upfloor

From Middle English upflor, from Old English upflōr, equivalent to up- +‎ floor.

upfloor (plural upfloors)

1. (architecture) An upper storey, especially a gallery or arcade above the arches of the nave, choir, and transepts of a church.
   • 1819, Charles Clarke, Architectura Ecclesiastica, page 8:

     At Westminster, a part of the shafts set about the columns support the arches and groinings of the ailes, while others in the cluster ascend the upfloors and sustain the munions of the vaulting of the nave.

   • 1878, Edward Hungerford Goddard, The Wiltshire Archaeological and Natural History Magazine:

     […] and, as the upfloor was used for the transaction of business, attended by many witnesses, we may suppose that it was a large, wide, lofty upper story, such as is found in many early Norman minsters.