Wind Resistance Testing

For most of the above-mentioned systems, different wind uplift tests and ratings are utilized, and various specification sections are applied to the different panel types.

The main product specifications for metal roofs are American Society for Testing and Materials’ ASTM E1514 Standard Specification for Structural Standing Seam Steel Roof Panel Systems and ASTM E1637 Standard Specification for Structural Standing Seam Aluminum Roof Panel Systems. In addition, Underwriters Laboratories’ UL 580 Standard for Tests for Uplift Resistance of Roof Assemblies, UL 1897 Uplift Test for Roof Covering Systems, and ASTM E1592 Standard Test Method for Structural Performance of Sheet Metal Roof and Siding Systems by Uniform Static Air Pressure Difference are used to determine a metal roof system’s uplift rating. These are all a series of tests.

Starting with ASTM E1592, this test is used to measure the bending capacity and attachment strength for standing seam, trapezoidal, ribbed, or corrugated metal panels in the thickness range of 0.012 inches to 0.050 inches, when the roofing system is subjected to a uniform static pressure. Air pressure is applied beneath the panels and attachments with a loosely installed plastic bag beneath the panels to maintain the pressure.

Deflections in the test panels are measured in at least six locations. Here, baseline measurements are taken at a nominal pressure and then at each of the specified test pressures or until the point of failure. Before moving to the next higher pressure, the test pressure is relieved so that the roof can be examined for permanent deformation. ASTM E1592 test results are then used along with wind design standards to determine required spacing of supports or attachments.

UL 580 assigns three classes—30, 60, and 90—based upon a roof deck and coverings’ level of uplift resistance. Suitable for systems where a structural panel is installed over open framing without the need for a solid deck, or where the roof covering is attached to a solid substrate when the two are specified as a system.

To achieve an ordinary class 30 or class 60 rating, pressure is applied beneath the system and a vacuum from above in an oscillating manner so that the maximum total pressures equal 45 pounds per square foot (psf) and 75 psf respectively. To qualify for the UL 580 class 90 rating, the system must withstand a maximum positive pressure—pressure from below—of 48.5 psf combined with a maximum negative pressure—a vacuum from above—of 56.5 psf, yielding a combined pressure differential of 105 psf.

First, the test is only run to the specified limit, not run to failure. In the second scenario where the roof covering is fastened to a solid deck, the deck aids in resistance to the pressure from below (positive), and the roof covering may only resist the vacuum from above of 56.5 psf. To aid in capturing the positive pressure below and transferring it to the metal panel itself, an air bag system is used between the panel and roof deck and/or purlins. Due to this requirement, and to distribute pressure to underside of panel, underlayments are not used within the test assembly.

Once the UL Class 90 test is completed, it’s common practice to transition to the UL 1897 test protocol to obtain ultimate pressure resistance values.

UL 1897 is used to evaluate the attachment of the roof covering systems to the roof decks by simulating negative pressures.

The test can be conducted by either pulling a vacuum above the assembly or, more commonly, by pressurizing an air bag placed loosely between the deck and the roof covering. The results are reported as the highest uplift pressure achieved prior to failure, usually in pounds per square foot. The test does not consider the strength of the roof deck and does not necessarily simulate the actual dynamic uplift pressures encountered by roofing systems.

In some cases, the tested panels are instrumented for observation of deflection and reaction during the panel loading process. This information can then be fed into high performance modeling computer programs which can assist in analyzing panel performance and panel clip interaction, and consequently predict the modes of failure.

With the aid of these key tests, architects will better understand how a metal roof system can withstand both external and internal pressures, which will than help guide the roof design and specification. The test result information will better inform decisions with regards to substrates, material gauges, clip spacing, fastener type, and panel profile.

While these tests can go a long way toward analyzing performance, it’s important that they don’t take the place of site-specific analysis by a design professional.

In a Metal Construction News article “Don’t be blown away by metal building requirements for high-wind areas,” MCA’s Director of Codes and Standards Andy Williams states, “The contractor should seek the assistance of a local design professional to ensure the project is designed to withstand the anticipated wind loads.”

Additional Metal Roofing Tests

A number of other tests can provide important performance information for metal roofing systems.

