The hearth is the floor on which your stove sits. It is always made from a non-combustible material and is subject to Building regulations (you do not need Building Control permission to fit one but the hearth will be inspected by Building Control when they inspect the overall installation).
There are two different types of hearth:
The Constructional hearth: a minimum 5″ thick concrete base that makes up the floor of a fireplace recess (usually hidden under a decorative hearth known as a superimposed hearth of anywhere between 1cm and 5cm thickness).
The superimposed hearth: the decorative area above the constructional hearth. Superimposed hearths can be made from many materials: brick, concrete, slate, granite, tile, metal etc. although slate, tile or granite are the most common.
Note that the constructional hearth can be sunk into the floor or raised above the floor or even be part under floor level and part above floor level. Note also that a stove not in a fireplace recess might not need a constructional hearth (hence why you might see a stove sitting on a 12mm glass hearth on top of carpet in a posh stove brochure).
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By Julian Patrick