Measuring roof pitch and slope

Slope gets written three different ways depending on who is asking. Roofers speak in rise per 12 units of run, civil drawings use percent, and a phone's inclinometer reads degrees. All three describe one angle, and moving between them is arithmetic you can do once and keep.

The three notations

Degrees are the angle from horizontal. Percent is rise divided by run times a hundred, so a slope that rises one metre over four is 25%. The x-in-12 form used for roofs states the rise for every 12 units of horizontal run, so 6-in-12 rises half a unit for each unit across. A 45° roof is 100% and 12-in-12 at the same time — the notations diverge quickly above that, since percent has no ceiling while degrees stop at 90.

Taking the reading

You do not need to be on the roof. Rest the phone's long edge against a rafter in the attic, against the underside of a gable overhang, or on a straight board held flush to the roof plane. Zero the tool on a level surface first, then read and lock the value before you move the phone, so you are not squinting at a screen at arm's length.

Why the number matters

Material choice hangs on it. Most shingles want a slope of at least 4-in-12 without special underlayment, and some standing seam and membrane systems are made for slopes so shallow a bubble barely notices them. Gutter runs, snow load and how visible the roof plane is from the street all follow from the same angle.