FM Global Property Loss Prevention Data Sheet 1-29, revised October 1998
(This is the second article in a series on the revisions that FM Global has made to several of its Property Loss Prevention Data Sheets.)
The stated scope of the revised Data Sheet 1-29: “This data sheet provides recommendations for the proper design and installation of above-deck roof components. Items covered include roof covers, insulation, vapor retarders, fasteners and recover assemblies. This data sheet is intended to be used in conjunction with Data Sheet 1-28, Wind Loads to Roof Systems and Roof Deck Securement.” (See FM Global Update, Roofing Contractor, February 2001).
“Roof systems are only Factory Mutual Research-Approved for certain wind uplift ratings. All recommendations are for systems used within their Approval limits.” This caveat is extended repeatedly in FM Global literature as a warning that tested and Approved above-deck systems have their limits, and it is incumbent on the user to determine that the appropriate information is used and that the system selected is suitable for its intended use.
Design RecommendationsParagraph 2.1.1 Design Recommendations: “Except for ballasted systems which are not Factory Mutual Research-Approved, all installations should utilize components and systems (roof deck, insulation, fasteners, roof cover, etc.) that are Factory Mutual Research-Approved for use together on a particular deck. Use of individually Factory Mutual Research-Approved components, not Factory Mutual Research-Approved for use together, does not constitute a Factory Mutual Research-Approved or recommended assembly.” Sound familiar? This message should come across loud and clear.
Paragraph 22.214.171.124 again stipulates the only “requirement” contained in FM Global literature — that “All Factory Mutual Research-Approved materials are required to have the Factory Mutual Research Approval mark on the packaging of the material itself.” And you can’t build a Factory Mutual Research-Approved roof system without appropriately labeled material.
Paragraph 2.1.3 Maintenance Recommendation: “Roof systems and flashing should be inspected semi-annually and after storms …. Written records should be maintained.” Sounds innocuous, but “healthy” roof systems are less likely to produce a casualty loss for the FM Global insureds.
Paragraph 2.1.4 Wind Uplift Resistance — Design Recommendations includes information about recommendations for actual and factored roof wind uplift pressure, guidelines for additional securement at perimeters and corners, and a Prescriptive Enhancement Option which separates “mechanically fastened” components from “adhered” components. Pay special attention to the additional fasteners and additional adhesives prescribed for perimeter and corner attachment; 75 percent more fasteners and 160 percent more adhesive are prescribed for corner areas. This prescription might seriously impact your labor and material costs on an irregularly shaped building.
Of particular note in Paragraph 2.1.5 is the stipulation that “ASTM E108 Class C rated assemblies are not recommended on any roofs >10,000 ft2.” Fire-resistant roof membranes are obviously favored and recommended for larger roof areas.
There are new recommendations for hail-resistant roof membranes (Paragraph 2.1.6), and a map delineating the “Hail Hazard” geographic areas. Essentially, roofs in the central United States and the extreme southeast are in “harm’s way” from hail damage. It is generally recognized that aggregate-surfaced roofing membranes tend to fair better in heavy hail than smooth-surfaced roof membranes.
Vapor RetardersThe FM Global basic recommendation for vapor retarders remains technically flawed, although the “acceptable option” will produce an effective vapor retarder in virtually any roof system.
In Paragraph 2.2 Vapor Retarders (Barriers): Design and Installation Recommendations, Factory Mutual Research indicates that “These recommendations outline vapor retarder applications that maintain Class 1 and appropriate wind uplift ratings.” Then in Paragraph 2.2.1, “A vapor retarder, if used, should be Factory Mutual Research-Approved and be applied in a single layer placed directly on the deck. Side and endlaps are sealed using the Factory Mutual Research-Approved adhesive. The remaining roof components are then installed with fasteners driven through the retarder into the deck.” My personal contention has always been that if a vapor retarder is really necessary, then the best one you can build will not be good enough. Perforating what might otherwise be an effective vapor retarder with literally thousands of fastener holes can’t possibly result in creating an effective vapor retarder. This recommendation might work if you only “think” you need a vapor retarder.
Paragraphs 2.2.3 and 2.2.4 provide technically sound recommendations for building a truly effective vapor retarder with any roof cover option, and I submit that the “acceptable option” should be the rule rather than the alternate. The “acceptable option” is described in 2.2.4 as follows:
Mechanically secure a thermal barrier of minimum 1in. thick perlite or 5/8in. thick Type X or Type C core gypsum board to the deck. Securement of the thermal barrier in the roof field should be per the Factory Mutual Research Approval for the particular board, assuming a BUR cover.
Mop a one-or two-ply organic felt vapor retarder.
Adhere a single layer of insulation with asphalt.
