A method of making a multilayer gravity-laid inorganic fiber web, comprising: introducing first inorganic fibers into a first forming chamber that comprises a plurality of first fiber-separating rollers provided in at least one row within the first forming chamber and that comprises a first moving endless belt screen; mechanically separating at least some of the first inorganic fibers with the first fiber-separating rollers; capturing any remaining agglomerates of the first inorganic fibers by the first moving endless belt screen and returning the captured agglomerates to the first fiber-separating rollers to be mechanically separated by the first fiber-separating rollers; collecting the mechanically separated first inorganic fibers as a first gravity-laid inorganic fiber mat layer; removing the first gravity-laid inorganic fiber mat layer from the first forming chamber; introducing the first gravity-laid inorganic fiber mat layer into a second forming chamber, on a carrier at the bottom of the second forming chamber; introducing second inorganic fibers into the second forming chamber, wherein the second forming chamber comprises a plurality of second fiber-separating rollers provided in at least one row within the second forming chamber and that comprises a second moving endless belt screen; mechanically separating at least some of the second inorganic fibers with the second fiber-separating rollers of the second forming chamber; capturing any remaining agglomerates of the second inorganic fibers by the second moving endless belt screen and returning the captured agglomerates to the second fiber-separating rollers to be mechanically separated by the second fiber-separating rollers; collecting the mechanically separated second inorganic fibers as a second gravity-laid inorganic fiber mat layer atop the first gravity-laid inorganic fiber mat layer, to form a multilayer gravity-laid inorganic fiber mat; removing the multilayer gravity-laid inorganic fiber mat from the second forming chamber; and, consolidating the first gravity-laid inorganic fiber mat layer and the second gravity-laid inorganic fiber mat layer to respectively form first and second gravity-laid inorganic web layers of a multilayer gravity-laid inorganic fiber web; wherein the second forming chamber is in tandem with the first forming chamber in a single production line. introducing first inorganic fibers into a first forming chamber that comprises a plurality of first fiber-separating rollers provided in at least one row within the first forming chamber and that comprises a first moving endless belt screen; mechanically separating at least some of the first inorganic fibers with the first fiber-separating rollers; capturing any remaining agglomerates of the first inorganic fibers by the first moving endless belt screen and returning the captured agglomerates to the first fiber-separating rollers to be mechanically separated by the first fiber-separating rollers; collecting the mechanically separated first inorganic fibers as a first gravity-laid inorganic fiber mat layer; removing the first gravity-laid inorganic fiber mat layer from the first forming chamber; introducing the first gravity-laid inorganic fiber mat layer into a second forming chamber, on a carrier at the bottom of the second forming chamber; introducing second inorganic fibers into the second forming chamber, wherein the second forming chamber comprises a plurality of second fiber-separating rollers provided in at least one row within the second forming chamber and that comprises a second moving endless belt screen; mechanically separating at least some of the second inorganic fibers with the second fiber-separating rollers of the second forming chamber; capturing any remaining agglomerates of the second inorganic fibers by the second moving endless belt screen and returning the captured agglomerates to the second fiber-separating rollers to be mechanically separated by the second fiber-separating rollers; collecting the mechanically separated second inorganic fibers as a second gravity-laid inorganic fiber mat layer atop the first gravity-laid inorganic fiber mat layer, to form a multilayer gravity-laid inorganic fiber mat; removing the multilayer gravity-laid inorganic fiber mat from the second forming chamber; and, consolidating the first gravity-laid inorganic fiber mat layer and the second gravity-laid inorganic fiber mat layer to respectively form first and second gravity-laid inorganic web layers of a multilayer gravity-laid inorganic fiber web; wherein the second forming chamber is in tandem with the first forming chamber in a single production line.