Wind uplift ratings are a critical safety metric for building and testing roof systems for superior wind performance. Wind uplift is a near constant force on roofs with wind movement from zero to over 170 mph.
Several rating systems have been developed to measure the stability of various parts of a roof in high winds. Most ratings were developed with standing seam metal roofs in mind, and no rating system can predict roof failure 100% of the time. The tests and simulations typically use steady wind pressure which does not echo real-world wind speeds and variables.
Although none of the systems devised are fool-proof, they can help you compare roofing construction and the probability of failure for each roof system you build.
Defining Wind Uplift
Wind uplift is the suction created by wind forces as the air moves parallel to the roof’s surface. As a gust of wind meets the side of a structure, the part of the air is directed upward then across the roof, creating a pressure differential. The air pressure atop the roof is less than that below, and as the differential attempts to equalize, a suction is created that pulls at the roof panels. The faster the wind, the more forceful the uplift will be. In extreme wind events, uplift can pull panels and shingles off buildings.
Wind uplift pressures vary depending on the location on the roof, meaning not every point on a roof experiences the same uplift pressure at the same time. The roof corners and perimeter, which are not fastened directly to the structure, are the weakest points.
Risk and Exposure Categories
Engineers use several factors when calculating wind uplift pressures per the ASCE 07-10 standard. One factor is the building category identifying the level of risk to human life if the roof should fail.
- I - a building representing a low risk to human life in the event of failure. Example: agricultural storage building.
- II - any building not covered by categories I, III or IV.
- III - buildings representing substantial risk to human life. Example: school, arena, water treatment facility.
- IV - buildings designated as essential facilities. Example: emergency shelters.
A second factor is where a building is positioned in relation to nearby structures, which may cause the wind to change direction or speed.
- D - buildings in a flat, unobstructed area not prone to hurricanes.
- C - buildings in areas with scattered obstructions, which are less than 9.1 meters (30 feet) high.
- B - buildings in an urban or suburban area where structures are close together.
Other factors include circumstances that can worsen or diminish wind effects or disrupt normal air flows.
- Large, open structures or cantilevers can make wind effects worse.
- Parapet walls can reduce the impact of wind effects.
- Roofs with unusual geometric surfaces may worsen or reduce wind effects depending on the design.
- Adjacent structures with extreme geometric designs may disrupt normal wind flow.
- Other roof additions, such as tilted solar panels, may affect wind uplift.
The engineer uses the building category, building position and other considerations to calculate design wind uplift values per square foot in three different zones of a roof.
Building design varies the wind speed at which each risk category is assigned. An example from ConstructionSpecifier.com shows that a Risk Category I building in the central plains of the U.S. would require a design to withstand a three-second wind gust of 170 km/h (105 mph, at 10 m (33 feet) above the ground to meet the standard for an Exposure Category C building.
Attempts have been made to provide a standardized rating system showing how well a roof will withstand wind uplift.
Underwriter Laboratories UL 580 Rating
UL 580 is a well-regarded test that has been used since 1973. A 10 X 10-foot sample of roofing material is installed on a test platform and subjected to static uplift pressure for five minutes and oscillating pressure in 10-second intervals over one hour. Results determine the class of roofing.
- Class 30 - resists nominal static pressure of 30 psf and range of oscillating pressure between 22 and 42 psf.
- Class 60 - resists nominal static pressure of 60 psf and range of oscillating pressure between 44 and 83 psf.
- Class 90 - resists nominal static pressure of 90 psf and range of oscillating pressure between 66 and 90 psf.
This test does not determine the failure rate in winds of rapidly changing speed and direction. Also, it does not show the failure rate of the anchors used in actual construction.
ASTM E 1592
ASTM E 1592 tests both panels and anchors. A 5-panel sample (10 feet wide X 25 feet long) with intermediate purlin support at various intervals and covering several spans is subjected to pressure from underneath to simulate wind load. The panel edges are allowed to move freely along the edges.
The pressure shows slowly developing failures such as seam separations and shows how a roof performs under uniform and non-uniform wind pressure. The test is run until the panels fail to determine the ultimate uplift load capacity.
FM Global Standard 4471
Factory Mutual Global is an insurance company, not a governing code body. In-house engineers conduct and write its standards. It has no control over the buildings it insures.
However, FM Global Loss Prevention Data Sheets and its online calculator are the most referenced data sheets used when designing a roof system. To use the data sheets, the engineer must know certain data about the building, including the height, terrain surrounding it, type of roof deck to be installed and whether it is a special use facility such as a hospital. The height of parallel walls must also be known.
The test set-up is highly specific and begins with the construction of a 5 X 9-foot frame with a 22-gauge steel roof deck attached to purlins. Insulation is attached to the deck and covered with a single-ply membrane. The test frame is clamped to a pressure vessel and pressurized to 15 pounds psf for one minute.
As long as the roof does not fail, the pressure is increased in 15 lb psf increments until the first component failure. The approval rating is established at the last level before failure. FM Global states their panel ratings as 1-60, 1-90, 1-120 and so on. The second number denotes the wind pressure in pounds per square foot.
The resulting rating is used to apply a classification to the roof panels. Class 1 is rated at 1-90, the default standard for roof system not located in a coastal or high wind area. Other considerations may also apply. A recent revision of the standard requires the roof's perimeter, and corners have separately assigned ratings based on the field rating.
For example, with an FM 1-90 field rating, the system must meet FM 1-150 along the perimeter and FM 1-225 on the corners.
Wind uplift ratings do not show what happens under true field conditions, they merely simulate performance in a few defined circumstances. Rating systems may measure wind speed or wind pressure only. However, the ratings allow engineers to compare roof panels and attempt to help innovate better roofing systems.