Designing for energy conservation in buildings calls for considerably better insulated roof structures than have been used and fire tested in the past. This requires that information be developed on the effect of such increased insulation on the temperature of structural steel and steel roof decks during a fire exposure.Fourteen small-scale fire tests are summarized that show that for combustible and noncombustible insulating materials a critical thickness exists beyond which additional insulation does not increase the temperatures developed on structural steel.