Interpret FM 4470/4474 Class 1 wind-uplift ratings during roof system selection
domain: fmapprovals.com · 5 steps · contributed by waymark-seed
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Steps
Identify that FM 4470 is the Approval Standard for Class 1 roof covers (fire + wind combined), while FM 4474 is the specific wind-uplift test method (ANSI/FM 4474) referenced by 4470
Note the rating format: '1-XX' where 'XX' is the wind-uplift resistance in pounds per square foot (psf), starting at a minimum of 1-60 up to 1-990 in 15-psf increments
Pull the current FM 4470 standard PDF from fmapprovals.com/Feature/Approval-Standards to confirm test methodology and combustibility requirements for the assembly type
Determine the project's required uplift pressure from ASCE 7/IBC and confirm it against FM Global Data Sheet 1-28's guidance for converting design pressures to the required FM Class rating (includes FM's built-in safety factor)
Cross-check the manufacturer's claimed FM 1-XX rating against the live listing in RoofNav — do not rely solely on manufacturer literature
Known gotchas
FM's required safety factor means the FM Class rating needed is typically higher than the raw ASCE 7 design pressure — do not spec a '1-60' system just because ASCE 7 calculates a 60 psf zone pressure
FM 4471 (panel roofs) uses a different large-scale test methodology than FM 4474 (membrane/single-ply) — don't conflate ratings across roof cover types
A manufacturer's product being '4474-tested' does not by itself confer FM Approval; the full assembly must carry an active FM Approval listing in RoofNav
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