FM 4470 is an accreditation by Factory Mutual (FM). It is a series of tests which includes a determination of hail-resistance classes. Class 1-MH can withstand moderate hail, with a Class 1-SH rating, the roof can tolerate severe hail, and a Class 1-VSH is given to roofs capable of handling very severe hail. Following simulated hail impact, the roofing material cannot show any signs of cracking or splitting under 10x magnification. The roof seams cannot show any signs of cracking, splitting, separation, or rupture when examined under 10x magnification. For a roof system with a VSH rating, FM also requires that the substrate below the roof material does not crack. Minor surface indentations in the substrate are allowed at the point of impact.

FM 4471 tests product performance for combustibility, wind uplift resistance, hail damage resistance, and foot traffic resistance. This test determines if the product meets certain quality control requirements. FM 4471 is a common test reference for insulated metal panel roofs.

ASTM E108 is the Standard Test Method for Fire Tests of Roof Coverings.

UL 790 is basically identical to ASTM E108 and assigns roofing classifications. Class A roof coverings can withstand severe fire test exposures, Class B roof coverings are effective against moderate fire test exposures, and Class C roof coverings can handle light fire test exposures.

ASTM E283 is the Standard Test Method for Determining the Rate of Air Leakage through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen for Steep Slope Roofs.

ASTM E331 is the Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure for Steep Slope Roofs.

ASTM E1680 is the Standard Test Method for Rate of Air Leakage through Exterior Metal Roof Panel Systems.

ASTM E1646 is the Standard Test Method for Water Penetration of Exterior Metal Roof Panel Systems by Uniform Static Air Pressure.

UL 2218 is the Standard for Impact Resistance of Prepared Roof Covering Materials. Roofs are classified from 1 to 4 with Class 4 offering the best resistance against hail. Most metal panel systems are capable of obtaining a Class 4 impact resistance.

ASTM E2140 is the Standard Test Method for Water Penetration of Metal Roof Panel Systems by Static Water Pressure Head.

Testing Metal Flashing Performance

A key component contributing to the resilience of metal roofing systems are the flashings which help prevent penetration of water. To better support metal roofing performance, the Metal Construction Association developed a new standard for testing metal flashing performance: ANSI/MCA FTS-1-2019 Test Method for Wind Load Resistance of Flashings Used with Metal Roof Systems.

Photos courtesy of ATAS International

The Metal Construction Association’s ANSI/MCA FTS-1-2019 Test Method for Wind Load Resistance of Flashings Used with Metal Roof Systems focuses on the flashings preventing water intrusion for better resilience in metal roofing systems.

“Metal roofing systems of all types have proven to perform well in high winds. Properly detailed and installed, metal roofing typically withstands the forces of nature better than most traditional roof systems,” state the experts at MCA. “However, when a roof failure does occur, research shows that flashing failure may be a main contributor to the event.”

In investigating roof failures from several hurricane events including Charley, Ivan, Katrina, Ike, and Irma, researchers found that metal flashing failures were cited as a leading cause of all roofing system failures of various roofing material types.

Installed at the perimeter and at the eaves, gables, hips, and ridges, these locations are where the highest wind uplift pressures occur, which is why the high-quality manufacturing, design, and installation of flashing is essential.

The International Building Code (IBC) currently requires that all Perimeter Edge Metal be tested per ANSI/SPRI ES-1 for low slope membrane roofing systems. In particular, a special code section in IBC 2012 and 2015 on edge securement for low slope roofs states that low slope built-up, modified bitumen, and single-ply roof system metal edge securement, except gutters, shall be designed and installed for wind loads and tested for resistance.

This new ANSI/MCA FTS-1-2019 standard consists of four key sections: Test Apparatus, Test Specimen, Loading Procedure, and Test Report. It is MCA’s hope that testing metal roof flashings per this standard will be included in the next version of the IBC in 2024. For the time being, the standard is available to specifiers and manufacturers of metal roof systems to assure proper performance of the edge flashings with metal roof systems and assemblies as a best-practice approach.

A related standard, ANSI/SPRI GT-1, is used to test gutters by measuring the resistance of the gutter system.

“Particularly in low slope metal membrane-type roofing, if gutters blow off, and if they’re tied into the roof system, that can propagate failure,” explained Bush.

In response, this standard was developed and applies to all material types and membrane installation methods for low slope roofs.