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.]