Use horizontal run, not slope length
For a symmetrical gable, horizontal run is commonly half the span from wall line to ridge centerline.
Different roof forms require the actual run for each rafter condition.
Translate pitch into rise
At a 6:12 slope, every 12 units of horizontal run corresponds to 6 units of rise.
Keep run and rise in consistent units before using the Pythagorean relationship.
Calculate the geometric line
Rafter line = sqrt(run² + rise²), before tail, ridge, bearing, or cut adjustments.
This geometry does not size the structural member.
Add stock-length requirements separately
Eave overhang, plumb cuts, birdsmouth, ridge details, and trimming can increase required board length.
Round procurement to an available stock length only after those details are known.
Verify structural design independently
Span, species, grade, spacing, loads, bracing, uplift, and connections are not determined by rafter-length math.
Use project documents and applicable structural requirements.