How do yo udetermine the slope of a roof?

It's very easy. If you level a line one foot at the bottom of your roof, and then plumb a line up that will tell you the pitch. Probably the best way to do this is to got into the attic take a level, a tape measure, and a marker. Your rafters slope down put your level up against the plywood of the roof hold it level and make a make on your level at one foot, then measure up from the one foot mark to the top of the plywood. Whatever, that measures will be your pitch.