Perspectives
A Quarterly Information Source from Benchmark, Inc.

Volume 59 September 2007

by Jeff Evans

If you hear grumblings from your roof designer, it could be because of an ill wind blowing through

the roofing industry. Recent changes in roofing standards have complicated roof design, have limited roofing selection choices, and have added steps to the process of wind design. More changes are most assuredly on the way.

One of the basic requirements in roof design is to provide enough resistance to a roof construction so that it will resist the winds expected to act on the building. As wind blows over the surface of a roof, an upward or suction force acts on the roof surface.

In addition, wind entering the doors and windows of the building produce a positive pressure in the building that acts to push up on the roof. The wind load on a roof is the sum of these pressures.

Wind design for buildings is addressed in the applicable building code, following the American Society of Civil Engineers standard ASCE - 7. This standard provides two methods for determining the wind uplift

forces on a roof, using look-up tables (simplified method) or mathematical formula (analytic method).

A third method, wind tunnel testing, is used to help determine wind uplift forces on buildings of complicated geometry or on unusual exposures.

The wind uplift on a roof is affected by a number of variables, including the wind design speed, building height, roughness of adjacent terrain, the number and location of windows and doors, and the type of occupancy. As such, the building code tells a designer what wind load must be resisted, but not how to design the roof to meet the required load.

Simply, the code requires the roof’s wind resistance must be greater or equal to the wind load. To determine the resistance side of the equation, a roof designer must rely on wind uplift testing.

For the past 25 years, roofing manufacturers have predominantly tested their roof systems for wind resistance at FM Global’s Norwood, Massachusetts testing facility. Some wind uplift testing has been done at Underwriters Laboratories, and other labs, but generally, most wind uplift testing and wind design recommendations have been driven by FM Global.

The roofing industry accepted FMG’s leadership in wind design for several decades, in the absence of any other wind design standard. FMG’s primary interest in establishing wind standards was to reduce their insured’s losses in wind events. But over the years, the roofing industry allowed FMG’s Loss Prevention Data Sheets and FM wind uplift testing (published in the ever thickening FM Approval Guide), to become the de facto standard.

Several years ago, FM brought to market ROOFNAV,

an on-line database of testing approvals. This database allows the user to search for testing results on hundreds of thousands of roofing systems.