The floor seems to be very sturdy and strong and straight, so was it common
to build attic rooms in this way before building regulations came in?
I don't think it was built as a room, or at least the 'floor' wasn't.
It sounds like the joists are just enough to support the ceiling with
the 50mm x 50mm members as strutting to stop any lateral movement.
AFAIR 100mm deep floor joists will span about 1.8m. The windows may
have been for decorative effect only. Are you certain that the
boarding and the windows weren't part of a later illegal loft
conversion?
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