The structural collapse of the Twin Towers of the World Trade Center occurred in a time period far shorter than its statutory Fire Code rating. Speculation gave rise to theories that the steel had buckled because of the substitution of the original asbestos-containing fireproofing with a non-asbestos-containing vitreous fibre formulation. The substitution was thought to have come about due to New York City’s 1972 ban of such materials in construction. The use of substitute material in the Towers actually pre-dated the New York City ban by almost 2 years. Although, the substitute material may have had the same thermal rating for fire, its characteristics concerning durability during service are now known to have been inferior. At the time of application and construction, no performance criteria were available concerning application thickness, adherence character, compressive strength, or density. These factors are now thought to have played a major role in the heating of the structural steel and its subsequent failure. The ban of asbestos-containing spray fireproofing was probably a reasonable conclusion for the time, only if the substitute material performed as well as the material which it replaced. However it did not. The issues today focus on the necessity to re-inspect buildings sprayed with asbestos substitutes during the time period 1972–1977, especially those structures that employed the same light-weight floor truss design, and measure their serviceability according to now established standards.