The slant, or pitch of a roof is commonly expressed as a fraction: 4/12, 6/12 which is shorthand for how much the roof drops down from horizontal in one foot.
To get this measurement you’ll need a yard stick, ruler or stick you’ve marked off 12" from one end and a level to keep it horizontal while measuring down to the roof surface.
One more time: Climbing around on your roof is a very dangerous business. Do take every precaution and pay attention to what you are doing, where, every second you’re up there.
Place the end of the ruler on the roof surface. Make sure it is horizontal using the level. From the 12" mark, measure straight down to the roof surface and note that number.
Say that distance was 4", your roof pitch is 4/12. Keep the number handy, along with the inside diameter of your plumbing pipe for ordering the correct roof vents for your application.