Online tool simplifies roof design process

April 04, 2012— Ottawa, Ontario

NRC has adapted a complex formula for determining the wind load on commercial roofs into a simple online calculator to help construction engineers and designers determine the best roof covering for any construction job.

Wind damage is a common cause of roofing failures, but the effects of wind on roofs are complex and difficult to calculate. The design load for wind uplift is a function of several variables including roof structure and slope, wind speed, building height, roof areas, building terrain, building type and building openings.

“Wind load calculations for roof cladding are critical to varying the design of commercial roofs,” says Dr. Bas Baskaran, who leads the roof systems and insulation group at NRC.

The National Building Code of Canada provides parameters and coefficients for manually calculating the wind uplift loads on roof coverings, “which involves going through various sections and applying the appropriate coefficients, formulas and procedures,” says Dr. Baskaran. “To change a single variable, you have to go through the whole wind load calculation process again.”

Request from roofing community

Wind damage is one of the common causes of roofing failures in Canada.

In response to a request from the Canadian roofing community to automate the calculation process, NRC researchers developed the Wind-Roof Calculator on the Internet (Wind-RCI). “The industry members wanted to be able to access an online calculator from a project site using their smart phones,” says Dr. Baskaran. “This would allow them to reach an immediate consensus with the architect or building owners to determine, for example, the impact on wind loads if the height of a building changes or the roof changes from a gable roof to a steep slope.”

The Wind-RCI duplicates the building code but simplifies the calculation process, he stresses. To use the calculator, one inputs the location, dimensions, exposure and type of roof configuration. The tool then displays a wind load diagram for the field, perimeter and corner zones of the roof assembly.

Dr. Baskaran says the calculator is limited to buildings of less than 100 feet (30 metres) in height. For taller buildings, additional performance evaluations are recommended. The Wind-RCI is also not designed for buildings located on hills and escarpments, or those with hipped roofs or roofs with overhangs.

Access the Wind-RCI tool.

Designing a wind-resistant commercial roof

To ensure a commercial roof can withstand wind loads, construction engineers follow a three stage process. First, they calculate the design load — this can be done either manually or using the Wind-RCI tool. Next, they evaluate the wind resistance of the roof assembly, using the national standard CSA A123.21-10 developed by NRC. Third, they ensure that the wind resistance of the roof assembly exceeds the design load specified by the National Building Code of Canada.

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