When properly designed, installed, and maintained, a commercial roofing system provides building owners and managers with value and long-term dependability. Most commercial roof coatings have outstanding performance records against just about any type of weather situation. However, it’s possible for a roof to be damaged or blown off if winds are stronger than what the roof was designed to withstand.
The best way to test the strength, durability, and resistance of your commercial roof to wind dynamics is to perform a pull test. Read on to understand what a pull test is and why your commercial roof needs to have them done.
Wind and Building Interactions
When ground-level wind flow comes into contact with a building, it is forced to go up and over the building. Wind uplift occurs when the air pressure below the roof deck is higher than the air pressure above the surface material.
When wind blows over a roof, it causes a decrease in the air pressure above the roof. This is referred to as “negative” (suction) pressure. Simultaneously, the wind causes the infiltration of air below the roof materials through cracks and openings around doors and window frames. This creates “positive” pressure.
The combination of positive pressure below and negative pressure above the roof surface creates a “push and pull” force which increases wind load on the roof. These forces can grab and tear the surface materials if they are not fixed firmly enough to the roof deck.
Wind uplift is affected by:
- Geographical location. Most of the U.S. has a basic wind speed of up to 90 mph, but other areas have much higher speeds – as high as 170 mph.
- Building height. The taller the commercial building, the greater the wind speed and wind loads.
- Building openings. A crack, window, or an open door can build internal pressurization and depressurization during a wind storm.
- Topography. A commercial property near a ridge, hill, or large water body would receive higher wind loads than a property on a relatively flat area. Also, neighboring buildings and other obstructions reduce wind loads by breaking wind flow.
A pull test determines the negative and positive pressures applied by wind to the roof and how much the roof can stand before it is blown off.
How a Pull Test is Conducted
There are two recognized pull test methods for testing wind uplift resistance: Field Verification of Roof Wind Uplift Resistance (FM 1-52) and Field Testing Uplift Resistance of Adhered Membrane Roofing Systems (ASTM E907).
Both test methods are similar and involve the use of a five by five foot dome-like chamber which is placed over the roofing membrane surface. A controlled negative (uplift) pressure is then applied inside the chamber to the roof’s surface using a vacuum pump. During the test, a deflection bar placed in the center of the chamber is used to monitor and measure the deflection of the membrane to determine whether the roofing passes the pull test or is suspect.
Using FM 1-52 test, a roof is considered suspect if the deflection measured is between ¼’ and 15/16’. With the ASTM E907 testing, a roof system is suspect if the measured deflection is 1 inch or greater.
FM 1-52 and ASTM E907 differ in their maximum test pressures and test cycles. While both tests are started at an initial pressure of 15-pounds per square foot, pressure intervals for ASTM E907 are based on the calculated design wind pressure for the specific roofing system being tested. With FM 1-52, the pressure is increased by 1.75 psf every minute until 1.25 times the design test pressure is reached.
A pull test should be done when roof level wind speeds are below 15 mph and the roof surface temperatures are between 40 F and 100 F.
Why Have a Pull Test Done
You may be thinking your fasteners are corrosion resistant and designed to withstand great stress. While this may be correct, the damage caused by temperature fluctuations can cause them to become loose or weak. Conversely, deck materials could lose their ability to hold adhesives and fasteners over time.
The results of a pull test provide insight to estimate a roofing system’s structural resistance and ability to withstand potential wind events. A pull test for roofing helps ensure your commercial roof meets wind load design requirements.
By checking the strength and durability of fasteners and deck materials, areas of weaknesses can be identified, helping roofers address them before they malfunction and cause catastrophic failure. Roof designers can also use pull test results to assess the adequacy of venting for your roofing system.
While pull tests are usually performed on new constructions, they should also be performed on roof replacement projects to determine the condition and durability of the deck. This is especially important because fasteners, adhesives, and other roofing materials can be affected by age and roofing problems such as moisture penetration. To get a pull test scheduled in the Mid-Hudson Region, contact Vanguard Roofing.
