asce 7-05 pressure coefficients

huangyhg

asce 7-05 pressure coefficients
it is unclear to me when figuring uplift pressures around the perimeter of a roof how to sum the overhang and the distance "a" together.  i'm looking at figures 6-11b, 6-11c, and 6-11d.  
is "a" calculated from edge of roof line or wall line of building?  in other words, if my distance "a" is 6' and i have a 2' overhang, can i use the coefficient for the overhang for 2' and the coefficient for zone 2 or zone 3 for 4'?  or.....do the overhang pressures need to be summed with the full pressures over the "a" distance from wall line in?
i'm probably talking in circles.  i'd appreciate some input.
thanks.
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i have a copy of "guide to the use of the wind load provisions of asce 7-02.  there is an example problem of a monosloped roof with a 7 ft overhang.  distance "a" was calculated as 4 ft and they show the first 4 feet of the overhang as zone 2 and the remainder 3 feet as zone 1.
perfect bendog!  just what i needed.  thanks.
the way i understand it is "a" is calculated from the edge of the roof line.
based on this, you can map out your correct zones on the roof and determine the correct gcp to use.  you would use the overhang coefficient table only on the overhang part. therefore it is possible for a zone to use different coefficients (1 from each table) if it occurred on both the overhang and the roof.