The wind design calculator is used to calculate the roof edge design pressure required to meet the ANSI/SPRI ES-1 Wind Design Standard and the 2003 International Building Code Section 1504.5 “Edge Securement for Low Slope Roof.”
Why calculator results do not exactly match results using charts in ES-1 document
Differences between values for the Design Pressure (P) listed on this site and those computed using the ANSI/SPRI ES-1 documentation are the result of generalizations of structure heights in producing the ANSI/SPRI ES-1 velocity pressure charts.
Both the ANSI/SPRI ES-1 charts and the Wind Calculator use the same formulas in the SEI/ACSE 7-02 standard to produce velocity pressure values (qz). However, the ANSI/SPRI ES-1 charts show velocity pressure values for a single, generalized height within a range of heights supplied. This method is less exact then by using the true height with the formulas in the SEI/ACSE 7-02 standard for calculating velocity pressure, as the Wind Calculator does.
The most dramatic difference in these two approaches is in exposures B & C since the ANSI/SPRI ES-1 charts use a broad range of 0-60 ft. for its lowest values while using 10 to 20 ft. jumps for the rest of the charts. Less dramatic are the generalities in the charts for exposure D since it uses 0-15 initially then ranges 5 to 20 ft. there after. The smaller the range used, the more similar the Wind Calculator and ANSI/SPRI ES-1 final Design Pressure values will be.
Since ANSI/SPRI ES-1 is a guide to the SEI/ACSE 7-02 standard, and as such does not reflect the standard in whole, every effort was made to be more exact to the SEI/ACSE 7-02 standard in calculating out values using the Wind Calculator. For this reason, the results provided by the Wind Calculator should supersede those provided by the ANSI/SPRI ES-1 documentation.