Masonry Magazine February 1983 Page. 27
Discussion of Corbeling
A discussion of corbeling is provided in Technical Notes 19A Revised.
Extended Hearth
The extended hearth may be formed by placing a reinforced concrete slab on top of the corbeled base assembly. Non-combustible or removable forming should be placed so that it spans from the corbeled masonry assembly to the floor joists forming the opening for the brick masonry heater. Double joists should be used around the entire perimeter of the opening with a nailer to support the edge of the non-combustible extended hearth, as discussed in Technical Notes 19A Revised, except that the extended hearth for the brick masonry heater should be at least 20 in. (500 mm) in front of the firebox and 12 in. (300 mm) around the remaining perimeter of the heater. However, the extended hearth may be eliminated on one side, or the back of the heater if it is positioned against a non-combustible wall with a minimum fire rating of 1 hr. Once the reinforced concrete slab is installed, it may be finished with brick masonry pavers. If removeable forming is used, the concrete slab used to support the extended hearth must be designed as a cantilever.
Ashpit
Once the extended hearth is installed, the brick masonry heater is laid out. The dimensions of the brick masonry heater are determined by the size of the available firebox doors and the number of baffles used in the masonry heater. The masonry heater, because of its efficient combustion of wood, does not require a large ashpit. The ashpit is usually formed by providing an opening, usually three courses of brick in height, and as wide as the firebox. This results in an ashpit which may be accessed from the front face of the brick masonry heater. The ashpit may be modified so that it is accessed from either side, or even from the rear of the masonry heater. Directly above the ashpit will be the base for the brick masonry heater firebox. Thus, the ashpit should be formed with corbeled brick masonry which will support the firebox base or a reinforced concrete slab may be used to serve as the top of the ashpit and the support for the firebox base. Using the reinforced concrete slab requires a formed opening in the slab for the ash drop.
Firebox Assembly
The dimensions of the firebox will depend on the size of the firebox doors, and either the length of the baffle chambers in the horizontally baffled masonry heater or the number of baffle chambers in the vertically baffled masonry heater. Thus, the baffle chambers need to be sized prior to laying out the masonry heater dimensions of the firebox.
There are several alternatives for constructing the firebox. The firebox base, sides and back should be lined