In the construction of masonry buildings, a fire cut or fireman's cut is a diagonal cut along the end of a joist or beam 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 fail and leave the masonry wall standing.
Without firecut joists, the burnt joists would fail and the ends of the joists would rotate as the joist deflected downwards. The rotation would damage the masonry wall at the connection point and possibly pull the wall inwards.
- Brannigan, Francis L., and Glenn P. Corbett. Brannigan's building construction for the fire service. 4th ed. Sudbury, MA: National Fire Protection Association :, 2007. Print.