Damaging floors are floors (sectors or groups of sectors) that will damage the player when he crosses them. They form part of the "dangerous Doom environment" described in the game booklets:
- Slime and other radioactive waste: Many of the areas in Doom contain pools of dangerous liquids that will damage you if you walk through them. There are several varieties of waste, each doing increasing damage. If it looks fluid, beware!
Since the damaging effects are determined independently from the flats used on their area, they may be used to stand for different types of hazards and substances, according to the graphics chosen for the floor, such as green slime or lava, and any associated hints. Flaming wall textures may reinforce the idea of a very hot area, radioactive warning signs that "nukage" has leaked, and a level name like Toxin Refinery that hazardous organic substances abound.
Despite the damaging effects, it is often necessary to cross a damaging floor during play. Crossing as quickly as possible minimizes the damage. If the player has armor, then the damage is split between health and armor. Also, the radiation suit protects the player from damaging floors for a limited time.
Damage is applied periodically, on every 32nd gametic, i.e. in intervals of 32/35 (approx. 0.91) of a second. The check is performed against the global level time; thus it is equally possible to receive damage immediately upon touching the sector's floor, or to cross whole sectors completely unscathed. Because of their strict periodicity, the damaging effects can be avoided by continuous jumping in those engines that employ it (such as Hexen or some source ports).
Monsters are not affected by damaging floors.
The fluid appearance is implemented using animated flats, which include nukage, lava, blood, and the like. Flats are also defined to implement blue water and sludge water, but these are not usually damaging.
In Doom and Heretic, it is not the flat appearance that makes a floor damaging. The level designer assigns a special type value to make the floor of a sector damaging when the player is within the typed sector. Thus, there can be (and are) damaging floors that do not have the usual fluid appearance, and sometimes a floor expected to be damaging because of its texture is not damaging. For instance, in E2M2: Containment Area and E2M7: Spawning Vats, there is a rare example of a non-animated damaging floor, using a row of intense blue lights in a trench.
In Hexen certain flats are hardcoded to do damage instead of also needing a sector type (allowing the sector to have another unrelated sector type).
The following damaging floor types are found in Doom:
|Sector type||Damage per second||Doom source code comment||Other effects|
|16||20%||Super hellslime damage|
|4||20%||Strobe hurt||Light blinks each 0.5 second|
|11||drops steadily until below 11%; Same as Type 5||Level ends when health is below 11%. God mode cheat is nullified.|
(Damage is halved, rounded down, at skill level 1.)
A specialized damaging floor type (11) is occasionally used in the last room of a level (e.g. E1M8: Phobos Anomaly). Damage is inflicted as with type 5, but when the player's health drops below 11%, the level ends. The player's health cannot drop below 1% while within this type of sector, a fact that has strange consequences when the player is not touching the sector floor. These sectors cancel the invulnerability bestowed by the God mode cheat, although the invulnerability powerup is not affected. This type of damaging floor is featured once again in Final Doom's The Plutonia Experiment, Map 11: Hunted.
"Leaky" radiation suits
For the 20% damage types (4 and 16), there is a small chance (6/256 ≈ 2.3%) that the player will be damaged during a given second even if he is wearing a radiation suit. If each of the 60 seconds is considered independently, probability theory gives the cumulative effect as:
|160% or more||0.01%|
Due to the periodicity of Doom's pseudorandom number generator, however, it can be shown that the player instead takes damage as follows:
The latter result presumably should not be accepted at face value, since it assumes that 60 numbers are pulled from the table consecutively, whereas in a real level, other phenomena (such as roaming monsters and blinking lights) would sometimes call the generator as well. Moreover, in stock maps, it is rarely necessary to remain on the same damaging floor for 60 seconds. Testing seems to indicate that a single suit rarely "leaks" more than once.
The damaging floor effect is perfectly synchronous with the weapon bobbing logic, because both of these employ global leveltime checks for timing. Specifically, the damage is applied at moments when the weapon is in either top point of its cycling motion.  It is unclear whether this was incidental or not.