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So my brother and his friend got the D&D starter set and wanted me to DM for them and their friends for the first time. With only an hour to learn the rules, it was hectic and chaotic. We probably broke a lot of rules, but we had tons of fun. We played through the start of The Lost Mines of Phandelver, and a lot of times the players wanted to burn stuff. Set fire to houses to trap enemies and try to burn people, burning down large trees that goblins are hiding in, or just setting someone's clothes on fire.

My question is, how should I handle people who want to set the world on fire? I understand as DM it is really at my discretion, but I wanted the opinions of others on what to do. Mainly, what kind of DC should I be using?

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Setting DCs

First of all, if there is no time pressure and no consequence of failure and the action is possible then they should just succeed. This applies for everything, not just lighting fires.

However, in the specific instance of lighting fires: if they have a tinderbox and something to burn and its not pouring with rain then they can start a fire, be it a torch, a campfire or a cigarette. Setting fire to something else under the same circumstances when you already have a fire should be quicker and (possibly) easier.

Its only when there is time pressure, or there are consequences of failure or there is some question mark over possibility that you need to set a DC.

When setting a DC you should look and see if the rules have a section like "How to set the World on Fire" - they don't. Pity.

So you have to make them up. Its always a good idea to look at other rules to see if you can nick their DCs. Offhand nothing springs to mind so we are going to have to wing it.

So what is the action the PCs want to accomplish? Well, its unclear from your question if they are trying to kindle a fire from scratch or use an existing fire (like a torch) to light something else. Let's see if we can deal with both.

Lighting fires from scratch

Hunting through my PHB my eye lights on this (p.153):

Tinderbox. This small container holds flint, fire steel, and tinder (usually dry cloth soaked in light oil) used to kindle a fire. Using it to light a torch—or anything else with abundant, exposed fuel—takes an action. Lighting any other fire takes 1 minute.

So here is an item that gives us our staring point. So if we have something highly combustible it takes 1 action to light, if not it takes 1 minute. Now, 1 minute is like forever in combat (10 rounds) - the longest combat I have ever seen in D&D 5e was over in 5-6 rounds, so if you are planning to do this in combat then whatever you are lighting had better be as combustible as hell.

I also noted (p.152):

Oil ... You can also pour a flask of oil on the ground to cover a 5-foot-square area, provided that the surface is level. If lit, the oil burns for 2 rounds and deals 5 fire damage to any creature that enters the area or ends its turn in the area.

So 1 action to pour the oil over the thing you want to burn and 1 action to use a tinderbox to set it on fire, or if you have something already burning you can light it for free (I would rule).

Now in neither case is a DC called for. Provided you have the tinderbox, you can light a fire given something to burn and enough time (1 action or 1 minute). Good, lighting fires is easy.

Setting something on fire

Remember the rules (this is basically all of the rules: everything else is just nuance :-)) (p.6):

  1. The DM describes the environment
  2. The players describe what they want to do.
  3. The DM narrates the results of the adventurers’ actions.

The DC swings on what you decide is in the environment:

  • What is the house made of: did the little piggies make it out of straw, sticks or bricks?

  • What type of tree: a willow, a pine, or a eucalypt?

  • Is it summer or winter or autumn?

  • Has it been raining?

  • Is there 3 feet of snow on the ground?

  • etc.

Thinking about all those things you do what experienced DMs do: lick your finger stick it in the air and make something up. Of course you can always use the Table on p.238 of the DMG:

Task      DC  Task              DC
Very easy  5  Hard              20
Easy      10  Very hard         25
Moderate  15  Nearly impossible 30

Even better, you can use this advice on the same page:

If the only DCs you ever use are 10, 15, and 20, your game will run just fine.

Which means that you only need to decide if the thing is easy to light, hard to light or somewhere in the middle.

Depending on what the players tell you when "The players describe what they want to do" they can earn themselves advantage ("I throw oil all over the thatched roof and shove my torch inside") or disadvantage ("I hold my torch against the timber wall until it lights") or neither ("I set fire to the house").

We already know how long it takes - if it is something easy to light (DC10-) it takes 1 action, if its harder than that it takes 1 minute.

Effects

The effect of a fire in terms of damage, growth and spread is a whole question on its own I think.

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    \$\begingroup\$ And if you ever tried to light a camp-fire yourself you will know, a solid block of wood like a tree or a wooden house takes forever to catch on fire, even if you pour oil over the stem of a tree, only the oil will burn for a while... \$\endgroup\$ – Falco May 31 '16 at 13:16
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    \$\begingroup\$ A person living in a setting inspired by medieval europe should know much better how to start a fire than someone who grew up in todays world, because back then open fire was a far more important part of daily life. \$\endgroup\$ – Philipp May 31 '16 at 13:55
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    \$\begingroup\$ Indeed, don't let your players con you that a tree or even a dry thatched roof contains "abundant, exposed fuel" and therefore only takes one action to get the whole thing burning. It might only take one action to light a fire in the thatch using a tinderbox, but you'll have to decide how long it then takes for the fire to spread to the whole roof. Outside combat the exact answer probably isn't important, so long as it's more than the time it takes to get back down off the roof... \$\endgroup\$ – Steve Jessop May 31 '16 at 15:30
  • \$\begingroup\$ There's a lot of great advice for setting a check DC here, but what kind of check is the player character making? I'd recommend Wisdom (Survival) for mundane tools or spellcasting ability check for applying a cantrip (e.g. produce flame, fire bolt). \$\endgroup\$ – Josh Clark May 31 '16 at 21:37
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Fire lighting should be easy. As DM, you need to be aware of the consequences of lighting that fire. Consequences also can make the game more enjoyable.

As far as DC, as long as the characters have unlimited time, you should really not require any rolls. If they use fire a lot, you may want to create a chart for yourself of the difficulty of starting various things on fire (wood, damp wood, wet wood, buildings, live trees). Use this chart to get an idea of how long these tasks will take rather than to introduce more dice rolling. As DM, you may not want to give them unlimited time and will have NPC characters, events and monsters interrupt them depending on the circumstances.

Other thoughts:

Fuel: If the characters are using oil or some other incendiary to help light that tree on fire, keep careful track of how much they are carrying.

I douse the tree with oil and set it on fire.

Sorry, you used the last of your oil to light up that zombie.

Carrying too much oil can turn them into giant timebombs. What sort of containers is it in: Are they breakable, leakable?

When you pull an oil flask from your backpack, you notice the sack is soaked with oil. One of the flasks on the bottom leaked or broke. When you light the building on fire, fumes from your knapsack reach the flames and your knapsack leaps into flames. You have about 5 seconds to get away before it becomes a fireball.

Location: Are there other buildings nearby? Other trees? Is there the possibility that while they are building the fire, the town watch will come by and ask what they are doing? Might they end up burning down the entire town or forest? Perhaps this will incur the wrath of the town watch, rich noble, forest rangers, tree shepherds, nature gods, etc.

Burns: If you want to push the players away from thinking fire is the solution to everything, if these guys are regularly setting up large fires, especially using oil, there's a possibility that they might get burned. Set a percent chance based on the situation that they will get burned (backflash, flying embers, intense heat, exploding pinecones, etc.). Maybe some burn scars will remind them that fire can be dangerous.

Stop, drop and roll

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    \$\begingroup\$ +1, but fire IS the solution to everything. As evidence, I present fireball \$\endgroup\$ – GreySage May 31 '16 at 20:48
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    \$\begingroup\$ @GreySage: when you absolutely positively got to kill every mf in the room, accept no substitutes. \$\endgroup\$ – Steve Jessop Jun 1 '16 at 1:15

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