Wind Speeds and Wind Zones

Cost Impact of Higher Wind Warranties
Building owners, specifiers, architects, and consultants have become concerned about protecting their investments by following the latest building codes. The International Building Code adopts standards that are based on studies of winds and the research of the damage that has occurred. This standard, developed by the American Society of Civil Engineers (ASCE) for determining the structural soundness, is referenced as ASCE-7.

Today there are three active versions of the ASCE-7 published in 2005, 2010, and 2016. Within these documents are wind speed maps that an engineer references based on the location of the building and are solely used with additional factors to calculate the uplift pressures in pounds per square inch.

The wind speed maps have gone through adjustments to help improve the strength of the structure.

• 2005 wind speed map was based on a 100-year Mean Return Incident (MRI).
• 2010 maps were separated into three sets, where the maximum MRI is now 1,700-year.
• 2016 maps were adjusted again into four sets of maps, where the maximum MRI is 3,000-year.

MRI means there is a 1 ÷ X-years of a chance wind will occur in 1 year. A 3000-yr MRI = 1/3000 (0.00033) chance.

The focus of the building code is the health and welfare of the people and the surrounding environment; and at face value, these changes should be a good thing.

Now in roofing, the ASCE-7 follows very similar consideration; so the designer of record would need to make sure that any chosen roofing assembly would meet the building code based on the pressures (pounds per square foot). The certification process is very simple, though not following the correct steps can cause confusion:

Step 1: (Both “a.” and “b.” should be confirmed by the designer of record)

1. Using the correct ASCE-7 for your state or location - State Adoption Chart
2. Calculate the uplift pressures resulting in pounds per square foot from local licensed engineer

Step 2:
Review the tested roofing assemblies that meet or exceed those pressures with the manufacturer (tested pressures by membrane and assembly can be found under Codes on the Carlisle SynTec Website). Building code requires systems to meet uplift pressure for that location. Pressure does not dictate warranty wind speed requirements. Standard wind speed warranties have no relationship to uplift pressures (psf) or the local wind speeds (mph).

After this has been accomplished, consideration for a warranty should be reviewed closely with the owner, because the cost impact of choosing a warranty without this consideration could be great. When one goes to discuss warranties, the owner will make the decision on the number of years of warranty based on the cost of the warranty and installation. This should also be the case when discussing the cost of the warranty wind speed coverage, since this has no relationship to ASCE-7 or building codes.

As an example, all of the following warranty options would meet the building code, but carry extra increase in cost because of additional labor, components, and coverage:

Example
Basic Installed Cost for a 20-year Warranty = $100,000.00

• 55-mph = no increase
• 90-mph = approx. 10% increase in *overall cost ($110,000.00)
• 120-mph = approx. 20% increase in *overall cost ($120,000.00)

*overall cost = basic installed + labor + components + coverage

Though the building code requirement has the potential of increasing the cost of installation for a roof assembly, the choice of the warranty wind speed could have a greater impact on cost. Therefore, the building owner should be properly informed in order to consider his/her budget for the length of the warranty and the overall impact of higher warranty coverage to make the best use of their funds.

To expand your knowledge on ASCE-7, warranties, and other industry issues and concerns, please check out Carlisle SynTec Systems “C.R.E.W.” learning courses “Understanding Design Criteria 401” and “Industry Challenges 501.”