In small fireplaces sagging of the arch over the opening seldom occurs,
but in fireplaces over 4 feet wide it is not uncommon. It is due to
insufficient support of the masonry. Except in massive construction
there generally is not sufficient masonry at the sides of the opening to
resist the thrust of arch construction; hence it is usual to support
the masonry with iron, which, if too light, will sag. Too small an iron
will become so hot that its tensile strength is lowered until it bends. A
heavy flat bar at least one-half inch thick is sometimes used or a T-bar
which has greater strength, but less metal; the wider the opening the
heavier the bar required.