The firebox is a critical component of every fireplace chimney, and ensuring that it is up to proper standard is important to making your chimney as safe as it can be. The firebox is a three (or two) walled structure inside of your chimney, which contains the direct heat of the fire and guides the products of combustion towards the smoke chamber and the flue. Along with the hearth, the firebox is what most people think of when they hear the word “fireplace”. While firebox construction is simple in concept, it is crucial that the box and hearth are built out of the proper materials and kept in good repair. Because the firebox is constantly exposed to the very intense, direct heat of the fire, it tends to deteriorate faster than other parts of a fireplace, even when it is properly constructed. If a fireplace is used regularly, even the best firebox will need to be rebuilt perhaps once every three or four decades, though many can and do last much longer than that. Of course, in our area, many homes are much, much older than that.
Hearth repairs fall into much the same category as fireboxes. The hearth is, very simply, the floor of the fireplace. Like the firebox, it is designed to contain the direct heat of the fire. Because the fire often sits directly on the hearth, hearths must be built of non-combustible materials and have strict clearances which they must meet for minimum amounts of insulation. Ensuring that the hearth is solid and well sealed is also important in order to prevent sparks or embers from falling into cracks and spreading fire to the areas below.
In our area, it is very common to see hearths that were built improperly or have fallen into disrepair. Many of the houses in more historic areas were built before modern safety standards were instituted and have inappropriate use of combustibles in their construction (Del Ray and Old Town are notorious for this, though by no means the only culprits).