Convert rise/run to degrees and check walkability safety.
Standard run is 12 inches.
Roof pitch is calculated as "Rise over Run." In the US, the run is standardized to 12 inches.
So a "4/12 pitch" means for every 12 inches the roof goes horizontally, it rises 4 inches vertically.
A standard gable roof usually has a pitch between 4/12 and 9/12. Anything below 3/12 is considered a low-slope or flat roof and requires special membrane roofing instead of shingles.
Take the total horizontal run of the rafter span and multiply it by the 'Length Multiplier' shown above. For example, if your run is 10 feet and the multiplier is 1.054, the rafter length is 10.54 feet.
Generally, pitches up to 6/12 are considered walkable for experienced workers. Pitches from 7/12 to 9/12 are steep and dangerous. anything over 10/12 is non-walkable and requires harnesses and roof jacks.
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