a. a factory built fireplace is like a car. It is built at a factory with specific parts that it has been UL tested with. It CANNOT have any random part installed. If a part is broken or damaged and needs replaced, it must be for that specific type and model of fireplace. The manufacture of these fireplaces that have specific clearances to combustibles that MUST be met. 

     Example: If your factory built fireplace requires 2 inches to all combustible materials and the chimney passes through the attic and is touching a wooden truss... what do you think could happen? yes, exactly that chimney gets warm and is in constant contact with a piece of wood, it could potentially ignite that truss. Fire could then rapidly spread through out the attic and into the living quarters of the home.  

b. a masonry fireplace is built on site with firebricks, a special kind of mortar and lined with a clay tile flue. This is all surrounded by bricks and mortar that have specific clearances to combustibles.  

     Example: a masonry fireplace should NEVER have wood under the firebox. It should Never have wood underneath the hearth extension.

     WHY? It doesn't matter if the firebox floor is 4 inches thick or 12 inches thick if there is wood underneath those bricks, heat can transfer down through the bricks to that wood where is could potentially ignite.