Worked like a charm.
That Ed Levin is one smart guy. # 39, 1996

Whether you come at it by the algebraic or the graphical method, it's possible to generalize a principal about thrust in simple rafter roofs: The resultant force always acts at an angle to
the level whose pitch is twice the roof pitch. So in a 12: 12 roof the vector sum of thrust and gravity load is pitched at 2412, in a 6:12 roof the resultant acts at 12: 12, etc. Hence roof thrust can be quantified as follows: For simple rafter roofs (no collars, struts,kingposts, etc. to muddy the waters),
with roof slope S (in degrees), the thrust is equal to the roof load divided by twice the tangent of the slope, or F, = F, + 2 tan S. Our 2,520 roof load induces thrust of 1,260 lbs. in a 12:12 roof. In a 9:12 roof, the same load would impart 1,680 lbs. of thrust. Load and thrust would be equal in a 6:li roof, and by the time you get down to 3:12 pitch, thrust has grown to 5,040, twice gravity load.