Smoke Hatch

[derekleffew]

aka, smoke vent, roof hatch, smoke door. Located on the roof of a proscenium theatre flyhouse, are usually operated by a fusible link to automatically open in case of an onstage fire. This creates an updraft and in conjunction with the fire curtain, helps protect the audience.

Viewed from outside:

http://www.journaultjourplex.com/products/roff hatches/FS.html