Wind Uplift Explained

Whether for life safety, insurance, or code compliance, wind uplift is a major concern for anyone who manufactures, designs or installs roofing products. In this post, we will provide an overview  of the design process, the two major testing agencies, and how Securock Roof Boards can help you protect against wind uplift in your next project.

When wind hits a building, pressure is exerted against the building as the air pushes against the sides and moves up and around the building.  Wind uplift is a force (pounds per square foot) that occurs when the pressure below a roof is greater than above it. This can happen from many different ways but is usually because pressure above the roof system decreases by high air flow (wind) or pressure increases inside a building from air pressure buildup. When wind uplift is greater than the system was designed for, the roof could potentially lift off the building.

There are many design considerations, but most codes (such as IBC 2012) and design professionals are using the 2010 edition of ASCE 7, or ASCE 7-10 to design a roof system (ASCE is the American Society of Civil Engineers). The old version of ASCE was ASCE 7-05, and with the new version of ASCE 7-10, some things remain the same with the new version, such as factors for design including building location, height and ground surface. However, with the new code, there have also been items that have changed such as the use of new wind speed maps based on risk categories and an expanded seismic area. In addition, there are tools such as www.roofwinddesigner.com that can help with the design.

Once a design professional understands the wind loads on the building, it is time to pick a roof system. For a rated system, most entities (such as IBC 2012) will look to Factory Mutual (FM) or Underwriter’s Laboratories (UL)  for guidance. Because there are differences in how the systems are tested, designers should always compare products using the same test agency numbers.

FM is the default testing standard for wind uplift. Roof assemblies that have been tested can be found on FM’s website. You should know that the testing is based on assemblies, not components. The two main FM  testing standards are FM 4450 and FM 4470.

1. FM 4450 tests Class 1 insulated metal decks.
2. FM 4470 tests all other Class 1 roof covers.

With FM Tests, the roof system is pressurized from below the deck to 30 psf, and held for one minute.  The pressure is increased by 15 psf increments every minute until failure.

UL has two standards for testing wind loads: UL 580 and UL 1897.  You may notice there is no safety factor in the UL numbers. The test is in pounds per square foot.  This is not a design number, but a test number.

1. UL 580 is a dynamic pressure test.  In addition to pressure from below, UL 580 draws a vacuum on top of the membrane. Pressure levels are varied throughout the duration of the test.
2. UL 1897 is a static pressure test in which an apparatus that creates a steady negative pressure is placed on the top side of the membrane.  Pressure is held for one minute and raised by 15 psf increments, until failure.

Wind uplift is a force that occurs when the pressure below a roof is greater than above it, and if the wind uplift is greater than the design of a roof system, it can blow the roof off the building. Design professionals will typically use ASCE 7-10 to find the proper wind load of a building.  Once that design parameter is found, the default testing agencies are FM (4450, 4470) and UL (580, 1897). Properly tested products that meet the design criteria, such as Securock High Performance Roof Boards, will help ensure that a safe building is constructed.
Now, we would like to give you some understanding of where Securock Roof Boards can potentially rate for FM testing.

Finally, using the very common FM 1-90 rating with a typical assembly (TPO adhered to a cover board that is mechanically fastened onto a metal deck), it is easy to see how costs can add up depending on the roof board that you select.  Below is a table of different roof boards for this assembly.  We assumed $.20 / fastener (screw and plate), average roof was 100’ x 200’ (625 4’x8’ roof boards), and it took an average of 2 minutes to screw in a fastener with a labor rate of $20/hour.

As you can see, using Securock Gypsum-Fiber on a 20,000 square foot job can save you as much as $4,333 just in fasteners and labor.  So, not only will you get a great product that will be easier to install, enhance the safety of your client’s building, and is made from 95% recycled material, you will also save money on installation.