Sludge metal generally combines the slow tempos, heavy rhythms and dark, pessimistic atmosphere of doom metal with the aggression, shouted vocals and occasional fast tempos of hardcore punk. As The New York Times put it, "The shorthand term for the kind of rock descending from early Black Sabbath and late Black Flag is sludge, because it's so slow and dense." According to Metal Hammer, sludge metal "[s]pawned from a messy collision of Black Sabbath’s downcast metal, Black Flag’s tortured hardcore and the sub/dom grind of early Swans, shaken up with lashings of cheap whisky and bad pharmaceuticals". Many sludge bands compose slow-paced songs that contain brief hardcore passages (for example, Eyehategod's "Depress" and "My Name Is God"). Mike Williams, a founder of the sludge style and member of Eyehategod, suggests that "the moniker of sludge apparently has to do with the slowness, the dirtiness, the filth and general feel of decadence the tunes convey". However, some bands emphasize fast tempos throughout their music. The string instruments (electric guitar and bass guitar) are down-tuned and heavily distorted and are often played with large amounts of feedback to produce a thick yet abrasive sound. Additionally, guitar solos are often absent. Drumming is often performed in typical doom metal fashion. Drummers may employ hardcore d-beat or double-kick drumming during faster passages, or through the thick breakdowns (which are characteristic of the sludge sound). Vocals are usually shouted or screamed, and lyrics are generally pessimistic in nature. Suffering, drug abuse, politics and anger towards society are common lyrical themes.
Many sludge metal bands from the Southern United States incorporate Southern rock influences, although not all sludge bands share this style. There is some controversy as to whether the term refers to only the style emerging from New Orleans and later the American South more broadly, or to "a complete consciousness in the heads of like-minded Black Flag/Black Sabbath influenced scenes and individuals all over the world". So-called "atmospheric" sludge bands adopt a more experimental approach and compose music with an ambient atmosphere, reduced aggression and philosophical lyrics. Due to the similarities between sludge and stoner metal, there is often a crossover between the two genres, but sludge metal generally avoids stoner metal's usage of psychedelia. Sludge metal also bears some musical and lyrical resemblance to crust punk, due to the usage of political lyrics and thick, "dirty" guitar sounds.