What does the T rating measure in fire stops?

The T rating, or temperature rating, specifically measures the thermal resistance of firestop systems. It indicates how well these systems can maintain the temperature on the unexposed side during a fire exposure, essentially helping to prevent the passage of heat through the assembly. This measurement is crucial for ensuring safety in buildings, as it provides insight into how long a firestop can withstand high temperatures before it fails.

In terms of context, the other options relate to fire safety but do not pertain to the specific function of the T rating. Resistance to smoke, for instance, is an important aspect of fire safety but is measured differently. The flame spread index addresses how quickly flame can move across a surface, which is also unrelated to the thermal properties of firestops. Ignition by conduction refers to how materials may ignite due to heat transfer, but again, it does not denote the specific applications of the T rating. Therefore, the accurate focus of the T rating is indeed on thermal resistance.