Install the roof cover.
Asphalt-coated glass fiber roofing felts will provide a more suitable reinforcement for the built-up vapor retarder, but by the strict “letter of the FM Global law,” organic felts are the only acceptable listed reinforcement. There appears to be no logic for specifically stipulating a “single layer of insulation” in the assembly. This is because FM Global recognizes multiple layers of Approved roof insulation boards adhered with hot asphalt in various wind uplift classifications, and some manufacturers of isocyanurate foam roof insulations are suggesting multiple layers for overall thickness in excess of 2 inches. And this is the only place in FM Global literature where acceptable “thermal barriers” are defined; Approved “thermal” barriers are listed in the current Factory Mutual Research Approval Guide.
And although Paragraph 2.2.1 indicates that the vapor retarder be “applied in a single layer placed directly on the deck,” Paragraph 2.2.5 takes away the option for an asphalt vapor retarder applied directly to the deck with the caveat that “Asphalt/felt vapor retarders should not be mopped directly to any deck except structural concrete as a Class 2 roof would result.” Designers and roofing contractors should read both paragraphs.
Roof InsulationDesigners and roofing contractors should be totally familiar with Paragraph 2.3 Roof Insulation. This paragraph defines acceptable attachment options for various combinations of Factory Mutual Research-Approved roof insulations. Probably the least understood concept in mechanical attachment of multiple layers of roof insulation is that the wind uplift classification is determined by the properties/Approval and fastener density of the top layer of insulation board if all insulation layers are mechanically attached. “Nail/mop constructions are not used unless the combination is Factory Mutual Research-Approved or installed as outlined in Recommendation 4, Section 2.2.”
Please Note: Paragraph 126.96.36.199: “The maximum recommended insulation board size if the board is adhered with asphalt or adhesive is 4 X 4 ft. Exception: flexible boards such as maximum 1/2 in. thick wood fiber or 5/8 in. gypsum board may be adhered at board sizes up to 4 X 8 ft.” This is an indirect recognition of the inherent dimensional instability of isocyanurate foam roof insulation boards.
Insulation and Roof Cover Fasteners Paragraph 2.4 Insulation and Roof Cover Fasteners: “Factory Mutual Research-Approved insulation fasteners are the only Approved and recommended method of securing insulation boards or other boardstock materials to steel deck. Some fasteners are Factory Mutual Research-Approved for other deck materials.” This paragraph also addresses fastener pull-out and fastener density.
Probably the most significant concession by FM Global is contained in Paragraph 2.4.2 steel deck: “For screw-type fasteners in new construction, the shortest screw that is at least 3/4 in. longer than the assembly being secured should be used. Fasteners should engage the deck top flange. For recover steel deck construction, the fastener must be long enough to be driven through the existing roof system and into the deck at least 3/4 in. While top flange engagement is recommended in all cases, for recover construction, it is acceptable for insulation fasteners to engage the bottom flange of the deck. Fasteners securing mechanically attached roof covers should engage the top flange of the deck in all constructions.”
This is the first time in print that FM Global has acknowledged the difficulty of placing all insulation fasteners in the top flange of steel roof deck in recover roof systems. But the necessity for installation of roof cover fasteners in the top flange of steel decks is well founded, since the extra lever dimension of fasteners installed through the bottom of the flute might adversely affect fastener performance and result in release of the roof cover.
Recover and Reroof ConstructionIn Paragraph 2.5 Recover and Reroof Construction: Design and Installation Recommendations: “When reroofing, the existing deck securement should be inspected and, if necessary, the deck fastened per Data Sheet 1-28 prior to installation of the above-deck components.”
When reroofing over a steel deck where the original assembly had been adhered with asphalt, one of the following should be done:
Use an insulation specifically Factory Mutual Research-Approved for reroofing, this will allow up to 15 lbs/sq of asphalt to remain on the deck.
Remove ALL asphalt from the deck and install a system Factory Mutual Research-Approved for new construction.
And another significant change to Property Loss Prevention Data Sheets: Paragraph 2.5.2 Recover Construction: Existing components remain. “All recover recommendations and Approvals are based on the assumption that there is only one existing roof system. If more than one roof system is in place (building has already been recovered), reroof construction should be done.” FM Global has taken the position of the Building Code Officials in limiting the number of roof coverings to remain in place. Reroof in FM Global-ese is removal and replacement of the existing roofing systems.
“Recover — mechanically attached recover systems. If the existing construction is dry, a Factory Mutual Research-Approved recover system can be applied over it. The entire assembly (insulation, fasteners and roof cover) should be Factory Mutual Research-Approved in combination for recover construction on the particular deck.” Consider the ramifications of this “emphasized” recommendation: How many recover roof systems have been tested and Approved over a myriad of existing roof constructions?
