There is no doubt that improper installations are a key component in roof damage from high winds. In this case, an improperly installed skylight lifted under wind load, causing separation of the flashing. (Photo courtesy of CRS Inc.)
The forum included charts showing the estimated costs of “increasing catastrophic losses” from 1970 through 2004. It should be obvious to anyone that over 34 years, the cost of losses would increase due to inflation, as well as growing population centers and additional industrial centers. There was no mention by FM Global that insurance rates have also increased, and no comparison of the increase in rates versus the increase in losses.
After an overview of FM Global’s organization, the next topic of the forum was headed by a graphic reading “about 70 percent of all property losses are due to human factors.” There followed several photographs of damaged roof systems, some comparing FM-approved to non-FM-approved roof systems, along with others showing obvious application deficiencies.
There was a chart with data from hurricane Katrina indicating that wind losses between “unimproved” locations and “improved” locations were reduced by 85 percent. However, the preceding version of Data Sheet 1-29 was in effect during that time. My last article (“Questioning the Changes to Data Sheet 1-29,” September 2006) included photos of roofs designed under the prior version of Data Sheet 1-29 guidelines that were virtually unaffected by wind, while adjacent roofs not installed in accordance with prior guidelines were severely damaged. That is a pretty good indication that we already knew how to build wind-resistant roof systems under the guidelines in effect before January 2006. We didn’t need the additional confusion created with the introduction of a new Data Sheet 1-29.
The forum contained information about “hurricane dynamics,” and referenced ASCE Standard 7-02, crossing out the word “minimum” and replacing it with “maximum” before the term “design loads.” FM Global then attempted to project “expected hurricane damage” as part of its justification for all of the changes to Data Sheet 1-29. (It’s interesting to note that the weather gods blew those predictions away with a record low number of named hurricanes over the past year). In the forum, FM Global stated, “Poor roofing-related workmanship is a recurring concern for many FM Global policyholders” and then indicated that their clients want wind damage “addressed through FM Global’s loss prevention engineering.”
This photo documents the failure of the plastic stress plates, which resulted in the loss of the membrane and insulation. (Photo courtesy of CRS Inc.)
Safety Factors
FM Global’s justification for increasing the safety factors is as follows: “Safety factor accounts for defects in materials and workmanship, effects of aging on material strength, loads in excess of design.” However, no amount of engineering enhancements or safety factors will eliminate problems associated with poor workmanship, and as long as FM Global’s policyholders are intent on using cheap roof systems installed by the low bidder and not following the FM Global recommendation for oversight by a competent person, their casualty losses will remain virtually the same. For years, the FM Global recommendation have noted, “It is essential that thorough supervision by the building owner’s qualified representative is provided during all roof construction to ensure quality workmanship and adherence to FM Approval standards and project specifications.” But there appears to be no incentive for policyholders to provide that service, but rather to expect that engineering enhancements and “safety factors” will solve their roof-related problems.FM Global representatives indicated that the intent for revisions to Data Sheet 1-29 were to apply only in coastal, hurricane-prone areas, suggested that 1-90 wind uplift classifications were not generally necessary, and noted that 1-60 and 1-75 uplift classifications should suffice for most buildings not located in the Caribbean islands or the southeastern coastal areas of the United States. The forum included the following updates to Data Sheet 1-29 January 2006 revisions.
Under 2.2.1.5.1, the forum listed “prescriptive enhancements no longer recommended where a 1-90 system is needed.” These included:
• Most installations in non-hurricane prone areas do not need a 1-90 system, design wind pressure 38-45 psf in field-of-roof for 1-90.
• 90 mph wind, less than 90 foot high roof - Ground roughness B or C, only need a 1-60 or 1-75 system for enclosed building.
• Only very tall buildings (usually not steel deck roof) or start of hurricane prone regions need a 1-90 system. If a 1-90 system is specified, but only a 1-60 or 1-75 system is needed, prescriptive enhancements may be used to meet intent of Data Sheet 1-29.
• Reasons for change - prescriptive enhancements (fastener increases) dealt only with failure modes related to fasteners.
• At higher pressures, other failure modes like delamination occur, testing allows all failure modes to be considered.
• A review of approved assemblies in RoofNav® verified concerns that, in some cases, resistance is only 150 psf, vs. 225 psf now recommended for corners.
The inference during the forum was that this information would be clarified by an addendum to Data Sheet 1-29.
Modifications to the Data Sheet
FM Global representatives released the new, modified version of Data Sheet 1-29 in February of 2007, and they made the “revised recommendations” available on their Web site (www.fmglobal.com/pdfs/roofing.pdf) as a “courtesy” to “roofing professionals” in January 2007, prior to the release. The implications of this “modification” are profound and onerous for the roofing contractor and the roof system designer - especially when it comes to roof replacement proposals. Few of the clarifications alluded to in the forum are to be found in the modifications.FM Global includes “design and installation recommendations” for lightweight insulating concrete (LWIC) roof decks in the modifications, but it again fails to differentiate between the types of LWIC, refers to fastener pull-out testing but provides no information about required minimum withdrawal resistance for base ply fasteners, defines no distinction between the typical 1-inch head on base ply fasteners and the available 1-90 base ply fastener with a 3-inch diameter disc, and no defined minimum thickness for LWIC by type.
