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FIELD TECHNIQUE: Roof In The Wind

   Author: Dave Fulton


HURRICANES AND WIND are widespread, costly hazards to people and buildings everywhere. Adequate treatment of wind effects in design and installation are essential to the safety and economy of all structures.

Hurricanes, typhoons and cyclones are different names for the same type of severe storm. In the air-flow patterns of these storms, air spins outside around the eye and spirals inward at low heights with increasing speed until it reaches the "eye wall". At that point, it rushes upward then outward in the upper regions. This upward pressure at the eye wall causes the terrific uplift forces that create problems for the roofing industry.

Hurricanes are classified by the Saffir/Simpson Scale, which has five categories. A Category One hurricane will primarily damage shrubbery, trees, foliage and unanchored mobile homes. Category Two hurricanes will inflict major damage on exposed mobile homes and poorly constructed signs while causing some damage to roofing materials, windows and doors. Category Three hurricanes will blow down large trees and cause some structural damage to small homes. Category Four hurricanes will blow all signs down and inflict extensive damage to roofs, windows and doors. They also will cause complete failure to roofs on many homes. The highest rating is a Category Five storm, which will cause complete failure of roofs on many industrial buildings and overturn or blow away small buildings. There have been 31 Category Five hurricanes since 1928, eight of which have occurred since 2003.

This upward pressure at the eye wall causes the terrific uplift forces that create problems for the roofing industry.

DESIGNING FOR WIND
A roof can be designed to withstand just about any wind speed. The problem is that material and labor costs increase proportionally with the roof's designed strength. The majority of metal roofs are designed to meet the wind-load requirements specified in the building codes for that area.

Unfortunately, not everyone pays close attention to the details-specifically, the edge details. When failure occurs, the process usually begins at the perimeter with the flashing attachment. Once there is a breach in the building envelope, pressure gets under the roof system, resulting in failure.

{The damage from flying debris was examined during the 2004 RICOWI investigation of Hurricane Charley. A metal building displays little to no damage; however, a trailer in front of the building was blown onto its side.}

Attention must be paid to the installation of fasteners and their spacing at the roof's perimeter. If one fastener is stripped out during installation and not replaced, the adjacent fasteners are required to handle more than they are designed to withstand when a wind event occurs. This can result in failure, as well.

It is critical that all stripped-out fasteners be replaced by the installer as the roof is installed. This applies to all varieties of roofs. The Florida Building Code lists its High-velocity Hurricane Zones, or HVHZs, also known as Wind-borne Debris Regions, as areas with potential wind speeds of 120 to 150 mph (193 to 241 km/h). Metal roofs are approved for installation in the HVHZs and listed on the Tallahassee-based Florida Department of Community Affairs' Florida Product approval listings, but attention must be paid to proper installation. If the installer leaves out or strips out fasteners during installation, the roof does not meet the installation requirements and is subject to failure during a wind event.

AN OUNCE OF PREVENTION
Hurricane Charley hit the Florida coast on August 13, 2004, as a Category Four storm with maximum sustained winds of 149 mph (240 km/h). After this storm, the Roofing Industry Committee on Weather Issues Inc., Powder Springs, GA., conducted its first hurricane assessment in Punta Gorda, Fla.

When roof failure occurs, the process usually begins at the perimeter with the flashing attachment. Once there is a breach in the building envelope, pressure gets under the roof system, resulting in failure.

During the investigation, it was determined that the majority of the damage could have been prevented had more attention been given to the installation details, particularly around perimeter areas. Almost every failure started at the perimeter corners.

Another extremely dangerous consideration during a hurricane is flying debris. The wind picks up items of debris and turns them into flying missiles. It is because of these flying missiles that the Dade County, Fla., approval process requires a missile-impact test be done on all listed roofing products. However, even with all the testing, there still are impact issues. Metal roofs function no only as an aesthetic highlight for the building, but also as a highly effective cover that is resistant to high winds and impact.

This year's hurricane season officially began this month. Experts are predicting seven hurricanes with three of those strengthening into storms that are Category Three or higher. If you live in a hurricane zone, you should know your home and/or business's vulnerability to surge, wind and flooding. Walk the perimeter of your home and office and assess places you need to make repairs or increase protection. A metal roof is a good choice if you are planning to reroof your existing home and/or business. You can never be too careful. Have a happy and safe summer.

David Fulton is the vice president of research and development for a metal-building and -component manufacturer. He is on the board of directors for the Pittsburgh-based Cool Metal Roofing Coalition and holds several U.S. patents related to the metal industry.


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