Behind the bar build a wooden form extending back several inches and sloped slightly upward. Then lay a course of stone on top of the iron and fill in between the stone and the form with a rich mixture of concrete reinforced with iron rods. This concrete should rise at such an angle as to come flush with the edge of the damper. The reinforcing rods should extend the full width of the arch and tie into the stone at the sides.
Buy a Good Damper
Since the fireplace is probably the most important feature of your cabin, don’t skimp on materials. Buy the best dome-type damper that you can afford. Some fireplace “experts” will say this is nonsense, but don’t let them fool you. The damper is to a fireplace what a mainspring is to a watch.
The dome damper is set on top of the fire brick lining where the throat enters the smoke chamber. The handle to control it may be set in the front or side as you wish (make sure of this when you buy it) and may project or be set in a recess in the masonry.
The throat, as can be seen in [Fig. 6], is considerably narrower from front to rear than the fire box—5½ inches is maximum—and should come as near the front as possible. It should begin 6 to 8 inches above the top of the fireplace opening, extend the entire width of the fire box and slant downward toward the back. At the same time the back surface of the fire box has been slanting forward until it reaches the same height at the beginning of the throat. At this point construct the smoke shelf, which is literally a shelf set back to the vertical line of your masonry.
An arch like this helps prevent downdraft.
DIRECTION OF PREVAILING WIND CHIMNEY CAP
Proper construction of the smoke chamber is often the factor that determines the success of your fireplace. The amount of taper may vary, but it is a good general rule to slope the walls inward one foot for every 18 inches of height. The smoke chamber walls must be as smooth as possible, the best finish being about ½ inch of mortar—or a commercial steel unit. This cuts down the amount of friction and makes it draw better. After the smoke chamber walls are tapered up to the desired dimensions, the flue is straight from here on up.