Roof Damage from Wind Uplift

I carried out an interesting structural assessment yesterday of an industrial roof on which a corner had lifted, crashing onto the the adjacent roof (apparently due to high winds).  The roof was comprised of a 25,000 sq. ft., pitched, metal clad, wood frame overlay, installed over a 20+ year, built up, 4 ply, flat roof which had exceeded its life expectancy.  The overlay had been installed 8 years ago because it was a more economical alternative than replacing the 4 ply asphalt roof.  Until a few days ago, the overlay had performed well.  Our investigation revealed that the cause of the roof failure was not high winds, as was initially assumed prior to our involvement, but the failure of the contractor to secure the roof overlay to the original roof it was covering.  What is truly surprising is that the roof overlay did not take flight years ago.

Although building component failures are normally predictable, once the installation characteristics are known, I am still amazed at how long the failure can take to occur.  The application of engineering principles in the field of structural forensic investigations can be most revealing.