FM Global Update 2000
In the years 1999 and 2000, commercial and industrial property insurer FM Global (formerly known as the Factory Mutual System) issued revisions to several of its Property Loss Prevention Data Sheets that directly affect roofing systems and roofing contractors. Property Loss Prevention Data Sheets 1-7, 1-28, 1-29, 1-31 and 1-49 were revised, and Data Sheet 1-32 issued to cover the examination and evaluation of existing PVC roof coverings.
The former Factory Mutual System became FM Global in July 1999 following the merger of Allendale Insurance, Arkwright and Protection Mutual Insurance companies, and their jointly owned affiliate Factory Mutual Research Corporation. FMRC, now called Factory Mutual Research, remains an independent, non-profit scientific organization and is an affiliate of FM Global. The research and product testing conducted by Factory Mutual Research is done primarily to help insurance policy holders of FM Global to better understand the property hazards at their facilities and find new ways to reduce their risks of property losses. FM Global and Factory Mutual Research, whether by chance or choice, have become a dominant influence in the roofing industry, with their recommendations for construction referenced by designers, trade associations and insureds. Some of their engineering guidelines have found their way into model building codes.
Since the changes and implications for the roofing contracting community are numerous and the subjects, though interrelated, diverse, a discussion of each individual revised Property Loss Prevention Data Sheet may help to minimize confusion. Not included in any of the quotes from Data Sheets are the metric references that are universally included in Factory Mutual Research guidelines simply because I don’t relate to the metric measurements — no matter what the government thinks I should comprehend. You may refer to the actual FM Global Data Sheets if you only understand metric measurements.
Wind Loads to Roof Systems and Roof Deck SecurementThis first article discusses Data Sheet 1-28, revised in January 2000. The original Factory Mutual Data Sheet 1-28 referred only to Steel Roof Deck criteria. The information contained in the original Data Sheet 1-28 was separated in the 1998 revisions, with then current information divided between Data Sheets 1-28 and 1-29. The original Data Sheet 1-29 addressed Ballasted Roof Systems, but the sequential numbering of new Data Sheets was favored by FM Global over the potential confusion created by renaming/renumbering revised editions of the wind information.
The newly revised FM Global Property Loss Prevention Data Sheet 1-28 is entitled Wind Loads to Roof Systems and Roof Deck Securement. The stated scope of the document is: “This data sheet provides recommendations for selecting the appropriate Factory Mutual Research roof system wind-uplift rating and for the proper securement of various roof decks to supporting members.” The document covers the procedure for determining field of roof uplift pressures, and includes charts/figures necessary to determine which wind-uplift classification may be necessary for a new or replacement roof system. The same wind-uplift information provided in Data Sheet 1-28 for various types of roof decks is also used in determining wind loads on above-deck roof system components covered in Data Sheet 1-29.
In highlighting the contents, begin with Paragraph 2.1 Procedure: “The following (2.1 - 2.5) indicates the procedure to be followed for determination of the anticipated wind uplift pressures on the field (center portions) of roofs, and the selection of Factory Mutual Research Approved minimum wind uplift ratings.
“Because the applicable reduction in pressure to above-deck components for monolithic decks is already taken into account during Factory Mutual Research Approval testing, the pressure calculated is applicable for determining the recommended rating for the entire roof system (roof deck and above deck components). The pressures calculated are not applicable for the roof perimeter or corners.”
Essentially, by referring to tables, isotachs and graphs included in Data Sheet 1-28, a roof system designer can determine the ground roughness coefficient, the field of roof-uplift pressure, and determine necessary “adjustment” to uplift pressures to allow selection of a roof system wind-uplift classification.
Adjustments to Uplift PressuresSomething new to pay attention to is Paragraph 2.1.4 Adjustments to Uplift Pressures. “The field of roof-uplift pressures from Table 1, 2, or 3 should be increased by 50 percent for buildings that meet all three of the following conditions:
a. In the floor immediately below the roof, the total area of openings in one wall exceeds the sum of the total area of openings in the remaining walls and roof by 10 percent or more; and
b. In the floor immediately below the roof, the total area of openings in the wall in question is >4 ft2; and
c. The percentage of openings in the balance of the building envelope (not including the wall in question) in the floor immediately below the roof is less than or equal to 20 percent.
