In FTL, higher levels of the door subsystem provides three purposes (in addition to interacting with an event or two):

  • Higher levels of the door subsystem give more durability to the subsystem, making it harder to destroy (which causes you to lose control over your doors.)
  • Higher levels of the door subsystem make doors require more melee attacks to force open by intruders; level one doors require no such force.
  • Higher levels of the door subsystem slow the spread of fire from one room to another, when there is a shut door(s) connecting them.

This question is about that third point. Specifically:

  • How quickly does fire spread from a room filled with fire to another if there is a level one door between these rooms?
  • How about a level two/three/four door? (Level four doors are attainable in the Advanced Edition with someone manning a level 3 door subsystem.)
  • Does it matter how large the room is that is on fire?
  • 17
    Science needs to be done. Time to set my ship on fire. Commented Apr 24, 2014 at 0:43
  • from my experience with FTL, if there is a fire, the level one doors are actually sufficient to stop fire from spreading so long as the door remain closed. My fire started in engineering and in order for it to stop from going into life support(i think, it was my base ship) I simply closed the doors and vented engineering by opening the blast doors to the vacuum of space. This is actually my goto strategy in midgame as it also becomes an important factor to kill off enemy combatents.
    – Cole Busby
    Commented Apr 24, 2014 at 2:58
  • @ColeBusby I agree. Of course, without venting atmosphere (which can happen if either the doors or the O2 system is down), level 1 doors may not be enough.
    – IQAndreas
    Commented Apr 24, 2014 at 4:16

1 Answer 1


Level 2+ doors prevent fire from spreading in MOST cases. Otherwise; every (x) seconds, fire performs a check to see if it can spread. This check goes something like:

  • Is there an open space nearby?
    • No: Do nothing;
    • Yes: Is there a level 2+ door between me and this space?
      • No: Spread to space;
      • Yes: Can I more easily spread to a different space?
        • No: 1/100(approx) chance to spread through door;
        • Yes: Spread to new space.

Fires will continue to spread until either; they run out of oxygen, or they are extinguished. With level 2 Blast Doors, most fires will burn themselves out, or be held off long enough for the player to air-lock them. Increasing the level of the doors will likely further increase effectiveness. Unlike actual fire, the fires in FTL do not run out of combustible material, meaning they can burn forever if left unattended.

Because of this, a smaller room is more 'at risk' of having a fire leave the room, because there are fewer spaces for the fire to occupy within in the room. However, since Oxygen is measured in units per space, this means the smaller room can run out of burnable oxygen sooner. Say each space has 100 units of air, burning 5 units per second with 2 unit-per-second recovery (O² subsystem), then with both squares on fire in a 2-square room, the fire would burn out in 33.33_ (rounded down) seconds.

Fires are most easily extinguished by opening a nearby airlock and draining all of the Oxygen from the room, but this can be hazardous if your Oxygen subsystem is damaged or destroyed in this time, or if they door Subsystem is destroyed, preventing you from closing the airlock.

  • 4
    pffft, my lanius crew thinks oxygen is a weakness. All doors on the ship are completely open starting in sector 1. Makes short work of boarders and fire. Though the constantly recloning engi might have something to say about it.
    – l I
    Commented Apr 24, 2014 at 11:15
  • 1
    I don't know if this is true. Level 2 doors completely stop fire? Commented Apr 24, 2014 at 11:16
  • @BlackVegetable its true, I've tested it. Use a fire bomb on your own ship with level 2 doors and see that it eventually burns itself after using up the oxygen in the room out without spreading anywhere else.
    – l I
    Commented Apr 24, 2014 at 13:55
  • 1
    If someone could check with level 3 oxygen running, I'd be satisfied and would accept (and upvote) this answer. Commented Apr 24, 2014 at 14:40
  • 2
    @BlackVegetable I just ran into this situation again and tested this. The fire did spread (though I was afk so I didn't see how) and my crew died. This was with level 2 doors and oxygen set to max. I guess level 2 blast doors do not completely prevent fire spreading but lowers it down to the point where most should die out due to oxygen deprivation unless your oxygen is bumped to 3 for some reason.
    – l I
    Commented May 13, 2014 at 12:48

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