soffit

noun2

1. •
   The visible underside of an arch, balcony, beam, cornice, staircase, vault or any other architectural element.

   • From 1945 onwards, deterioration in the condition of the brickwork in the arch soffits was kept under close observation, and in 1951 it was decided that strengthening must be undertaken. — 1954
2. •
   The top point of the inside open section of a pipe or box conduit.

   • The elevation of the pipe soffit is 10.4 meters above mean sea level.
   • The vents on the bottom of the soffit should be open, and the vents on the side of the soffit should be closed.