How this instrument works
Roofers describe a roof's steepness as a ratio: how many inches the roof rises for every 12 inches of horizontal run, written as something like '6/12' or '6-in-12'. That convention is convenient on a job site because a framing square's 12-inch arm makes it easy to check directly, but it is a ratio, not an angle — a '6/12' roof and a '12/24' roof describe the same steepness even though the numbers look completely different, and neither figure is directly the angle a level or protractor would read.
This instrument reports both quantities from the same two inputs. The pitch ratio is simply rise divided by run — 6 divided by 12 gives 0.5, regardless of what run you actually measured it over. The roof angle in degrees comes from the arctangent of that same ratio, the inverse-tangent function that converts a rise-over-run slope back into the angle it represents, the same relationship a level's digital angle readout or an old-fashioned protractor placed against a rafter would confirm.
Knowing both figures matters for different reasons. The ratio is what shows up on blueprints, in material specification sheets and in a framing square's own rafter tables, while the angle in degrees is what a digital angle finder reads directly and what determines whether a roof qualifies as 'low-slope' or 'steep-slope' for code and material-warranty purposes, since those thresholds are usually stated in degrees or as a minimum ratio rather than the other way around.
- Enter Rise (in, per 12 in of run) — the inches of vertical rise for every 12 inches of horizontal run, the '6' in a '6/12' roof.
- Enter Run (in) — normally left at 12, matching the standard rise-per-12-inches convention; change it only if you measured over a different run.
- Read Pitch ratio (rise / run) for the plain ratio, and Roof angle (degrees) for the equivalent angle a level or protractor would show.
- Run must be greater than zero — a flat run of 0 in has no defined slope and the instrument will ask for a value above zero.
- To compare against a rafter length, feed the same rise and run (converted to feet) into this site's rafter length instrument.
Worked example — a common 6/12 roof pitch
Enter 6 into Rise (in, per 12 in of run) and leave Run (in) at its default of 12 — a '6/12' pitch, one of the most common residential roof slopes in the US. Pitch ratio (rise / run) reads 0.5000, simply 6 divided by 12.
Roof angle (degrees) reads 26.565 — the arctangent of 0.5, converted from radians to degrees. That is the actual angle a digital angle finder or protractor would read against this roof's slope, distinct from the 0.5 ratio a framing square measurement gives directly.
Questions
What's the difference between roof pitch and roof slope?
In careful trade usage, pitch is rise divided by the full span (both sides of the roof together), while slope is rise divided by run measured to the ridge's centerline — but in everyday practice on a job site and in this instrument, 'pitch' and the rise-per-12-in ratio are used interchangeably, following how most roofers and suppliers actually talk about it.
Why does a '6/12' pitch give a 26.565° angle instead of something rounder?
Because the angle comes from the arctangent of 0.5, and there is no reason for that particular inverse-tangent value to land on a round number of degrees — only a handful of ratios, like rise equal to run (a 12/12 pitch, exactly 45°), happen to produce a clean angle.
Can I enter a run other than 12?
Yes — Run (in) defaults to 12 to match the standard 'rise per 12 inches' convention, but the ratio and angle are computed from whatever run you enter, so a rise measured over a different span still returns a correct result as long as rise and run are both in inches.
How steep does a roof need to be before it's called 'steep-slope'?
Definitions vary by code and material warranty, but many roofing standards draw the line around a 4/12 to 6/12 pitch — roughly 18° to 27° by this instrument's angle figure — with anything shallower generally requiring different underlayment and material specifications suited to low-slope roofing.
Is the pitch ratio the same thing as a percentage slope?
Not quite as usually written, but closely related — multiplying the pitch ratio by 100 converts it to a percentage slope, so a 6/12 pitch (ratio 0.5) is also a 50% slope. Civil and drainage work often uses the percentage form, while roof framing sticks with the rise-per-12-in ratio.