Crushing ceilings are a hazard that appears in Doom, Heretic, Hexen, and Strife. A crushing ceiling cycles between its starting height and eight units above its sector's floor. It damages any players or monsters that happen to be in the space between the two planes. In Doom, barrels explode when crushed, corpses become pools of blood, and weapons and ammunition dropped by monsters will disappear. Crushing ceilings also destroy projectiles, whether fired by the player or by monsters.
Crushing ceilings are set in motion by specific linedef types. Once started, they remain running for the remainder of the level, even if temporarily suspended with a "stop" linedef type. Most other actions in the sector will be locked out afterward (this may vary depending on the source port used for play). A stopped crusher can be restarted, but not changed, with any crusher "start" linedef.
A crusher can be either slow or fast. A slow crusher does more damage, simply by being applied longer. When a slow crusher is moving down and hits a player or monster, it slows down even more, by a factor of eight. This persists until it reaches the bottom of its stroke and starts upward again. Fast crushers do not slow down.
A crusher does 10 hit points of damage every 4 tics, equivalent to 87.5 points per second. The total amount of damage per cycle will vary depending on the height of the object being crushed. In vanilla Doom, given the height of the player (56), a fast crusher will deal 60 hit points of damage before returning to a height at which the player can escape. A slow crusher will inflict 940 points of damage.
Crusher damage is cumulative so self-referencing sectors can be used to create more damaging crushers. ZDoom "fixes" the crusher damage, so this technique will not work in zdoom-derived ports like GZDoom or Zandronum.
Crushing ceilings are listed as part of the "dangerous Doom environment" in the printed game booklets.
Certain types of rising floors can also inflict crush damage: see Varieties of floors.
In Hexen, a crushing ceiling can inflict any amount of damage based on a parameter to the linedef action or script, still at a fixed rate of once every 4 tics. Unlike Doom, the crusher stops descending if there are objects in the way, allowing those trapped to escape or even to allow a player to slip under the ceiling while it crushes.
- Ghost monsters and their corpses are unaffected by crushers. Due to having zero height, they are never measured as touching the ceiling.
- Exploding barrels will turn into gibs if destroyed by a fast crusher in Doom.
- Monsters killed by fast crushers will not execute any special actions in their normal death sequence. If the thing is a boss required to finish the level, this can be game breaking.
- Some crusher action types in Strife deal no damage. The reason for this is unknown.
- Boom source code: boomref.txt