wind: in the field of the roof

by William J. Hope, CSI, CDT, PE, RRC


as the height of the roof increases, the wind velocities increase relative to same wind at ground level

wind: why is it a problem? 

In the last wind article, we investigated  the origin of wind and the “lifting effect” or negative pressure moving air has on a surface. This month we will look at the effect wind has on the field of low slope roofs and how important it is to correctly classify the exposure.

As wind passes over the roof surface, the negative pressures stress the membrane, membrane attachment, insulation, insulation attachment, decking and the structural supports in an upward direction. An increase in wind velocity will result in an increase these upward stresses. What parameters are more likely  to cause high winds and high uplift pressures? Building location, ground roughness and roof height above ground are the main culprits. As you probably know, coastal locations have higher basic wind speeds than interior continental locations. Ground roughness addresses how sheltered or exposed the roof is to its surroundings. This culprit can influence design uplift pressures by as much as 75% depending on which ground roughness category applies.

As the height of a roof increases, the wind velocities increase relative to same wind at ground level. This can result in an increase of 35% in the design uplift pressures for a roof 15 feet height above ground relative to a roof 50 feet height above ground. The aggregate effect of all these parameters determine the design uplift pressure for a given roof. It is this uplift pressure that we use to determine the attachment level for the field of the roof. Once the attachment level for the field is known, the most critical areas, the perimeters and corners, are empirically designated for attachment.

We can see why knowing the interplay of location, roughness and height can drastically effect a roof’s design wind uplift pressures and subsequently determine the wind classification and attachments. These are all crucial if we are going to have any chance to keep our roofs in place when they are tested by the elements.

© 2000 William J. Hope, CSI, CDT, PE, RRC on line at Reprinted with permission. 
Want to know about light colored roofs? Read what Dick Fricklas has to say in his article “Reflections on a Cool Roof” 

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