Why do masons place fire bricks on edge when laying up a fire box? I always assumed this was a cost-cutting measure, but I have heard two other explanations. One holds that, because there are fewer joints, the box will be more fire-resistant. The other explanation is that the 21_4-inch face doesn't expand and contract properly and would make the chimney crack and spall.
Placing fire bricks on edge increases the interior dimensions of the combustion chamber and can reduce costs. The Brick Institute of America, in Technical Note on Brick Construction 19 ("Residential Fireplace Design"), however, recommends using nominal 4-inch-thick brick to line the combustion chamber. BIA further states that many building codes allow the thickness of this wall to be reduced when refractory brick or firebrick are used. The BOCA code, for instance, states that fireplace walls must be lined with at least 2 inches of low-duty refractory brick conforming to ASTM C 64 and must be a minimum total thickness of 8 inches. The BIA also recommends providing a 1-inch air space between the combustion chamber wall and the backup wall, although this is not required by building codes. Whenever possible, I recommend following the BIA recommendations by using 4-inch nominal units with a 1-inch air space when constructing the combustion chamber of a fireplace.