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Hi, I'm James Pease and this is Take5, Building Operating Management's podcasts on topics of interest to building and facility executives.

We don’t think much about the roof over our head until something goes wrong and water gets into a building. And although a properly designed roofing system shouldn’t be affected by high winds, certain forces placed on a roofing system can create problems.

With us today is Helene Hardy-Pierce, Vice President of Code & Industry Relations with GAF/Elk Corporation. Helene has over 20 years of experience in Technical and Guarantee Services’ functions for both Steep and Low Slope commercial roofing systems. Among her many awards of distinction, she was named Fellow of the Institute by the Roof Consultants Institute in 2005.

Thanks so much for being here with us, Helene.

[Pease] How do high winds affect a roofing system on a building?

[Hardy-Pierce] hen a strong wind “hits” a building, it “races” up the side or corner of the building and tops over the roof – this creates a pressure on the roofing assembly that causes it to want to pull off of the building. We refer to this force as “uplift pressure” and there has been a lot of research and engineering work devoted to being able to estimate what uplift pressures the roof on a particular building will see over its lifetime.

As an industry, we’ve become pretty sophisticated with regards to wind design and roofing system performance – and can calculate the uplift force that a given wind speed will create on the roofing system using the engineering principles detailed in a standard put out by the American Society of Civil Engineers called ASCE 7, The Minimum Design Loads for Buildings and Other Structures.

[Pease] What does the building code require for wind resistance?

[Hardy-Pierce] The International Building Code (or the IBC) requires that most buildings be designed in accordance with ASCE 7.

The IBC has now been adopted for use in all 50 states, although there are modifications to the IBC that may be specific to different jurisdictions, such as the state of Florida.

Building owners should talk with their roofing professional and local code officials to make sure that any roofing activity is code compliant.

[Pease] What are some of the things that can affect the uplift pressure on a given roofing system?

[Hardy-Pierce] Wind forces vary depending on 4 factors: surrounding terrain, building height and geometry, the presence of a parapet wall, and the presence of large openings – such as truck bays – in the building.

In the case of the surrounding terrain, if the building is out in the open, wind will have a greater affect than if the same building is surrounded by similar buildings which would have the effect of “breaking up” the wind forces.

As for building height and geometry… when the wind races up the wall of a taller building, when it tops the roof, it has a higher velocity than if it only had to go up a few stories.

[Pease] Why? Does the larger surface area resisting greater air pressure translate directly into increased velocity?

[Hardy-Pierce] A parapet wall protects the roofing system, and when the wind tops the roof, a parapet wall reduces the uplift pressure on the membrane by moving the fastest wind above the membrane.

Finally, large openings, such as large truck bays, in the building can really cause problems for the roofing system because a wind, blowing into the opening has to “go” somewhere”… when this happens, it can greatly increase the pressure on the roof because not only is the wind topping the roof edge and applying pressure, but the “wind” from inside is pushing up from the underside of the deck at the same time.

[Pease] How are roofing systems designed today to resist these wind uplift forces?

[Hardy-Pierce] We’ve learned a lot from our experiences… from looking at what has worked in severe winds, such as hurricanes, we tend to look at the whole system and how it works together – from the deck up through the roofing membrane and including the wall and edge details.

First, how the deck itself will perform comes into play – for example, if it’s a steel deck, then it needs to be properly attached. The deck also determines the best way to attach the roofing system, including the insulation… concrete decks are well suited to fully adhering insulation using hot asphalt or an insulation adhesive whereas with a steel or wood deck, insulation (or base sheet for a wood deck) is mechanically attached.

Generally, the roofing membrane can be attached using 3 different methods: fully adhered -- such as a hot mopped built-up roof or a single ply membrane put down in adhesive -- mechanically attached, or a system that uses “ballast” to hold them in place.

Regardless of which installation method is preferred, the “design” of the roofing system involves calculating the uplift pressures for the specific building and then selecting a roofing system that can provide the required uplift pressure resistance.

[Pease] How can a facility owner or manager know that they are getting a roofing system that will meet their needs with respect to wind?

[Hardy-Pierce] If you know what the design uplift pressure is for your building and the type of roofing system you want installed, then a roofing system manufacturer should be able to provide an insulation and membrane assembly that has been tested to meet those uplift pressures.

It’s important to understand that reputable manufacturers such as GAF spend a great amount of time developing and testing roofing system for wind uplift resistance. When you consider all the permutations and combinations of insulation, fasteners, membrane components, etc., we literally have thousands of roofing systems that will provide resistance to wind events.

Yet even with all of the testing and subsequent listed systems, you should be flexible about what is their best and safest choice. For example, if a property owner has a steel deck and wants to install a mechanically attached TPO membrane – they will get better wind resistance if they mechanically attach the membrane on 6” centers instead of value engineering the same installation and attaching the system on 12” centers. Or they may have a preference for perlite insulation as a coverboard and want a cold applied membrane installed over the perlite, but they may get better wind uplift resistance if they change to a woodfiber coverboard.

[Pease] What else can affect how well a roofing system will perform?

[Hardy-Pierce] Even the best designed roofing system can have problems performing when subjected to wind uplift forces if a few peripheral items are not up to standard.

Investigating how roofs have performed in hurricanes has shown us that how well the roof edge is attached is a key variable to how well the roofing system will perform. Property owners should make sure that the wood blocking anywhere the membrane is terminated is properly attached with the correct fasteners.

As we’ve already discussed – a parapet wall will greatly reduce the uplift pressures on the roofing system.

Also, a large amount of damage can be caused by things that are “on” the roof bouncing across the roof if they become airborne. So particular attention should be paid to making sure that roof top units and penetrations are properly secured to the structure to prevent them from being able to cause damage in a wind event.

[Pease] Thanks for the useful information, Helene.

Our guest today has been Helene Hardy-Pierce, Vice President of Code & Industry Relations with GAF/Elk Corporation For more information on GAF commercial roofing products, click the More Info button or the GAF text link on this page.

I’m James Pease and this has been Take5.

Helene Hardy-Pierce, GAF/Elk Corporation

"Take5," a podcast series from Building Operating Management magazine, interviews Helene Hardy-Pierce Vice President of Code & Industry Relations with GAF/Elk Corporation, on the effects of high winds on roofing systems.