Obviously, the “increased 50 percent” should get your attention, because those kind of adjustments can cause some serious heartburn if improperly interpreted!
And immediately following in Paragraph 2.1.5 Factored Wind Uplift Pressure and Recommended Roof System Approval Rating: “Multiply the actual field of roof pressure from 2.1.3 or 2.1.4, if applicable, by 2.0 to determine the factored pressure.” Does a multiplier of 2.0 get your attention? And pay attention to the examples of the factored pressure following the direction of 2.1.5.
Design RecommendationsA common misunderstanding in the roofing industry is clarified in Paragraph 3.1.1 Design Recommendations: “Use of individually Approved components, not Factory Mutual Research-Approved for use together does not constitute a Factory Mutual Research-Approved nor recommended assembly. Use of Factory Mutual Research-Approved components and systems is universally recommended and is implied for all applicable assemblies whether or not specifically stated.” Be very careful when assembling materials for any specified Factory Mutual Research Approved roof system, especially if the specified roofing system cannot be identified in the current Factory Mutual Research Approval Guide. It is always prudent to obtain the blessing of the local FM Global engineering office when selecting components to construct an FM-Approved roof system if the intended system varies the least from the listing/description in the Factory Mutual Research Approval Guide.
To construct a Factory Mutual Research-Approved roof system, all materials to be installed must be shipped with appropriate Factory Mutual Research labels. Paragraph 126.96.36.199 states “All Factory Mutual Research-Approved materials are required to have the Approval mark on the packaging of the material itself. Materials without proper labeling are not Factory Mutual Research-Approved and should not be accepted.” You can bet the farm that if an FM Global engineer visits the project site during the time of construction and discovers non-labeled materials being installed that the picture won’t be pretty. The owner is likely to receive a letter from FM Global’s engineer indicating that the roof system being installed is “inferior” and that the insurance classification for the roof will be downgraded.
It’s hard to collect money on a project that the owner’s insurance carrier considers “inferior.” The owner doesn’t understand that the roof may perform perfectly satisfactorily as constructed, and that the FM Global term “inferior” applies to roof systems that are not Approved or to roof systems constructed with components that do not bear the Factory Mutual Research Approval mark on their packages.
Corners and PerimetersPay particular attention to Paragraph 4, which deals with “several acceptable methodologies for increasing the uplift resistance of the roof deck in the building corners and perimeter.” There are criteria for defining “corners” and “perimeters.” Keep in mind that these criteria are listed for installation of roof decks . . . and unless you are a roof deck installer, you needn’t pay attention to deck attachment criteria. Right? Wrong! Think about the clause contained in most specifications/contracts that stipulates something to the effect that “installation of the roof system constitutes the installer’s acceptance of the roof deck as suitable for installation of the roof system,” which may well include whether or not the deck installation complies with Factory Mutual Research recommendations.
It is almost incumbent on the roofing contractor to look at the specification criteria for roof deck installation. If Factory Mutual Research criteria are included, check the deck for compliance before commencing construction. The General Contractor may not want to hear the bad news, but better to have the understanding before the work is rejected by an inspecting agency. And keep in mind that most General Contractors don’t know that FM Global Property Loss Prevention Data Sheets exist!
Paragraph 188.8.131.52 clarifies what Factory Mutual Research considers to be a perimeter: “Where multi-level roofs meet at a common wall, the edge of the upper roof is treated as a roof perimeter if the difference in height is >3 ft. The lower roof strip where it meets the higher wall is not treated as a perimeter. See Figures 3a and b.”