It is absolutely imperative to note that the title of Data Sheet 1-29 has been changed from “Above Deck Roof Components” to “Roof Deck Securement and Above-Deck Roofing Components.” Design and attachment criteria for roof decks previously included in Data Sheet 1-28 are now contained in Data Sheet 1-29.
By including “Roof Deck Securement” with “Above Deck Roof Components,” FM Global is indirectly making the roofing contractor and the roof system designer responsible for the way the deck is installed and secured to the structural members, even though the roofing contractor may not be the roof deck installer. If your contract makes you responsible for “accepting” the roof deck by commencement of installation of the roof system, you will be responsible to ensure that the roof deck complies with the FM Global recommendations (if specified) for installation. But it’s not as simple as ensuring that the steel roof deck has sufficient welds or fasteners, like in the good old days. A determination of the deck type will be necessary. FM Global has included Table 11 in Data Sheet 1-29, and the table defines “allowable uniform uplift pressure” in pounds per square foot (psf). It should be noted that no steel roof deck supplier provides information about uniform uplift pressure, and that “allowable uniform uplift pressure” is dependent on the quality of fastening the steel roof deck sections to structural members. This information is contained in FM Global Data Sheet 1-29 for Grade 33 (Type C) steel roof decks. There is no equivalent table for Grade 80 (Type E) roof decks, but “multipliers” are provided in the notes for Table 11. So, you must obtain the recommended uniform uplift pressure for the installed steel roof deck and determine whether or not the uniform uplift pressure listing matches the assembly proposed or installed.
Roof Replacement
These guidelines for allowable uniform uplift pressure may be suitable for new construction because support spacing, deck type and deck fastening method may be incorporated during construction. But what about roof replacement projects? At least half of the roofing projects undertaken by FM Global clients will be roof replacement or roof recovery, and guidelines for these conditions are not provided in Data Sheet 1-29.So, it may well be the responsibility of the roofing contractor to determine if the roof deck is Grade 33 or Grade 80. The only way this requirement can be satisfied is to take representative coupons from the existing roof or the deck delivered to the site for transfer to a lab capable of performing testing for kps, or “yield strength.” It is not possible to tell by visual inspection what the yield strength (kps) of the delivered deck may be. You must pay particular attention to the type of steel roof deck used in testing of any given roof system to achieve higher uplift resistance; the higher wind uplift listings are invariably acquired using Grade 80 steel roof decks in roof system testing. It could easily be construed that the roofing contractor should be responsible to determine the uniform uplift pressure of steel roof decks to include configuration (A, F, or B), grade, gage of the steel roof deck and the joist spacing for the steel deck sections prior to commencing installation of the roof system if FM Global recommendations are referenced in the project specifications.
Paragraph 2.2.13.1.7 of the modified Data Sheet 1-29 states, “For new construction, the structural design engineer of record is responsible for determining if more stringent design criteria is needed with regard to the deck type and securement, such as providing design diaphragm resistance in conjunction with wind uplift resistance.” This information will not be readily available to the roofing contractor, and therefore no judgment can be made as to the effectiveness of the roof deck. In roof system replacement applications, this information may not be available, nor can it be easily determined. No guidelines from FM Global about roof replacement/roof recovery options, and the “safety factor” in roof membrane systems cannot compensate for existing roof decks that do not comply with FM Global’s criteria. (That is, we can’t provide a 1-90 roof system over a non-1-90 structural system.)
Most of the criteria contained in the modified Data Sheet 1-29 refer to new construction, but there is virtually no acknowledgment or recognition to reroofing and how these “modifications” apply to the roof restoration or replacement projects. What about replacing a roof system over an existing steel roof deck in Mobile, Ala.? Must the existing roof deck be tested for yield strength before roof replacement begins? Must joist spacing be altered to comply with recommended Uniform Uplift Pressure? If all of the calculations in Data Sheet 1-29 indicate that a Grade 80 roof deck is required, must the existing roof deck (assumed to be Grade 33 or less) be replaced with new roof deck with higher yield strength? And who is responsible for making these determinations? No engineer who values his license would venture into this morass.
Paragraph 2.2.19.1, “Field Uplift Tests,” dictates, “Conduct field uplift tests at FM Global insured client locations, where practical, on all new, re-roofed or recovered roof installations in hurricane-prone regions where the basic wind speed is at least 100 mph (45 m/s), except where otherwise noted in Data Sheet 1-52, ‘Field Uplift Tests.’ Conduct tests in accordance with Data Sheet 1-52.”
Data Sheet 1-52 explains why the test is not valid for mechanically attached single ply roof membranes, but no mention is made of invalidity of the test on roof membranes with mechanically attached base plies or newer tested roof membrane systems containing fasteners/fastening apparatus within the roof membrane itself. Nor are the qualifications contained in the modified version of Data Sheet 1-29. RoofNav, which FM Global has recently announced will be available at no charge, was supposed to simplify roof system selection, but in this regard it has been lacking.
Sooner or later FM Global clients will recognize that increasing the safety factors and the cumulative effect of “rounding up” to the next wind uplift category is costing them more than they can justify for their roof systems, and the FM Global reputation for providing cost-effective loss prevention engineering may be tarnished. The backlash from the roofing industry is not yet over. Designers and contractors will soon have other valid engineering options with which to design and construct wind-resistant roof systems, and it would be no surprise to see FM Global exerting much less influence on those other than FM Global clients. And it would be no surprise to see more of a backlash from FM Global clients.
