Revised wind uplift designs can lower job costs

Is an 1-90 system really necessary on your next job?

Time flies. While an old phrase, it's appropriate given that SPRI recently wrapped up its 19th annual conference. And plans are already underway for the association's 20th anniversary meeting next January. This marks a milestone in the history of flexible roof membranes, the industry SPRI represents. Naturally, many manufacturers and marketers of flexible membranes pre-date the founding of our organization. To document that long history, SPRI is asking contractors to tell us about 20-year-old flexible membrane systems that they've installed. Share the good news about your team's workmanship. E-mail details of successful, long-term projects to lkspri@aol.com. Surveying just the past year, SPRI has accomplished a lot that contractors may want to look at. High on this list would be SPRI's updating of its "Wind Load Design Guide for Low-Sloped Sheet Membrane Systems." (This guide is available from SPRI as both a stand-alone document and in SPRI's Flexible Membrane Roofing: A Professional's Guide to Specifications.) We revised our design guide to bring it in line with the latest recommendations outlined in ASCE (American Society of Civil Engineers) 7-98.

Noteworthy savings

If a contractor uses calculations outline in these tables, he'll see that he may not have to use the 1-90 roof assembly that he thought, notes SPRI Technical Director Dave Roodvoets. In reality, he may need only an 1-60 design in a lot of cases. These reduced design and installation costs could be significant to a contractor's bottom line. After all, an 1-90 roof system is not necessarily better than an 1-60 if all you need is a 1-60, Roodvoets explains. Safety factors are already designed into that rating. Most roofs in the U.S. should be installed using a system that's been tested to 60 pounds per square foot (psf), i.e., Factory Mutual 1-60, Underwriters Laboratory Class 60. However, most architects spec a Class 90, 90-psf or 1-90 roof. Using ASCE 7-98 data, all roofs on buildings 60 feet tall or less in the urban/suburban type (exposure B) surroundings can be installed with a 60 psf system--even buildings considered essential facilities like fire houses and hospitals. A 60-foot-tall essential building, like a hospital, will have a windload in the field of the roof of 30 psf. Using Factory Mutual's required safety factor of 2, the field of the roof system should be tested to 60 psf or the FM 1-60, UL Class 60 system. Installing the system to a 90 psf design doesn't make economic sense.

Going by the numbers

We will illustrate the cost issues using the example of a 60-foot tall hospital in a typical suburban setting in Ohio. This example uses a generic flexible membrane with a sheet width of approximately five feet. Manufacturers with sheet widths of more or less would offer different cost comparisons. If a mechanically fastened flexible membrane system is installed to the 1-60 design, it requires a five-foot-wide sheet membrane to be fastened with rows of fasteners at five feet and within row spacing of 18 inches. By comparison, the 1-90 design for that membrane requires the rows of fasteners to be within five feet and in-row spacing of fasteners to be on 12-inch centers. This means there is a 50% increase in the number of fasteners to go from a system resistant to 60 psf to one resistant to 90 psf. The resultant costs are simple: 50% more fasteners, 50% more labor. The benefits for 1-90 over 1-60: none. No benefits, just increased cost. A 60 psf tested system is twice the projected wind load for the field of the roof for a storm that has a 2% probability of occurring in any given year. In fact, it would take a wind speed of 140 mph to provide enough uplift in the field of the roof to equal the tested load. The probability of that wind speed is determinable, but very, very small. To create the tested load for the 1-90 system, the wind gusts would need to be blowing at 170 mph. Installation of 60 psf systems applies to buildings in the vast central area of the U.S. This area is a 90-mph gust wind zone. The buildings are in cities or urban areas and are less than 60 feet tall. Of course, that's where most roofs go. The Factory Mutual 1-28 requirements are also for a 1-60 system for this area of the country and ground roughness B. FM recommends that perimeters of these roofs be installed to 1-90 and the corners installed to 1-135 requirements. However, this does NOT apply to the 100-mile wide area that follows the Atlantic and Gulf Coasts or special mountain areas. There, you will need ASCE 7-98 or a similar guide. Bildings over 60 feet tall also must have wind loads calculated by ASCE 7-98 methods.

SPRI represents sheet membrane and component suppliers to the commercial roofing industry. SPRI can be reached at 200 Reservoir St., Ste. 309A, Needham, MA 02494; Tel.: 781-444-0242; fax 781-444-0611.

This column is previously appeared in the March issue of RSI Magazine.