The deck is at the 8' level, where the base of the oven needed to be. A structure was needed to get the base up to 8'. This became the room and the reason for the sauna.
To clear things up early, the sauna does not share a heat source or any infrastructure with the brick oven other than the walls that support the oven structure.
The following photos may help illustrate how this works:
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The installation of the sauna apparatus is yet to come...by others.