Meteorologists are predicting between 13 to 20 named tropical storms this year with between six to 10 becoming hurricanes. With this storm season off to a quick start, now is the perfect time to prepare commercial buildings for hurricane force winds.
Although no systems are fool-proof, uplift testing can help to compare the construction and the probability of failure for each roof tested. Uplift testing is especially important in parts of the country that experience high winds such as the eastern coast of the United States, tornado alley, and southwestern areas affected by monsoon storms.
Understand Uplift Testing
The dynamic uplift resistance (DUR) of a roofing system that includes the roof material, insulation, clips, and fasteners but not the roof deck, is determined by testing for resistance to outward pressure. Understanding this rating is crucial in selecting the right roof assembly that can withstand certain pressures of wind uplift loads. The magnitude of wind uplift loads that must be resisted is determined by a building’s height, type of occupancy, style of roof geometry, and geographic location.
Several test protocols are used when evaluating the wind uplift resistance of commercial roofing systems. UL1897, ASTM E1592, UL58- and FM 4470 are a few of the industry accepted protocols. These tests are typically performed using a cup-shaped rig and placing the open end on the roof. Once attached, the test begins working much like a vacuum. The apparatus removes atmospheric pressure creating suction that lifts the roof, and ratings correspond with the magnitude of suction resisted. This rating will determine if additional securing to the supporting structure is needed. Following these protocols will ensure that the roofing testing is certified for that region and meets all the safety requirements necessary.
The results from a wind uplift test will provide insight to the structural resistance of a roofing system and ensure it is meeting the wind load design requirements. Uplift testing also helps identify weaknesses in the roof, helping manufacturers, contractors, engineers, and builders to address the problem areas before a failure can occur.
What is Wind Uplift?
In the 18th century, Swiss mathematician Daniel Bernoulli discovered a fundamental behavior of fluids that would later be named the Bernoulli effect. The Bernoulli effect states that the faster a fluid is moving, the lower its pressure. This is the same effect that creates lift under an airplane’s wings since the air is acting as a fluid. The Bernoulli effect is also exhibited when strong winds are tearing off parts of a roof.
Buildings obstruct the flow of wind moving over the Earth’s surface, causing a change in pressure over a building’s surface as the wind warps around the obstruction. Typically, the direction of airflow is associated with pressure in a few different ways; positive pressure occurs on wind facing surfaces, negative pressure occurs on leeward surfaces and negative pressure occurs at eaves, corners, rakes, and at changes in roof geometry. Even windward roof faces can experience negative pressure for roof slopes under approximately 26 degrees (6:12 pitch).
Wind uplift occurs when the pressure below the roof is greater than the pressure above and is measured in force per unit area, similar to pounds per square foot. Uplift can be increased during storms with high winds as air enters the building, causing an increase in pressure below the roof by effectively inflating the building while the fast-moving wind over the roof reduces the pressure from above.
Wind uplift introduces stress to the roofing system and can cause a breakdown in the materials, which can result in severe damage or even failure of the roofing system. When wind uplift is greater than what the roofing system was designed for, then fasteners can withdraw and pull free, removing the roofing and leaving the fasteners intact, or roofing materials can tear, delaminate, or disintegrate. When wind uplift is greater than what the roofing system can resist, the supporting roof deck could possibly detach from the structure. Engineers and architects follow codes to prevent this from happening. Codes vary by building location and their exposure to high winds.
Timeframe for Uplift Testing
Wind uplift testing cannot be performed until the roofing is installed on a building, and is ideally performed prior to completing construction of the building so that any issues may be readily addressed. Registered professional engineers or building contractors need to follow the codes and standards that are in effect for the region of the country they are working in when performing the test. The test results must meet or exceed the standards set in place for that region.
It is also suggested that every few years (seven to 10) to retest the roof to account for degradation and any wear and tear that has occurred over the years. Also, if the area is prone to major storms like hurricanes and tornadoes year after year, retesting interval should be shorter. Note that fasteners, insulation, and other roofing materials may need replacing after testing.
Safety is a key priority when it comes to building and designing commercial buildings. Conducting uplift testing can help contractors, engineers, and builders determine where a roofing system’s weak points are located, allowing them to be fixed. Uplift testing also allows owners of these buildings to better understand how resilient their roofing system really is in case of a storm.
With hurricane season just beginning, now is the time to promote uplift testing so roofing issues can be identified and fixed if needed.
