Hopefully, it is also the ONLY place in your chimney's structure that you
experience a fire. The firebox is built to hold wood fires unless it was
constructed to burn only gas appliances. Masonry fireboxes use "firebrick"
that is supposed to be held together with refractory mortar. This is a mortar
that can withstand higher temperatures than regular mortar. Unfortunately, it
is quite rare in the Miami Valley, to find a firebox that has refractory mortar.
That is why the joints between the bricks tend to deteriorate. This is
particularly evident in what we term as the "facing joints". The face, or
fascade of a fireplace is the brick, or stone, or various other material that is
seen in the room around the opening of the fireplace. The fascade expands
and contracts when heated, at a different rate than the firebrick. The stress
put on the joint between these two materials causes small cracks to appear.
Over time, the continual heating and cooling expands the cracks to the point
that the mortar will often fall out of the joint and leave a rather large gap. Heat
and smoke can easily be pulled into this area, which can become a fire hazard.
I have looked into these openings and seen building lumber within four to five
inches from the inside wall of the firebox! Does that help you to understand the
importance of the integrity of these joints?
That leads to a statement that is very often repeated in our industry. "There
are codes for building chimneys, and then there is the real world chimneys."
Which ones do you think we see almost ninety nine percent of the time?
The firebox is one of the most visible parts of your system. This is an area that
the homeowner can check regularly to determine when mortar joints need to
be repaired, which is known as tuck pointing. If too many joints need attention
and numerous bricks have become loose, the entire firebox may need to be
taken down and rebuilt. This is a major repair.