile in the wall mosaic give them 20 seconds to get past the scythe trap before it re-arms ), or a test of possession ( the Red Monks all have arcane amulets which allow them through the shocking barrier without taking damage ).

In an upcoming scenario, the PCs will very likely search a storage room. So far they just say "I want to do an investigation to see if there's anything worth taking," and if they roll high enough I reward them with some kind of loot. This time around, I want to have a trapped container that springs a net on them.

Every DM does things differently... However for me, whether there's something worth taking or not doesn't depend on how high the characters roll, but on what is there. I've already decided if there is anything valuable in the storage room and, if so, where it might be, before they walked in (obviously, I don't just make stuff up as they go, most of the time). Even if I hadn't decided ahead of time, I'd use some sort of Fate-chart (Mythic GM Emulator) type roll behind the screen to see if there was anything. How likely is it that there's something valuable in here? Let's say 25%. Roll that and see... before they even ask.

Second, "If there's anything worth taking" is an entirely subjective opinion. I'd ask my players to clarify... "Are you looking for weapons? Food? Supplies? What is it that you are searching for?" After all, a crate filled with bolts of nice cloth is potentially worth quite a lot, but unless the PCs have some mechanism to transport it, probably aren't worth actually taking at that particular moment (or maybe ever).

Of course it is entirely up to you how you run this. For me... I am wary of introducing situations in which the players think that if they just roll high enough they will get what they want. This is the same reason I don't allow a nat 20 to be a "crit success" on a skill check. And I also don't let players call for checks... I have one player who sometimes says "I'm going to roll arcana to see if I know anything about this" before I ask, but I wouldn't just allow her to succeed if she rolls high. It depends if I'd have called for the roll there or not... if not, then I guess she just failed. So my worry on doing it this way is that the players, knowing you'll give them "kewl loot" if they roll high enough, will just search everything, including places where no cool loot should be, just in the hopes of rolling well and then you have to give it to them. Just a word of warning.

1) Should I have it so finding the trap is responsive to just that initial investigation roll?

I wouldn't if they're just looking for treasure. "I search this room for treasure," is specific enough to mean that you are not searching for traps (Otherwise you'd say you were searching for traps). So my players would need to specify -- but I'm kind of from the old school, in which if you don't say your character's doing it, they aren't doing it. If they say "We walk into the room" and there is a trap on the floor, they get passive perception against seeing something "off" (floor tile of a different color or slightly off-center maybe), but that's it. On the other hand if they say, "We're going to check around the doorway as we walk in to see if there are any traps" then we may be talking about an active roll where they can roll high and whatnot.

As a general rule if they are just searching to see what is "around" or "valuable," well, that is ordinarily the very activity that traps are designed to catch -- i.e., someone rifling through belongings -- so that's highly liable to set off the trap. If they say they are looking for traps, that's another story - it implies greater care, and the idea of looking under and behind things rather than inside of them (at least, before looking inside).

2) How would one of the PCs find the trap? Does it need to be with a spell or action like "Find Traps" or something more like the PC saying "I want to check for traps/check this container for traps" – which might require me to clue them that a trap is around?

That depends on the trap. Some traps are hidden; some are easy to spot. If they are specifically looking for a trap on a container, then if they roll high enough to beat the DC, they will find it. If they are just examining the container without specifically looking for traps, and roll well enough, they will see something that clues them in... you notice that there seems to be something inside the lock. Then on further inspection they see the poison needle or what have you. If it's something under a rug, you notice a bump, like something is under the rug. Etc. Then when they lift up the rug, they see whatever it is.

3) We have one rogue who has thieve's tools. To disable a trap it's like a high DC roll usually - let's say 25. Do thieves tools give the rogue advantage in this?

No, they let the rogue use his proficiency bonus with the roll, assuming he has proficiency. Without proficiency with the tools you can't use your proficiency bonus.

I usually ask for a perception check to see if they see something odd, preferably with specificity. Then they can investigate it further. 

If it’s just investigation, I would consider that taking longer in-game. 

Thieves tools with proficiency if they’re proficient. 

Also no one can lend help to picking a lock but others could potentially figure out how a trap works and help that way. 

Traps are often a game element that I find to be a bit misunderstood.

