Proper ballast selection for wind uplift resistance is a complex procedure involving the determination of the rooftop wind speed based on:  the local wind speed, air temperature and pressure, gust factors, ground conditions, and building and parapet height.

In order to simplify this, ROOFBLOK has developed criteria that enables the designer to establish the rooftop wind speed (V_R) with only three variables:
 

Basic Wind Speed:  The basic wind speed may be determined by several methods including the ANSI 100-Year wind map (Figure 3).  Research by ROOFBLOK into the local storm conditions of various locations in the United States indicates that the 50 and 100 year storm conditions for a given locality are, in fact, exceeded by 10 to 15 percent in approximately 80% of the cases analyzed.  Consequently, ROOFBLOK encourages the designer to select the design wind speed at ground level from either the 100 year criteria or an analysis of local wind conditions, whichever is higher.



 

2.  The Ground Terrain within 1/2 mile of the site (Table 1 and Figure 2).

The Ground Terrain:  The topography and height of building within 1/2 mile of the proposed structure must be identified.  For simplicity, 4 zones are generally utilized to depict the exposure coefficient of a building.  From Table 1 and Figure 2, select the ground characteristics which best describes the proposed structure.  The designer can now reasonably select the proper wind zone and designated wind exposure as depicted in the Uniform Building Code.  If designing in a central area of large cities (Exposure A), ROOFBLOK should be consulted due to complexity of wind on high-rise buildings and wind tunneling effects on lower buildings.
 

3.  Building Height.