What is the formula for figuring a roof slope starting at a wall height of 12' and then 28 feet later the wall height is 9' in height, utilizing a shed roof design?

Would it be a 1:12 pitch?

28' run divided by 12' = 2.33 12' runs

3' rise divided by the 2.33 12 foot sections = a 1.29:12 pitch

Pretty shallow

Roof pitch = rise/run, typically expressed as feet of rise in 12' of run. The case above is an oddball as it isn't a whole number feet of rise in 12' of run or even a half fraction. 3' rise / 28' run = 0.107. 1/12 pitch = 0.0833. 1.5/12 pitch = 0.125.

I usually use this - just make sure you get the length and w idth on the correct field or it will not be correct (i.e. the eave and the gable)

http://final-analysis.com/calculators/roof_area.htm