File:Shaftwall corner fireproofing.jpg

English: This picture shows a shaftwall assembly immediately before a successful 3 hour fire test conforming to ASTM E119, ULC-S101 and NFPA 251. Factory-made boards are straight. Test frames are not always straight due to repeated and extreme temperature fluctuations. It is common practice in North America to fireproof the edges of all such assemblies. The labs call this "monokoting" after a popular commercially available brand of fireproofing plaster. This common corner fireproofing is a prudent item to consider in grandfathering drywall shaftwall systems for fireproofing ductwork or electrical services because not even the straight wall corners, all four of them in a test, are actually tested because they are protected by fireproofing plaster. A pressurisation or grease or smoke exhaust duct or cable tray, conduit or wiring that needs to be fire-resistance rated necessarily requires corners. Joints are the weakest link in fire-resistance testing of wall and floor assemblies. The corners are also never equipped with thermocouples (which determine pass-fail criteria for the test). Even though the corners are protected with fireproofing plaster that is not mentioned in any certification listings in North America, not a single thermocouple has to go there. If there were a joint in a grandfathered 3-dimensional drywall shaftwall system around any ductwork, mechanical or electrical services, there is zero evidence of the temperature going through that system at this location.