When evaluating a commercial roofing system, building owners and facility managers may encounter specifications such as FM 1-90, FM 1-120, or FM 1-180. These are Factory Mutual (FM) wind-uplift ratings used to identify the tested performance of roofing assemblies against wind forces. They are especially important for commercial buildings where strong winds, hurricanes, and severe weather can create significant roof uplift.
FM Approvals is a third-party certification program associated with FM Global. It evaluates building materials and complete assemblies, including commercial roofing systems, to verify that they meet established performance standards. FM-approved roofing assemblies are listed through FM’s certification resources and RoofNav.
What Does FM 1-90 Mean?
The “1” refers to the FM Class 1 classification, while the “90” identifies the roof assembly’s FM wind-uplift rating. Importantly, FM 1-90 does not mean the roof is rated for 90 mph winds.
The 90 refers to a tested uplift-pressure level measured in pounds per square foot (psf). FM ratings therefore describe the performance of a roofing assembly under uplift pressure rather than directly specifying a wind speed. Industry guidance specifically warns that an FM 1-90 rating should not be interpreted as a 90-mph wind rating.
Historically, FM 1-90 systems were associated with roof assemblies tested to withstand substantial uplift pressures, with the FM rating incorporating a safety relationship between tested pressure and allowable design pressure. The actual wind resistance required for a building depends on factors such as building height, location, geometry, exposure, enclosure classification, and roof zone.
What Does FM 1-180 Mean?
FM 1-180 represents a higher FM wind-uplift classification than FM 1-90. The 180 indicates a higher tested uplift-pressure rating, making it appropriate for roofing assemblies where greater wind resistance is required.
FM ratings can include classifications such as 1-60, 1-75, 1-90, 1-120, 1-135, 1-150, and 1-180, depending on the tested roofing assembly and project requirements.
A higher rating does not automatically mean that every building needs an FM 1-180 roof. The correct rating should be determined from the building’s calculated wind pressures and the applicable design criteria.
FM Rating vs. Wind Speed
One of the most important distinctions for Florida commercial roofing is that FM wind-uplift ratings and wind speeds are not interchangeable.
For example, a roof specification requiring FM 1-90 should not be interpreted as saying, “This roof can withstand 90-mph winds.” Wind speed is only one factor used to calculate the pressure acting on a roof. Building height, shape, location, exposure, roof geometry, and perimeter and corner zones can significantly change the required resistance.
For projects governed by building codes, engineers typically calculate required wind pressures using the applicable code and ASCE 7 methodology. FM Global requirements may introduce additional criteria when a building is FM-insured or when the project specifications specifically require FM compliance.
Why FM Ratings Matter in Florida
Florida commercial roofs face elevated wind exposure from hurricanes and tropical storms. A roofing assembly must therefore be selected based on the actual wind-uplift requirements of the building rather than simply choosing the highest available rating.
The roof assembly matters as a complete system. Membrane type, insulation, fasteners, plates, adhesives, deck type, attachment spacing, and perimeter and corner enhancements can all affect wind performance. An FM-approved assembly should be installed according to its approved configuration rather than mixing components without verification.
Bottom Line
FM 1-90 and FM 1-180 are wind-uplift performance ratings for commercial roofing assemblies—not wind-speed ratings. FM 1-180 provides a higher uplift classification than FM 1-90, but the appropriate rating depends on the building’s calculated wind pressures and applicable project requirements.
For a Florida commercial roof replacement or restoration project, the safest approach is to have the required wind-uplift pressure determined first and then select an approved roofing assembly that meets or exceeds that requirement. This helps avoid both under-designed roofing systems and unnecessary construction costs.
Commercial Roof Replacement / Re-Roofing
Related Questions
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