Hurricane Katrina: observations from the field

• Photo courtesy of NRCA
• Photo courtesy of NRCA
• Photo courtesy of NRCA
• Photo courtesy of NRCA
• Photo courtesy of NRCA
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On Aug. 29, Hurricane Katrina swept through portions of Louisiana, Mississippi and Alabama. Katrina's hurricane-force winds and storm surge affected everything in its path-roofs, walls, cars, boats, trees, fences and signs. The hurricane's storm surge damage was limited to areas along the Gulf Coast, but the areas suffering from wind damage were considerably inland. The hurricane-force winds tested roof system performance, as well as recent building code improvements that had been instituted as a result of previous hurricanes.

On behalf of NRCA, we visited affected areas to investigate how roof systems performed. Following is a summary of our observations in the aftermath of Hurricane Katrina.

Hurricane conditions

Hurricane Katrina made landfall from the Gulf of Mexico, just southeast of New Orleans, and primarily traveled north into Mississippi. The hurricane's eye passed through areas near Picayune, Miss., up to Hattiesburg, Miss., essentially following northbound Interstate 59.

The Weather Channel reported Hurricane Katrina made landfall as a Category 3 hurricane, which can have a wind speed from 111 mph (50 m/sec) to 130 mph (58 m/sec) according to the Saffir-Simpson Hurricane Scale. Actual wind speeds along this path were about 125 mph (56 m/sec) in Slidell, La., 115 mph (51 m/sec) at Stennis Space Center in Mississippi, 115 mph (51 m/sec) in Picayune and 105 mph (47 m/sec) in Hattiesburg. These are recorded three-second peak gusts. Sustained wind speeds are about 80 percent of three-second peak gusts.

Because current design basic wind speeds in the hurricane's path ranged from 140 mph (62 m/sec) at the coastline to 125 mph (56 m/sec) in Slidell, 118 mph (53 m/sec) in Picayune and 105 mph (47 m/sec) in Hattiesburg, Hurricane Katrina provided real-life field tests of roof system performance in design high-wind conditions. These design basic wind speeds (three-second peak gusts) are from the International Building Code [IBC], 2003 Edition. Because the design basic wind speeds have been increased in recent years, there are many roofs that met previous building code requirements when the design basic wind speeds were less than those recommended in IBC 2003.

For example, design basic wind speed for New Orleans according to ASCE 7-93, "Minimum Design Loads for Buildings and Other Structures," was 100 mph (45 m/sec). ASCE 7-93 was revised in 1995; design basic wind speed for New Orleans now is 125 mph (56 m/sec). However, because it takes time for building codes to adopt new standards, the 1999 edition of the Standard Building Code still provided a design basic wind speed of 100 mph (45 m/sec) for New Orleans. Only as recently as 2003 did New Orleans adopt a code referencing the 125-mph (56-m/sec) design basic wind speed in ASCE 7-95.

Observations

During Sept. 19-22, we participated in the Institute for Business and Home Safety Post-Disaster Investigation (IBHS-PDI) team's investigation of Hurricane Katrina's damage. The IBHS-PDI's objective was to collect data on the performance of single-family residential construction in areas affected by the hurricane. As part of the IBHS-PDI team, we toured portions of the hurricane's path along its route from coastal Mississippi through Picayune to Hattiesburg; coastal Mississippi along Interstate 10 and Route 90 also was observed. We made ground-level observations of commercial buildings, as well.

Given the hurricane's high wind speeds and roof systems observed, roof system wind-resistance performance during the hurricane generally was good. With only a few exceptions, roof systems that appeared to be installed to meet current building code and insurance company requirements generally performed satisfactorily. In many cases, the damage we observed was not extensive enough to warrant more than minimal repair. We saw damage on a majority of asphalt shingle roof systems and a portion of metal panel roof systems. Also, our ground-level observations provided visual confirmation of damage to many built-up roof (BUR) systems. Asphalt shingle roof systems, metal panel roof systems and BUR systems appeared to be the prominent roof systems in the areas we observed.

Asphalt shingle performances varied greatly. Recently manufactured asphalt shingles (both three-tab and laminated) that appeared to meet current building codes seemed to perform well. However, blow-off of older, lightweight asphalt shingles that did not appear to meet current building codes was common. Also, the performance of asphalt shingles (regardless of age or style) installed by the racked method was noticeably worse than the performance of asphalt shingles installed using a conventional method. (For more information, see "The hurricanes of 2004," September issue, page 22.)

We observed edge conditions once again to be an important factor for roof system damage from high winds, as well as many variations of roof system edge performances. Some metal edges—copings and perimeter edge metal—were blown off or damaged during the hurricane; this damage occurred mostly where installation did not appear to meet current building code requirements. Specifically, we observed damaged copings and edge metal with no securement on the front face of any kind; in some cases, there were no continuous cleats, and in other cases, there were no clips. We observed edge metal that appeared to be building code-compliant and generally performed satisfactorily.

Where roof system damage was extensive or categorized as a complete failure, our observations generally provided a rational basis for the blow-off. For example, in a number of cases, roof systems failed when outer walls had openings that allowed wind to enter the building. These openings were either man-made, such as garage doors or louvers, or caused by objects creating breaches in walls or soffits. From these observations, we visually confirmed internal pressurization is a key element that contributes greatly to roof system blow-offs.

We noted roof system type was not the predominant factor in the success or failure of a building's roof system. We observed successful and failed asphalt shingle, BUR and metal panel roof systems.

Interestingly, we observed roof systems on hip roof construction generally performed better than roof systems on gable-end construction. And because most of the gable-end structures did not have gable-end vents, the shape of the hip construction appeared to be beneficial in high winds.

Closing thoughts

Hurricane Katrina proved to be a real-life test of roof system performance in high-wind conditions.

A majority of roof systems that appeared to meet current building code requirements performed satisfactorily and as expected. However, systems that did not appear to meet current building code requirements—such as old lightweight asphalt shingle systems, racked asphalt shingle systems, certain metal panel roof system edges and improperly attached low-slope metal edge details—did not perform well.

We believe government agencies, insurers and other parties interested in roof system performance likely will use the assessment of Katrina's damage to attempt to increase the current building code requirements. Based on our observations and the fact that the higher design basic wind speeds only have existed since these jurisdictions adopted a code referencing higher wind speeds, we caution this attempted increase.

James R. Kirby, AIA, and Chuck Scislo are NRCA senior directors of technical services.

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