Wind Uplift at Roof Corners and Edges: FM Classes, ES-1 Edge Metal and ASCE 7 Loads

Why corners and edges go first

After Hurricane Ian, RICOWI's teams inspected seven multi-ply bituminous roofs. Two were damaged only at the roof corners, "where wind uplift forces are commonly the greatest," and all but one showed the membrane peeled back at corners and perimeter edges (RICOWI Hurricane Ian report).

FM Approvals explains the mechanism in its edge-system standard, Class 4435. An edge that is not properly secured can bend outward and upward, exposing more of its area to the wind; the prying and pulling forces can then fail the edge, and once it goes, the roof cover can be pulled back to expose the insulation and deck. The standard notes that even an adequately secured roof cover is vulnerable if it is improperly terminated at the building edge.

The same Ian report shows how small lapses start big failures. On a building finished in 2021, a mechanically attached single-ply system was nearly blown off entirely; the failure began at a roof corner after ground-floor window damage breached the metal deck and pressurized the roof, and investigators found large areas where induction welds to fasteners had been missed or poorly made. On another roof, too few edge-metal fasteners let the coping detach and strike the roof, opening a path for water between the TPO and the deck.

Design loads: ASCE 7

ASCE 7 sets the wind loads that the International Building Code adopts by reference (ASCE). The 2022 edition, released December 1, 2021, superseded ASCE 7-16 and added the standard's first tornado criteria (ASCE). ASCE's Civil Engineering magazine reports that the tornado chapter covers tornadoes of roughly EF2 intensity or less, depending on location, and that a new standard is not binding until a city, county or state adopts a code that references it.

For roofing, NRCA's Roof Wind Designer is available at no cost. It calculates design wind loads for many common building types under ASCE 7-05, 7-10, 7-16 or 7-22, then derives minimum recommended design wind-resistance loads for the roof system using a safety factor that relies on ASTM D6630, with edge metal handled through ANSI/SPRI ES-1.

Assembly ratings: FM 4470 uplift classes

Under FM 4470, wind uplift is tested to ANSI/FM 4474 using a 12-by-24-foot pressure test, a 5-by-9-foot pressure test or an uplift pull test. Ratings start at Class 1-60 and run to Class 1-990 in steps of 15. The number is the highest simulated uplift pressure, in pounds per square foot, that the assembly held for one minute without failure, and the assembly must also carry its service wind load, defined as half the rated load, for one minute without visible cracking or creasing of the insulation or cover board.

So a Class 1-90 assembly survived 90 psf in the test and has a 45 psf service load. The rating belongs to the assembly as tested, and FM 4470 bars swapping its components without the certification agency's prior authorization.

Edge metal: ES-1 and the building code

NRCA quotes Section 1504.6 of the 2024 International Building Code: metal edge systems, other than gutters and counterflashing, on low-slope built-up, modified bitumen and single-ply roofs must be designed for wind loads under Chapter 16 and tested to the RE-1, RE-2 and RE-3 methods of ANSI/SPRI ES-1. Gutters that secure the membrane edge must be tested to methods G-1 and G-2 of SPRI GT-1.

FM 4435 sorts edge systems into fascia, coping and gutter types and uses ES-1's three methods: RE-1 for membranes terminated by the edge itself, RE-2 for edge flashings and gravel stops with an exposed horizontal leg of 4 inches or less, and RE-3 for copings wider than that. FM's own minimums for certification are 58 psf for edge flashings and gravel stops, and 101 psf vertical plus 58 psf horizontal for copings.

Who fabricates the metal matters. SPRI's position is that ES-1 is a manufacturing standard, so each fabricator must test its own product at an independently accredited laboratory or be licensed and audited to produce a tested system. NRCA runs such a program for shop-fabricated edge metal and gutters, with certifications through UL Solutions and Intertek.

For contractors

Crews who install tested edge systems and document their uplift ratings have a story worth telling owners. Claim your National Roofing Directory listing, review the membership levels, and subscribe to The Roofing Wire for weekly industry news and Storm Watch alerts. SPRI and NRCA both have NRD directory pages.

Frequently asked questions

What does an FM Class 1-90 rating mean?

The tested assembly held a simulated uplift pressure of 90 pounds per square foot for one minute without failure under FM 4470. Its service wind load, the load FM expects it to resist in a windstorm, is half that, or 45 psf.

Can a local sheet metal shop bend its own ES-1 edge metal?

It can make edge metal, but SPRI's view is that the product is ES-1 compliant only if that fabricator's own product was tested at an independently accredited lab, or the shop is licensed and audited to fabricate a tested design. Copying a tested profile is not enough.

Does ASCE 7-22 address tornadoes?

Yes. It is the first edition with tornado load criteria, and ASCE's magazine says the chapter covers tornadoes of roughly EF2 intensity or less depending on location. The criteria apply only where a jurisdiction adopts a code referencing ASCE 7-22.