The walls of fireplaces should never be less than 8 inches thick, and
if of stone they should be at least 12 inches thick. When built of
stone or hard-burned brick, the back and sides are often not lined with
firebrick, but it is better to use firebrick laid in fire-clay. When
firebricks are laid fiat with the long sides exposed there is less danger
of their falling out. They are generally placed on edge, however,
forming a 2-inch protection, in which case metal ties should be built
into the main brickwork to hold the 2-inch firebrick veneer in place.
Thick metal backs and sides are sometimes used as lining. When a
grate for burning coal or coke is built in, firebrick at least 2 inches
thick should be added to the fireplace back unless the grate has a
solid iron back and is only set in with an air space behind it (fig. 37).