If you cannot measure the height of a roof accurately, the next best way to measure the roof pitch on an existing building is with a 48" level and a tapemeasure.

Roof pitch or slope is usually expressed in as "X/12" or "RISE over RUN"

Carefully climb onto roof (ensure to use proper tie-off methods or scaffolding) and hold a 48" level so the bubble is in the middle. This will be your RUN.

Next, measure from the end of the level down perpendicular to the top of the roof. This number will be your RISE. 

Take this number and divide mathmatically by 4 to get your pitch. This only works with a 48" level. If you are using a 36" level, divide by 3.

For example, if you measured 40" perpendicular from a 48" level then your pitch would be (40" / 4 = 10) and (48" / 4 = 12) or 10/12.

If you measured 24" perpendicular from a 36" level then your pitch would be (24" / 3 = 8) and (36" / 3 = 12) or 8/12.

For accuracy, we do not suggest using a level less than 36" long.