FM Global and Underwriters Laboratories typically restrict the total insulation thickness in recover roof assemblies because of the effect on fire ratings on the original and the recover roof systems. The current FM Global posture is stipulated to be: “The thickness of the recover roof system should be within the Factory Mutual Research Approval limits. Most systems are limited to a maximum 1 in. A greater thickness may affect the Class 1 fire rating of the completed system. Installation of a Factory Mutual Research-Approved Class 1 recover assembly over an existing Class 2 roof system will not upgrade the assembly to Class 1. However, it would be possible to upgrade the external fire resistance rating with the addition of a Factory Mutual Research-Approved roof cover system.”
Another recognition of potential problems associated with high fastener density has been formally recognized by FM Global. Paragraph 2.6.2 stipulates that “Fasteners driven into lightweight insulating concrete should not be closer than 4 in. on center to avoid cracking the deck.” Fastener location should be adjusted to allow for this provision; decreasing fastener density in corners and perimeters is not an option.
Single-Ply Membrane CoversIn Paragraph 2.7 Single-Ply Membrane Covers, it is noted that “Some Factory Mutual Research-Approved single-ply membranes are formulated with fire retardants on the top side only. These systems should be installed with that surface up. Membranes with this requirement are labeled on the underside “THIS SIDE DOWN/IN.” It would be embarrassing, to say the least, to have to return to the site to turn the membrane over, but stranger things have happened in the roof system installation business.
Pay particular attention to the perimeter and corner supplemental mechanical attachment criteria for mechanically attached single-ply membranes set out in this section. “Increased fastening density is obtained by using narrower sheets, or intermediate rows of fasteners installed through the sheet with a cover strip applied over the fasteners. Fastening increase is not obtained by increasing the number of fasteners along each row.” FM Global also makes provision for a fully adhered perimeter with mechanical attachment commencing inside the adhered perimeter. Insulation fastener density should be increased at roof corners and perimeters in accordance with Recommendation 3.
“Stone ballasted roofs should not be used on buildings >150 ft. high or in areas with design wind speeds >100 mph regardless of uplift pressure. These limitations do not apply to paver-ballasted systems.”
“PVC membranes should not be used in ballasted applications.”
“For ballast application only, the roof perimeter and corners are subject to a minimum dimension of 8.5 ft. rather than 4 ft. defined in Data Sheet 1-28. Also, the 10% lesser building plan dimension is not applied in determining corner dimension. The 4 ft. minimum and the 10% limit still apply to roof deck securement.” And FM Global makes provisions for a combination of pavers and ballast “provided that the appropriate weights of stone and pavers are used in each area.” Tables are included in the new Data Sheet 1-29 to assist in determining the weight of aggregate ballast and pavers under differing design conditions. Note that the ballast size recommended and listed in the tables is No. 3 Round Stone Ballast, not No. 4 ballast as indicated by some membrane manufacturers.
Mechanically Attached Base SheetsGeneral guidelines from FM Global for mechanical attachment of base plies has always been sketchy, at best. Paragraph 2.8 addresses Roof Covers Adhered to Mechanically Attached Base Sheets. The guidelines for fastening base sheets to lightweight insulating concrete roof decks are clearer than those for attaching base sheets to wood roof decks. Paragraph 2.11 Mechanically Fastened Base Sheet Assemblies over Wood Decks indicates that “Currently, there are no Factory Mutual Research-Approved combinations of a base sheet mechanically fastened directly to a wood roof deck followed by a BUR or modified bitumen cover. These assemblies are popular in some regions. They should be considered Class 2 and need automatic sprinkler protection regardless of the Approval status of the deck. They can be acceptable from a wind resistance veiwpoint if installed in accordance with this section [2.11].”
Fastener pull-out tests are essential to calculating the fastener density required to provide any wind uplift classification for roof systems installed over mechanically attached base sheets over wood roof decks. Pull-out test results can be used to determine the recommended fastener density for mechanically attached base sheets from Table 8, Data Sheet 1-29. Base sheet fasteners should have metal heads or discs. “Where non-Factory Mutual Research-Approved wood roof decks have been used and a Factory Mutual Research-Approved roof system (above-deck components) is installed, roof system fastening can be in accordance with the Approval Guide listings for FR-treated wood decks provided that the deck is nominal 3/4 in. thick plywood or nominal 2 in. thick lumber. If a thinner deck is used, fastener pull-out tests are needed. The fastener density should then be determined per Recommendation 3, Section 2.4.1.”
FM Global notes that for recover installations, “Roof vents are not considered to be effective in removing existing moisture,” a sentiment shared by most of the rest of the roofing community.