When Roofing in Accordance with Factory Mutual...The Devil is in the Details
By Ted Michelsen, PhD, President of Michelsen Technologies
Factory Mutual (FM) Global Research is a non-profit engineering and research affiliate of the FM Global Insurance Company, a large and respected commercial building insurer.
Each year, FM issues its Property Loss Prevention Data Sheets and listings of approved products found to reduce the potential of insurable losses by FM Global. As you might guess, FM Global Insurance awards the lowest insurance rates available to building owners who construct and maintain their building in accordance with FM requirements. Thereby, adhering to Factory Mutual requirements is of utmost importance when working on the roof of an FM Global insured building.
Since reroofing represents roughly seventy percent of the roofing contracts in the market each year, we’ll use it as a case study to illustrate the detailed knowledge required to meet Factory Mutual standards.
Factory Mutual does not have a Loss Prevention Data Sheet that deals only with reroofing. However, most of the information available on this subject can be found in the Recover and Reroof Construction section of FM’s Property Loss Prevention Data Sheet 1-29 under the section entitled, “Above Deck Roof Components.”
Generally speaking, the roofing industry uses the term reroof to describe services involving the “tear-off” of an old roof to be replaced, or a “recover” meaning existing components remain in conjunction with new. FM standards recommend a reroof (or tear-off) over a recover.
After an old roof system is removed during a tear-off project, the deck attachment should be inspected and refastened according to the guidelines found on Property Loss Data Sheet 1-28. Sounds simple enough. However, there are many levels of detail that must be understood, details that often complicate the process.
Consider this example: If a roof system is attached to a steel deck with asphalt there are two options for reroofing. One: use a brand and type of insulation specifically approved for reroofing. This will allow up to 15 pounds per square foot of asphalt to remain on the deck. Again, this sounds simple enough. However, a quick check of approved insulation listings will reveal the fact that several major insulation manufacturers do not have any FM approved reroofing insulations. Other manufacturers list approved insulations for some, but not all products. So, finding approved replacement insulation, though simple in theory, can require quite a bit of homework.
Option two in this tear-off scenario would be to remove all asphalt from the deck and install a system approved for new construction. However, in this case it would be important to note that if extra combustibles, such as asphalt, are added during the process it could affect the system under deck fire rating and should be addressed in the specifications to avoid compliance issues.
As might be expected, recover projects can pose even greater challenges than tear-offs. FM specifically outlines, “the entire assembly (insulation, fasteners and roof cover) should be approved in combination for recover construction.” Approved materials are listed by manufacturer in the FM Approval Guides. It is important to understand when referencing approved materials that FM approved systems for reroof (tear-off) may not be approved for recover.
Now consider the FM requirements, and obstacles, for a simple recover project. If the plan is to adhere a recover system to an existing roof, FM requires proof that the attachment is adequate and any gravel surfacing is removed. Data Sheet 1-52 suggests the field uplift test be used. If the roof fails the test, the old roof must be re-fastened using approved insulation screws and plates in prescribed numbers depending on the location on the roof and the needed uplift resistance. A problem in this scenario is the fact that the stress of an uplift test can damage a roof to the extent that it may pass the initial test, but not pass if the test is administered a second time.
These are just some examples of the detail involved in maintaining FM standards. It’s these little and detailed components that can make a big difference in the quality of a job. And it’s the contractor’s job to stay on top of the details and be knowledgeable of the specifications required to meet FM standards. Some contractors question whether adopting FM standards on buildings for which they are not required, adds unnecessary complications and expenses to the project. But, the truth is that many FM requirements are simply good roofing practice. For example, it is considered a standard, good practice to remove wet materials, cut out blisters and test fastener pull out of gypsum, lightweight insulating concrete and cementious wood fiber decks.
Required or not, efficient or not, many specifiers have adopted Factory Mutual standards over the years.
Ted Michelsen holds a BS and PhD in Chemistry from Rensselaer Polytechnic Institute. He is former Executive Director of the Roofing Industry Educational Institute (RIEI). Prior to working at RIEI he spent 26 years with Johns Manville Corporation in Marketing, Technical Service and Research. Currently, Ted Michelsen is the President of Michelsen Technologies, a roof-consulting firm.
*Note to Reader:
Chamberlin''s standard is to deliver the highest quality work at all times. As a matter of policy and integrity, Chamberlin taked into consideration the most stringent standards in all the work we do. Marty Stoud, President of Chamberlin commented, "Knowing the ins and outs of Factory Mutual can be difficult and detailed work. But, that''s our job. We understand our FM insured clients have a vested interest in meeting those standards."