What is the purpose of a trap? Initially, for a DM, it is often to drive suspense, however, the players do not know there is a trap. There is no suspense for them at all. The only suspense for the players occurs after they find a trap and before it goes off. 

What does a trap do? It imposes the possibility of a resource penalty on the character or party. They may lose hit points, they could die, they could be immobilized or suffer a status effect. If the party can remove the status effect then it is just an additional resource drain. If not, then some party members are weakened for subsequent encounters.

Mechanically, there may be a DC to notice some aspect of the trap. In addition, there may be a DC to disarm the trap that may or may not require proficiency with thieves tools. A DM needs to keep in mind that there are some traps that just can't be disarmed or deactivated from where the players are located. Deactivating a trap may require the players to already be on the other side of the trap or in an adjacent room or elsewhere.

The DM also has to decide what happens if the character fails to deactivate the trap. Can they try again? The rules in the PHB/DMG say yes but if the attempt changes the trap in some way like setting it off, they may not get another chance.

As a result, traps are often narrative elements that, once the players are aware of their general presence, tends to make the characters behave more cautiously when exploring and often add "I am checking for traps" to their general comments when entering any rooms. The DM might ask "What are you doing specifically" and the player will respond with detailed descriptions of how they approach every item in the room if that is what the DM wants. I find this confrontational approach where the DM wants to see if the character randomly comes up with actions to try to find and disarm traps to be a bit of a waste of time. The character, once aware that traps exist, are likely doing their best to avoid them and the DM only needs to know how the character approaches finding traps in general to adjudicate the result (e.g. The character does a far visual inspection, then a close one, front and back, then they closely examine locks, hinges and any openings, if they can't find anything they briefly touch the object, if nothing happens they cautiously touch back/sides/top/underneath if accessible - leaving the front for last, if it all looks ok they see if the container is locked - try to open it from a distance using mage hand or a stick, if locked, carefully probe the lock and try to unlock it while watching for anything that may indicate a trap on the lock - an arcane trickster would do this from 30' using mage hand). Does a DM really want to go through all of this every time a character says "I am checking the box for traps" ... personally, I don't think it is a good use of RPG time :)

Anyway, if there are no consequences for failure, then after spotting the trap, the DM can have the player make a roll - if they succeed then narrate success and if they fail, the DM can describe how it takes the character longer than expected to disarm it.

I don't completely disagree - and I think AngryDM covered this exact line of argument pretty extensively - but traps are a "waste of time" only if the Players are spamming "check for traps" everywhere they go.

Player only need to do this, if the GM has a history of designing and placing traps stupidly or maliciously.

If traps a) have a defined purpose ( see my points above about them being security devices: permeable barrier, complete barrier, or alarm ), and b) are placed intelligently / with verisimilitude, then Players don't need to spam "search for traps" in every room. They would only check for traps where it makes sense for such security devices to be placed. Banks don't put alarm systems on the janitor's closet, they put alarms on the bank vault; pyramid builders didn't trap the peak of the structure or on stone block #252,522 in the structure, they trap the corridor leading to the burial chamber.

If the Players approaches the situation intelligently ( although they can only do so if they can trust that the GM is designing and placing traps intelligently ),  the GM is not seeing "if the character randomly comes up with actions to try to find and disarm trap", they are seeing if the Player can intelligently solve a problem: look at an environment, estimate if it makes sense for this area to trapped, figure out what the possible nature of such traps might be for here, figure out the best ways to detect such possible traps. In short, they need to use their heads.

If the Players can trust their GM, and stop interacting with the rules - trying to find the right combination of rolls, abilities, and bonus that they can spam every 6 seconds that get them past the totally randomly placed GM "screw job" traps - and start interacting with the world, then traps are just a different flavor of encounter.

The example of a hyper cautious player is a bit reductio ad absurdum, and an competent GM is going to collapse that down into a single roll anyways: "OK, I get it, you're carefully examining as many elements of the chest as you can, and then using Mage Hand to open the chest? OK - give me a Slight of Hand roll" ( or "... that's covered under your previous roll, you still don't see anything ... ").

Your argument against use of traps isn't wrong - but it's not universally correct either. It's a good argument that not using traps is a better option than using stupidly designed and placed traps, but I think traps can be a viable adventure element if designed and placed logically - whereas they just become another flavor of encounter with which a competent Player can engage.