Letters

In your March issue, the article "Understanding wind-resistant design," page 43, by Mark S. Graham, NRCA's associate executive director of technical services, makes several good points, including that a safety factor of 2.0 is appropriate for wind design of roof systems.

However, I would like to clarify some points the article addresses.

First, the article does not reflect the most recent changes and enhancements published in the February 2007 version of FM 1-29, "Roof Deck Securement and Above-Deck Roof Components." The updated data sheet is available at no cost at www.roofnav.com along with updates to FM 1-28, "Design Wind Loads," and FM 1-52, "Field Uplift Tests."

Second, the article states FM Global recommends design wind loads up to 49 percent greater than those stated in ASCE 7-05, "Minimum Design Loads for Buildings and Other Structures." In fact, FM Global recommendations are at most 15 percent higher than ASCE 7-05 requirements for most buildings.

Category I buildings (to which the 0.77 importance factor in ASCE 7-05 specifically applies) include agricultural, temporary storage and minor storage facilities. So excluding such buildings as barns and tool sheds, FM Global uses the same importance factor as what ASCE 7-05 requires for more important buildings (Categories III and IV).

Third, the article implies FM 1-28 is not code-compliant because it is not based on ASCE 7-05. In fact, the February 2007 update to FM 1-28 is based on ASCE 7-05.

Fourth, though the code acceptability of wind-uplift tests is discussed, in addition to FM Approvals' wind testing, it is important to point out a 10- by 10-foot laboratory uplift test provides inaccurate results for certain roof systems with larger tributary areas and certain end restraint conditions—particularly most mechanically fastened single-ply and standing-seam metal roof systems. Such roof systems can be accurately tested with a 12- by 24-foot uplift test.

As always, FM Global is happy to work with Professional Roofing to ensure the articles you plan to publish related to FM Global and FM Approvals are technically accurate and up-to-date before you go to press.

Richard J. Davis, P.E.
FM Global
Johnston, R.I.

Following is Graham's response to the letter:

Thank you for taking the time to share your comments and opinions.

My article was intended to provide readers with the fundamentals of wind-resistant design applicable to low-slope roof systems as an alternative to using FM Global's and FM Approvals' guidelines. In my Tech Today column, "FM update," in the same issue of the magazine, I reported on FM Approvals' free distribution of RoofNav and FM Global's February 2007 update to FM 1-29. The latest revision of FM 1-29 in no way affects the accuracy, completeness or appropriateness of my article.

Regarding FM Global's conservative use of an importance factor, I stand by my statement that FM 1-28 results in design wind loads for many buildings to be 15 to 49 percent greater than results derived using ASCE 7-05. A vast majority of the building inventory roofing professionals regularly encounter qualify as Occupancy Categories I or II according to ASCE 7-05, resulting in importance factors less than the 1.15 FM 1-28 prescribes. Most building codes also recognize the lower importance factors for ASCE 7-05's Occupancy Categories I and II.

Also, it is important for roofing professionals to realize using FM 1-28 and FM 1-29 may result in minimum recommended design wind-resistance loads that far exceed those derived using ASCE 7-05 and ASTM D6630, "Standard Guide for Low Slope Insulated Roof Membrane Assembly Performance," or NRCA's online application, www.roofwinddesigner.com.

For example, for a hypothetical industrial building located in the 90-mph basic wind speed region (a vast majority of the U.S.) with a mean roof height of 20 feet, the minimum recommended design wind-resistance loads derived using ASCE 7-05 and ASTM D6630 are 38 pounds per square foot (psf) in Zone 1 (roof area field), 63 psf in Zone 2 (roof area perimeter) and 95 psf in Zone 3 (roof area corners). Using FM 1-28 and FM 1-29 for the same building, Zone 1 is 60 psf, Zone 2 is 105 psf and Zone 3 is 150 psf (58 percent, 67 percent and 58 percent, respectively, greater than the wind-resistance loads derived using ASCE 7-05 and ASTM D6630).

Regarding FM Global's representation that FM 1-28's latest edition is based on ASCE 7-05, this does not necessarily mean FM 1-28 is code-compliant. As I illustrated, FM 1-28 does not provide results identical to ASCE 7-05. Also, FM 1-28 is not a recognized consensus standard and is not referenced in the International Building Code (IBC) as an acceptable method for determining wind loads.

Regarding FM Global's claim that laboratory uplift tests using 10- by 10-foot test specimens provide "inaccurate results for certain roof systems," I agree larger test specimens (12 feet by 24 feet) sometimes are preferred. However, I need to point out the 10- by 10-foot test specimen size is contained in UL 580, "Tests for Uplift Resistance of Roof Assemblies," and UL 1897, "Uplift Tests for Roof Covering Systems," which are recognized consensus standards. IBC permits the use of these standards. FM 4450, "Approval Standard for Class 1 Insulated Steel Roof Decks," and FM 4470, "Approval Standard for Class 1 Roof Covers," which use larger test specimens, are not recognized consensus standards. If FM Global and FM Approvals believe 10- by 10-foot test specimens are inappropriate, I suggest they pursue the issue in the standards development community and attempt to gain consensus standard recognition for FM 4450 and FM 4470.

Finally, it is important Professional Roofing readers realize NRCA and FM Approvals and FM Global maintain a close working relationship. Also, Professional Roofing has provided more information regarding FM Global's and FM Approvals' recent changes than any other venue I am aware of. However, as you can see here, this does not necessarily mean NRCA and FM Approvals and FM Global fully agree.