Riku Ylipelkonen, Owner, Standard Building Advisors and FRSA Technical Advisor
One of the fundamental concepts in engineering is that context matters. Getting soaked with a garden hose is a fun summer way to cool down. Using a pressure washer to do the same is dangerous and not recommended. Similarly, wind at 5 miles per hour (MPH) is pleasant, but wind at 105 MPH is not.
Wind behaves predictably as it is scaled. This is useful because we can observe wind behavior at lower speeds and smaller dimensions to model how hurricanes and other wind events affect whatever gets in their path. As shown in Figure 1, the scale model of a building can be built along with the surrounding topography to learn how the wind loads act upon the building.
There are some important estimations shown in Figure 1. First, the fan at the end of the tunnel must have a path designed with features to mimic how wind is experienced on land. The surface roughness bumps help to shape the wind into a proper profile. Next, the turntable has the surrounding buildings assembled to scale because these features affect how the subject building
observes the wind. This turntable also spins 360 degrees or completely around, because the direction of the wind event cannot be known. The features and shape of the building affect the final wind loads on the building itself. There are more variables accounted for in the testing itself, but these basic facts get the point across. The scientific community that publishes ASCE
7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures has been getting better and better over time. The critical feedback of this process is the improved ability to collect data from wind events like hurricanes where concepts like surface roughness, topographical effects and more are validated with analysis. With advances in computers, data collection in wind events and communication abilities of researchers, the information and analysis to substantiate the standards have been revised with greater accuracy and precision.
This kind of wind tunnel testing has also been performed on steep-slope roofed building models, much like the projects Florida roofing contractors work on every day. Recent changes to roof zones on hip and gable roofs are a prime example of this work. When the modeling shows that the ridge end of a gable rake has a higher-pressure load than the eave end of a gable rake, the changes in roof zones take that into account. This is the same way we learned that the eave course of a steep-sloped roof covering is more susceptible to wind loads. Anyone that has seen the aftermath of a wind event in Florida can attest to that.
When a roof is modeled for pressure readings, there must be enough taps in the model to account for the changes and to predictably identify the shape of the pressure areas so we can apply that to real world roofs. The readout can show how the wind is creating negative pressures across the roof area. As shown in Figure 2, the output of a wind tunnel test shows the pressures and how it changes on a sample low-slope roof.
The areas of perimeter, corners and the new 1 Prime Zone are defined well in the results. Please bear in mind that this is just
one readout of the turn table. As the source direction of the wind is rotated around this roof model, the corner and 1 Prime
Zones become more defined.
This may be a little counterintuitive, but the problem with roofing is not just that windborne debris and rain are blown onto the roof, but the moving wind causes low wind pressure areas that cause uplift force on the roof covering trying to suck it up into the sky. Think of any time you have had a light load in the back of your pick-up truck and it gets sucked out as you speed up. That napkin or receipt wasn’t blown out, it was sucked out.
With all this in mind, there is a connection to how current roof coverings are tested at third-party independent laboratories. The fundamental science is there and so is a lot of work to verify that the test methods used not only represent what happens during a wind event, but also that the wind event levels and pressures reflect what happens in real life. The vacuum chamber testing for underlayments and low-slope roof coverings, as well as the various test methods for steep-slope roof coverings, all produce results that allow us to predict how well the products will perform in real-life wind events. All this effort comes together
to provide roofs that better withstand wind tests – and the test of time.
Riku Ylipelkonen, Owner, Standard Building Advisors has been in the roofing industry for 15 years working for Polyfoam Products. When Polyfoam Products was acquired by 3M, the name was changed to ICP Building Solutions Group. Riku worked at ICP as Technical Services Manager until March of 2023, when he left to begin his own company. Riku is an Engineer and is working as a consultant with FRSA. He is a member on FRSA’s Codes Committee, Codes Subcommittee, Tile Committee and on the FRSA-TRI Manual Rewrite Committee. Riku is also a member of the American Society of Civil Engineers (ASCE) and the Single Ply Roofing Industry (SPRI).
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