A back hearth is the section of a fireplace hearth that is actually inside the chamber where the fire will be built; is the essentially the floor of the inner hearth. A back hearth is typically made of concrete, though it can be made from other non-combustible materials such as brick or stone. It needs to be a flat surface that is wide enough to accommodate a fire or wood stove, and unlike the hearth that will be seen from the room in which the fireplace sits, the back hearth does not necessarily have to be aesthetically pleasing because it is rarely clearly seen.
Common issues concerning the back hearth are cleaning and cracking. A back hearth must be cleaned after several uses to prevent too much ash and ember from building up in the fireplace. Such a buildup can cause unwanted fires in the fireplace, and can also dirty the chimney above, which will in turn necessitate a professional cleaning. A back hearth can be cleaned easily using a small flat bladed shovel to scoop up embers and ash, an a small hand broom to further clean the ash.
A back hearth can crack after time if the foundation of a house shifts or moisture works into the concrete of the hearth itself. A cracked back hearth can be difficult to repair or replace, but it should be done, as a crack in the hearth can lead to other problems in the flue, chimney, or even the decorative front hearth. Ash and embers can also work into the crack, causing a potential fire hazard. A crack can be sealed using more concrete or mortar, though this is only a temporary solution and will not necessarily prevent the crack from spreading. The worst case scenario is one in which the hearth needs to be demolished and rebuilt.
In many cases, the inner hearth is built from brick and mortar so it can support the weight of the chimney. Concrete is laid on top of the bricks to form a flat surface for the fire box. The inner hearth itself needs to be built sturdily, so it is often quite thick to support the weight above it, especially if a wood stove is being placed upon it. It is generally built below the floor level of the hearth, and it can be built to include a gas starter or a built-in ash collecting system.