Guides/How roof pitch works
Why it is written over 12, how to measure it with a level, and what the slope factor is really for — which is the part that saves you money on material.
Roof pitch is a ratio, not an angle. 6/12 means six inches of rise for every twelve inches of horizontal run. Everything else — the angle, the rafter length, the real roof area — comes out of that one ratio.
Because it is a foot, and because it makes the number measurable with tools already in the truck. Hold a level out twelve inches from the roof surface and measure straight down to the roof: that distance in inches is the pitch. No calculation, no protractor.
It also makes a framing square directly useful. The square's blade is marked in inches, so a pitch is something you can set gauges to and walk down a rafter.
Measure in a couple of places. An older roof can sag, and one reading off a bowed rafter will send every downstream number out.
The angle is the arctangent of rise over 12. Worth knowing a few by heart: 4/12 is 18.43°, 6/12 is 26.57°, 8/12 is 33.69°, and 12/12 is exactly 45°.
That last one is a useful anchor. Past 12/12 the rise is greater than the run, and the roof is steeper than 45 degrees — which is the point where staging stops being optional. The full set is in the roof pitch table.
Every pitch has a slope factor: the length along the slope per unit of horizontal run. It is √(rise² + 144) ⁄ 12 — the hypotenuse of the pitch triangle.
Multiply any level measurement by it and you get the sloped one. Which matters twice:
Being short on a shingle order is not just an extra trip. Bundles from a different run can differ in shade, and on a visible roof that shows.
What you can put on it. Most shingle manufacturers set a minimum pitch — commonly around 4/12 — below which they require extra underlayment or will not warrant the installation. Check the product, not a rule of thumb.
How the job runs. Somewhere near 7/12 is where most people stop walking a roof comfortably, and staging turns from a nicety into a line item.
What is underneath. Pitch drives rafter length, which drives span, which drives the member size — and that is a span table question, not an arithmetic one.
To run the numbers for a specific roof, the roof pitch and rafter calculator takes building width, pitch, ridge thickness and overhang, and gives back the angle, slope factor and cut length.
Because 12 inches is a foot, and it makes the ratio directly measurable with a framing square and a level. Hold a level out 12 inches from the roof surface and measure down to it: that distance is the rise, and the pitch is that number over 12.
Hold a level against the roof line, mark 12 inches along it from the point where it touches, then measure vertically from that mark to the roof. That vertical distance in inches is the rise per 12 of run. It works on a rafter in the attic as well as on the roof surface.
In common trade usage pitch is the rise over a run of 12, written 6/12, and slope is the same relationship as an angle in degrees. Strictly, pitch once meant rise over the full span rather than over the run, which is why very old drawings can use the word differently.
Check it against the job. Building codes, span tables and product specifications vary by jurisdiction and manufacturer. Your approved drawings and your local code govern, not this page.