In buildings with wooden floors the hearth in front of the fireplace should be supported by masonry trimmer arches (fig. 34) or other fire-resistant construction. Hearths should project at least 16 inches from the chimney breast and should be of brick, stone, terra cotta, or reinforced concrete not less than 4 inches thick. The length of the hearth should be not less than the width of the fireplace opening plus 16 inches. Wooden centering under trimmer arches may be removed after the mortar has set, though it is more frequently left in place. Figure 36 shows a recommended method of floor framing around a fireplace. Wall Thickness The walls of fireplaces should never be less than 8 inches thick, and if of stone they should be at least 12 inches thick. When built of stone or hard-burned brick, the back and sides are often not lined with firebrick, but it is better to use firebrick laid in fire-clay. When firebricks are laid fiat with the long sides exposed there is less danger of their falling out. They are generally placed on edge, however, forming a 2-inch protection, in which case metal ties should be built into the main brickwork to hold the 2-inch firebrick veneer in place. Thick metal backs and sides are sometimes used as lining. When a grate for burning coal or coke is built in, firebrick at least 2 inches thick should be added to the fireplace back unless the grate has a solid iron back and is only set in with an air space behind it (fig. 37). Jambs The jambs should be wide enough to give stability and a pleasing appearance; they are frequently faced with ornamental brick or tile. For an opening 3 feet wide or less, a 12- or 16-inch width is generally sufficient, depending on whether a wood mantel is used or the jambs are of exposed masonry. The edges of a wood mantel should be kept at least 8 inches from the fireplace opening. For wider openings and large rooms, similar proportions should be kept. [Illustration: Figure 36.--Where a header is more than 4 feet in length, it should be doubled, as shown. Headers supporting more than four tail beams should have ends supported in metal joist hangers. The framing may be placed one-half inch from the chimney because the masonry is 8 inches thick.] Lintel Lintels of ½- by 3-inch flat iron bars. 3½- by 3¼- by ¼-inch angle irons, or damper frames are used to support the masonry over the opening of ordinary fireplaces. Heavier lintel irons are required for wider openings. Where a masonry arch (fig. 38) is used over the opening, the jambs should be heavy enough to resist the thrust of the arch. Arches over openings less than 4 feet wide seldom sag, but sagging is not uncommon in wider fireplaces, especially where massive masonry is used. Throat