Roof Pitch Calculator: Full Span vs Horizontal Run
Enter the horizontal run for one roof plane—not the full span—when your calculator asks for run. On a symmetric gable, one-plane run is half of the full horizontal span.
Label the line before doing the arithmetic
A roof-pitch calculator forms a right triangle from vertical rise and horizontal run. On a symmetric gable, the full span crosses from one outside support line to the other. The calculator's one-plane run reaches from the centreline beneath the ridge to one side.
That is why a field note can contain two correct horizontal lengths that should not be entered in the same field. The useful check is not “Which number is bigger?” but “Does this number belong to one roof triangle?”
Record the complete horizontal distance only when that is what was measured.
For a symmetric gable, split the span into two equal roof-plane runs.
Use the vertical rise and that one horizontal run in the calculator.
When you should not halve a measurement
Do not halve a number merely because the roof looks gabled. An offset ridge, unequal eaves, an addition, a dormer, or a multi-plane roof can make the two sides different. In that case, identify the actual roof plane and measure its own horizontal run before calculating.
The calculator can transform the pair supplied. It cannot decide that a roof is symmetric, find a hidden centreline, or verify that an existing span note belongs to the same plane as the rise.
One simple consistency check
Write the three labels beside the field measurement: full span, one-plane horizontal run, and vertical rise. If the run is being derived from a symmetric full span, label it as derived rather than measured. That makes the path to the pitch result reviewable later.
Correctly identifying run improves the input pair. It does not determine rafter cuts, member sizing, product compatibility, or any roof-system approval.
Should I enter full span as run in a roof pitch calculator?
Not for a calculator that defines run as the horizontal distance from ridge to eave. For a symmetric gable, calculate one-plane run from half the full span first. For any non-symmetric roof, measure the run of the exact plane instead of assuming half.
Sources and scope
- RoofSnap Roof Pitch Calculator — reviewed for its definitions of run and its distinction between slope as rise/run and pitch as rise/span; no wording or assets are reused.
This is an original measurement-label guide, not a structural layout or construction approval.