By Tom Irvine, RRC, CDT
The International Building Code (IBC) mandates that a roof’s wind uplift resistance be designed to satisfy the requirements of ASCE-7. ASCE-7 – Minimum Design Loads for Buildings and Other Structures is published by the American Society of Civil Engineers. ASCE 7-10 is the last published revision, and is referenced in the 2015 IBC.
ASCE reports that the latest revision to the standard, ASCE 7-16, is slated for release in early 2017. The Governmental members of the International Code Council (ICC) have voted to adopt the 7-16 standard within the current 2018 I-Code revision cycle, and it is widely predicted that full approval and adoption will follow. Subsequent state adoptions of the 2018 I-Codes will trigger the need for roofing professionals to use the 7-16 standard, likely beginning in 2019.
There are several notable changes to ASCE 7 being discussed in the roofing industry, which are generally based on reviews and commentary on the draft 2016 standard. Although the final standard has not been published as of this writing, the following notable changes are anticipated:
- ASCE 7-10 wind maps and associated wind speeds will be updated in 7-16. ASCE reports that wind speeds will be reduced for much of the U.S. where located outside of hurricane-prone regions. A separate wind speed map will be included for Occupancy Category IV buildings, where previously Category III and IV buildings were addressed on a combined map. Special wind regions will also receive clarification, including Hawaii.
- In ASCE 7-10 and earlier versions, widths of perimeter, corner, and ridge attachment enhancement zones for low slope roofs are calculated as 0.1 times the roof’s least horizontal dimension, or 0.4 times the building height – whichever is less, but not less than either 4 percent of the least horizontal dimension or 3’. In ASCE 7-16, calculating zone widths using 0.1 times the roof’s least horizontal dimension will no longer be allowed, and calculations must be based on 0.4 times the building height. This change will result in larger enhancement zones for many roofs, compared to the dimensions required under 7-10. For example, a 50’W x 200’L x 40’H roof would require 16’ wide perimeter zones and 16’ x 16’ corner zones under 7-16, as opposed to 5’ perimeters and 5’ x 5’ corners under 7-10.
- External pressure coefficients for components and cladding have increased; however, the final pressures will be offset by a reduction in the design wind speeds over much of the U.S. In hurricane prone regions, wind speeds were not reduced while pressure coefficients still receive the global increases. This will result in significant increases in design pressures in hurricane-prone regions.
- In ASCE 7-10 and earlier versions, there is one basic enhancement zone layout for low slope roofs with variations outlined for roofs higher than 60’, where parapet wall heights exceed 3’, and at perimeters and corners of monoslope roofs. In 7-16, there will reportedly be four model enhancement zone layouts for low slope roofs, which will require selection based on the relationship between the least horizontal roof dimension and the roof’s height. The zone dimensions will need to be calculated for the selected layout as prescribed by the standard.
- ASCE 7-16 will introduce a fourth enhancement zone for roof attachment, in addition to the traditional industry standard perimeter, corner, and ridge zones used in 7-10 and earlier versions. The new zone will reportedly apply to very large buildings.
- Hip and gable roofs will be separated under 7-16, with pressure coefficients based on revised slope criteria in some cases.
- In the new standard, design requirements for rooftop photovoltaic (PV) equipment will be addressed for the first time under ASCE-7.
- A ground elevation factor is added under ASCE 7-16, which adjusts calculations to account for variations in air densities at different altitudes. Building the roof “on paper” will no doubt become more complex for the design community. It will also be increasingly important to communicate the application requirements to the roofing technicians and monitor their work for compliance with the contract documents. There will be a learning curve as roofing contractors adapt to new and larger enhancement zones, with potentially different layouts than in years past.
Once ASCE releases the final standard, a more definitive analysis of the changes and impacts on roof design and construction will be possible. Benchmark will provide further information as the roofing industry reacts and adapts to the changes.
Building the roof “on paper” will no doubt become more complex for the design community. It will also be increasingly important to communicate the application requirements to the roofing technicians and monitor their work for compliance with the contract documents. There will be a learning curve as roofing contractors adapt to new and larger enhancement zones, with potentially different layouts than in years past.