Fire cut

In the construction of masonry buildings, a fire cut or fireman's cut is a diagonal cut along the top of a joist where it enters a masonry wall. If the joist burns through somewhere along its length, damage to the wall is prevented as the fire cut allows the joist to tilt out of the wall. Alternatively, the burnt joist would remain bound and cause extreme horizontal pressure, most likely resulting in the masonry wall crumbling.