Steel DecksParagraph 3.2 Insulated Steel Deck includes securement recommendations for steel roof decks. There are some new criteria for weld size, new reference to weld washers, attachment at corners and perimeters, and criteria for side-lap attachment. Note that welding of sidelaps is not recommended on 20-gauge or thinner deck, and welding of sidelaps on 18-gauge steel roof deck is acceptable only to Class 1-90 and lower.
Lightweight Insulating ConcreteMuch more clearly defined in the current Data Sheet 1-28 are the criteria for attachment of form decks for lightweight insulating concrete. Paragraph 3.3 includes design recommendations for attachment of various deck profiles. Note that sidelap securement is recommended on form deck just as it is on structural steel roof deck panels. There is also a reference to the minimum thickness of the slurry coat over the deck when EPS is incorporated into the deck system.
The reader is referred to the Factory Mutual Research Approval Guide for acceptable criteria. But the question is: How can you tell if the form deck is properly secured and the slurry coat is proper once the lightweight insulating concrete has been placed over the deck and EPS? It might be prudent to send your General Contractor a copy of FM Global Property Loss Prevention Data Sheet 1-28 with a cover note to be retained in your file before the deck is poured, in case the form deck is not properly secured and your roof and the roof deck leave the county in some high winds at some future date. At least you will have called attention to the deck requirements, which may mitigate any action against you by an unhappy owner in the future.
Cementitious Wood FiberParagraph 3.4 addresses securement of Cementitious Wood Fiber Roof Decks (Tectum, Petrical, InsulRock, etc.). The fastening options include clips, through-fastening, or a combination of clips and through-fasteners for securement of individual roof deck panels in corners and at perimeters.
Pay special attention to Paragraph 3.5 Lumber and Plywood Decks. FM Global is unequivocally recommending Factory Mutual Research-Approved fire retardant lumber and plywood for all new wood roof deck construction. “Approved” lumber is minimum 1.5 inches; “Approved” plywood is minimum 23/32 inch thick (nominal 3/4 inch). “In no case should plywood roof deck be thinner than 19/32 (nominal 5/8 inch).”
Conclusions“Where non-Factory Mutual Research-Approved wood roof decks have been used (acceptable for sprinklered buildings), roof deck fastening can also be as outlined . . . If the deck is thinner than the minimum Factory Mutual Research-Approved thicknesses, above-deck fastener pull-out tests are needed, as all above-deck fastening Approvals are based on the minimum Approved thickness.” Above-deck components on wood decks currently are only Factory Mutual Research-Approved for up to Class 90. Hence, deck-fastening recommendations are for these uplift ratings only. And the fastener size and density for varying deck conditions are continued in Paragraph 3.5.4.
Paragraph 3.6 stipulates that “Cast-in-Place and precast structural concrete decks are acceptable in all wind exposures provided the appropriate above-deck system is used.”
Paragraph 4.0 provides the “support” for recommendations included in Data Sheet 1-28. In part, Paragraph 4.1 stipulates that “Pressure coefficients are influenced by the type of roof deck. If the deck is of panel construction, such as steel, wood or cementitious panels, the internal building pressure is allowed to act on the underside of the above-deck components. Decks such as gypsum or concrete are cast in place from a cementitious slurry that hardens into a monolithic material. These decks have virtually no spaces to allow internal pressure to act on the underside of the roof cover and insulation. As a result, the net load on the above-deck components is less than that for panel-type decks. Precast gypsum and concrete plank decks do have joints that allow internal pressure to act on the roof underside. All necessary adjustments for monolithic decks have been considered during Factory Mutual Research Approval testing.”
Remember: The current FM Global Property Loss Prevention Data Sheet 1-28 deals only with roof deck installation, with reference to “above-deck” components that are discussed in the new Data Sheet 1-29 (to be discussed in a future issue). Roofing contractors need to be familiar with Data Sheet 1-28 to be able to determine wind uplift classes for roof systems and to insure they can call attention to inappropriately secured roof decks prior to commencement of installation of a new roof system, or to be able to make appropriate corrective action to roof decks when existing roof systems are replaced.