Letters

"Hurricane Katrina: observations from the field," December 2005 issue, page 22, was a great informative article about observations of roof systems after Hurricane Katrina. NRCA Senior Technical Directors Jim Kirby's and Chuck Scislo's observations that damage often was associated with edge metal, older installations and positive pressurization dovetail with those of my business partner and mine from Hurricanes Katrina and Ivan.

They mention basic design wind speeds have increased in ASCE 7, "Minimum Design Loads for Buildings and Other Structures"; Standard Building Code; and the New Orleans building code from the early 1990s to the present. They are correct. The wind speeds have increased. However, it may be misleading to some readers how Kirby and Scislo refer to the increase. To the casual reader, the article appears to say the increase is 25 mph (11 m/sec) when it really is much less than that. Basic design wind speeds for the New Orleans area have increased from 100 mph (44.5 m/sec) (fastest mile) to 125 mph (55.7 m/sec) (three-second gust).

Because a wind speed of 100-mph (44.5-m/sec) fastest mile is roughly equivalent to 120-mph (53-m/sec) three-second gust, the increase is more on the order of 5 mph (2 m/sec). Readers attuned to the fact that ASCE 7 and the various model codes changed from fastest mile to three-second gust wind speeds during the 1990s will consider this when reading the article. I'm afraid other readers may not.

Philip D. Dregger, PE, RRC
DNG GROUP Cos., Technical Roof Services
Inc. and Pacific Building Consultants Inc.
Concord, Calif.

Following is Kirby's reply to the letter:

Thanks for your comments. Actually, real-life wind speeds haven't increased. How we make note of the wind speeds has changed (fastest mile versus three-second gust). The effect of this is how roof systems are tested. So the point is that Mother Nature hasn't changed, but the resulting testing of roofs has changed based on our data recording. A roof in New Orleans now is tested at 125 mph (55.7 m/sec), not 100 mph (44.5 m/sec).