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
The sides of the fireplace should be vertical up to the throat, or damper opening (ff [fig. 34]). The throat should be 6 to 8 inches or more above the bottom of the lintel and have an area not less than that of the flue and a length equal to the width of the fireplace opening. Starting 5 inches above the throat, ee, the sides should be drawn in at tt to equal the flue area.
Proper throat construction is so necessary to a successful fireplace that the work should be inspected several times a day during construction to make certain that the side walls are carried up perpendicularly until the throat is passed and that the full length of opening is provided.
Smoke Shelf and Chamber
The smoke shelf is made by setting the brickwork back at the top of the throat to the line of the flue wall for the full length of the throat. Its depth may vary from 6 to 12 inches or more, depending on the depth, d, of the fireplace.
Figure 37.—Grates of this type are commonly used in fireplaces for burning coal or coke. This one has a metal back and ends and is only set in to permit proper circulation of air around it.