Warranty Wind Speed and Design Wind Speed
Every wind uplift design for a roofing project starts with the local wind speed and the type of building being designed. These two characteristics are needed for calculating the wind uplift. However, in gathering this information, some designers look at the American Society of Civil Engineers' ASCE 7 Wind Maps for their area, see a number like 90 mph or 120 mph, and think that is the likely wind speed their building will encounter. Therefore, they specify the same speed for their warranty (i.e 120 mph local speed means I need a 120 mph wind speed warranty). Rest assured, this is not the case.
ASCE 7 maps have contours with the local speeds in 10 mph increments. In previous editions, such as ASCE 7-2005 and ASCE 7-2010, the mph were relatively straightforward and the majority of the US was in a 90 mph zone. However, ASCE heavily reconfigured their calculation in 2016 and therefore deemed it necessary to have separate maps for each building risk category (i.e. Category I, II, III, and IV). This increased the wind speeds for the majority of the U.S., especially for projects with increased risk categories.
Naturally, designers saw this increase and thought since the local wind speed was increasing, they needed to ask for increased wind speed warranties (i.e. 130 mph or more). Again, this is not the case.
It’s true that warranted wind speed is the limit of (3 second) peak gust, which is recorded at the weather station nearest your building project and measured at 10 meters above the ground during a weather event that affects your building project. But to achieve wind speeds over 90 mph, a cyclonic windstorm (i.e. tornado, hurricane, etc.) is generally necessary. In the event your building experiences a cyclonic windstorm, there will be flying debris, broken glazing, and other envelope breaches that could contribute to a roof failure (such as over-pressurizing the building, detachment of decking from structural components, etc.) that would not be covered under a roofing warranty, no matter the wind speed coverage.
Keep in mind that a roofing warranty assumes the building remains intact, the decking remains solid, the inside pressure of the building is generally equalized, and that foot traffic is limited to maintenance and inspection of rooftop equipment. It is not building insurance. Critical weather events (i.e. tornado and hurricanes), fire, and vandalism to a building are covered by the building owner’s insurance carrier.
Choose a warranted wind speed that makes sense for you and your client, but don’t try to match that with your local wind speed. You will just be paying more for something you do not need.
Always verify your need for an increased warranty wind speed before inquiring about matching your local wind speed with the warranty. Contact Craig Tyler at [email protected] with questions.
ASCE 7 maps have contours with the local speeds in 10 mph increments. In previous editions, such as ASCE 7-2005 and ASCE 7-2010, the mph were relatively straightforward and the majority of the US was in a 90 mph zone. However, ASCE heavily reconfigured their calculation in 2016 and therefore deemed it necessary to have separate maps for each building risk category (i.e. Category I, II, III, and IV). This increased the wind speeds for the majority of the U.S., especially for projects with increased risk categories.
Naturally, designers saw this increase and thought since the local wind speed was increasing, they needed to ask for increased wind speed warranties (i.e. 130 mph or more). Again, this is not the case.
It’s true that warranted wind speed is the limit of (3 second) peak gust, which is recorded at the weather station nearest your building project and measured at 10 meters above the ground during a weather event that affects your building project. But to achieve wind speeds over 90 mph, a cyclonic windstorm (i.e. tornado, hurricane, etc.) is generally necessary. In the event your building experiences a cyclonic windstorm, there will be flying debris, broken glazing, and other envelope breaches that could contribute to a roof failure (such as over-pressurizing the building, detachment of decking from structural components, etc.) that would not be covered under a roofing warranty, no matter the wind speed coverage.
Keep in mind that a roofing warranty assumes the building remains intact, the decking remains solid, the inside pressure of the building is generally equalized, and that foot traffic is limited to maintenance and inspection of rooftop equipment. It is not building insurance. Critical weather events (i.e. tornado and hurricanes), fire, and vandalism to a building are covered by the building owner’s insurance carrier.
Choose a warranted wind speed that makes sense for you and your client, but don’t try to match that with your local wind speed. You will just be paying more for something you do not need.
Always verify your need for an increased warranty wind speed before inquiring about matching your local wind speed with the warranty. Contact Craig Tyler at [email protected] with questions.
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