Thank you for your question.
A 12mm hearth can be used when a stove is fitted without a fireplace around it, in an open plan room.
In order to use a freestanding stove with a 12mm hearth made of glass or slate, the stove must not cause the temperature of the upper surface of the hearth to reach or exceed 100°C
The Tiger could reach these high temperatures and so a "constructional" hearth is required.
If you are fitting the stove into an existing fireplace you will more likely than not already have a constructional hearth.
The full details are listed in page 37 of the current Building Regulations Part J document ( https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/468872/ADJ_LOCKED.pdf )
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