Masonry Magazine October 1982 Page. 21
Extended Hearth
The extended hearth should also be noncombustible and is that portion of the hearth that projects out into the room beyond the face. Most building codes require that it extend a minimum of 8 in. (200 mm) on each side of the fireplace opening and 16 in. (400 mm) in front. Some building codes require that, if the area of the opening is 6 sq ft (0.55 m²) or larger, the dimensions be increased to 12 in. (300 mm) on either side of the opening and 20 in. (500 mm) in front of the fireplace.
The extended hearth may also be designed as a reinforced brick masonry cantilever. The cantilevered extended hearth can be an attractive option.
Combustion Chamber
The shape and depth of the combustion chamber will influence the draft and combustion air requirements in addition to the amount of heat reflected and radiated into the room. Figure 1 illustrates the shape and Table 1 provides recommended dimensions for the combustion chamber. These dimensions may be varied slightly, but the information given is based on successful designs, and it is not recommended that any great changes be made.
The sides and lower portion of the back of the chamber should be vertical. Above the vertical portion of the back, the brick should slope forward towards the opening to support the damper. For the maximum heat to be reflected into the room, the sloped portion of the back should be plane rather than concave. If it is concave, more of the heat will be reflected back into the fire rather than into the room. A greater splay of the sides also increases the heat that is radiated and reflected into the room.
The combustion chamber should be constructed of brick, preferably 4 in. (100 mm) thick. Thin mortar joints of no more than 14 in. (6.4 mm) should be specified. There should be a space of at least 1 in. (25 mm) between the combustion chamber wall and the backup wall, see Fig. 1. This is to provide for expansion of the combustion chamber. It is recommended that noncombustible, compressible, fibrous insulation or similar material be wrapped around the combustion chamber to assure that this space is maintained. The backup wall should be not less than 4 in. (100 mm) in thickness around the back of the combustion chamber to support the loads from the smoke chamber and chimney assemblies above.
Throat
Because of its effect on the draft, the throat of the fireplace should be carefully designed. It should be not less than 8 in. (200 mm) above the highest point of the fireplace opening. Details are illustrated in Fig. I and dimensions for the throat are given in Table 1.
Damper
A metal damper should be placed in the throat, extending the full width of the opening. It should have a valve plate which opens toward the back of the fireplace. Such a plate, when open, forms a barrier to deflect any downdraft which may occur.