A roof pitch gauge for roofers, estimators and adjusters. Measures a plane three ways and reports pitch, angle and the slope factor on one line, with the tolerance that applied.
Roof Pitch Gauge & Factor is coming soon to the App Store.
Roof pitch is written as rise over 12 in of run, so 6/12 rises 6 in for every 12 in across. The angle is the arctangent of that ratio, which is why the relationship is not a straight line: doubling the pitch does not double the angle. Percent grade is the same ratio times 100. The slope factor is 1 divided by the cosine of the angle, and it is the number that turns a flat footprint area into actual roof area - it is also the feet of rafter per foot of run, because they are the same number. The hip and valley factor is the square root of 2 plus the square of the ratio.
| Pitch | Degrees | % grade | Slope factor | Hip/valley |
|---|---|---|---|---|
| 1/12 | 4.76 | 8.33 | 1.0035 | 1.4167 |
| 2/12 | 9.46 | 16.67 | 1.0138 | 1.4240 |
| 3/12 | 14.04 | 25.00 | 1.0308 | 1.4361 |
| 4/12 | 18.43 | 33.33 | 1.0541 | 1.4530 |
| 5/12 | 22.62 | 41.67 | 1.0833 | 1.4743 |
| 6/12 | 26.57 | 50.00 | 1.1180 | 1.5000 |
| 7/12 | 30.26 | 58.33 | 1.1577 | 1.5298 |
| 8/12 | 33.69 | 66.67 | 1.2019 | 1.5635 |
| 9/12 | 36.87 | 75.00 | 1.2500 | 1.6008 |
| 10/12 | 39.81 | 83.33 | 1.3017 | 1.6415 |
| 12/12 | 45.00 | 100.00 | 1.4142 | 1.7321 |
| 16/12 | 53.13 | 133.33 | 1.6667 | 1.9437 |
| 20/12 | 59.04 | 166.67 | 1.9437 | 2.1858 |
| 24/12 | 63.43 | 200.00 | 2.2361 | 2.4495 |
A 2,400 sq ft footprint on a 6/12 roof is 2,400 x 1.1180 = 2,683 sq ft of actual roof, which is 26.83 roofing squares.
| Mode | Posture | Tolerance | Where it is used |
|---|---|---|---|
| SURFACE | Phone laid flat on the roof deck | ±0.2° zeroed, ±0.8° not | The true dip of the plane, whichever way the phone is turned on it |
| EDGE | Long edge against a rafter, rake or straightedge | ±0.2° zeroed, ±0.8° not | A rake board from a ladder, or a rafter in the attic |
| SIGHT | Camera, sighted at a gable from the ground | ±2.0° always, tagged ESTIMATE | A roof that should not be walked |
How to measure roof pitch · Roof pitch to degrees · What the slope factor is · The roof pitch chart
All Roof Pitch Gauge & Factor guides
The two contact modes hold plus or minus 0.2 degrees once the phone has been zeroed against a known flat surface, and plus or minus 0.8 degrees when it has not been zeroed. SIGHT, the camera mode you use from the ground, holds plus or minus 2.0 degrees and every sighted reading is tagged ESTIMATE so it is never mistaken for a deck reading. Nothing is committed by hand either: a stillness gate arms once the phone has been steady and locks the median of that window, so you keep what the plane actually is rather than whatever your thumb caught.
No. SURFACE mode reads the dip of the surface normal, which does not change no matter how the phone is rotated flat on the deck, so you can set it down in any orientation and get the same pitch. EDGE mode is the same idea along a line: it ignores rocking about the rafter, rake board or straightedge the phone is resting on. Once it has the reading the app draws an arrow pointing down the fall line for you.
SURFACE is the phone laid flat on the roof deck, EDGE is the phone's long edge laid along a rafter, a rake board or a straightedge, and SIGHT is the camera sighted at a gable from the driveway when you cannot get onto the roof. The two contact modes are the accurate ones at plus or minus 0.2 degrees zeroed, so use one of them whenever you can reach the plane. SIGHT is the fallback at plus or minus 2.0 degrees and every reading it takes is labeled ESTIMATE; because a sighted reading under-reads as the phone tips back, it warns you at 10 degrees of tip and refuses the capture at 16 degrees, telling you to stand further back.
Zeroing is optional, but it is what takes the contact modes from plus or minus 0.8 degrees to plus or minus 0.2 degrees. You hold the phone against a surface you know is flat, and the app stores that offset as a vector and applies it to later readings. A stale calibration is not applied, so an old zero from another day cannot quietly bias a number you are about to price a job from.
Yes. Every reading shows degrees, x/12 pitch and percent grade together, plus the slope factor for that pitch and the fall-line arrow, and the app works out common rafter, hip and valley lengths from your run. It also snaps to the nearest standard whole x/12 when the reading is within half a degree of one, tells you that it snapped and what it actually measured, and lets you turn snapping off when you want the raw angle.
It works with no signal at all, because the pitch comes from the phone's own motion sensors and every calculation happens on the device. There is no account, no sign-in, no cloud sync and no server of ours. Saved jobs and readings stay on the phone, which is what you want on a roof behind a hill or inside a metal building.
Free forever: all three gauge modes, zeroing, degrees, x/12 and percent grade, the slope factor and fall-line arrow, snapping, rafter, hip and valley lengths, saved jobs and readings, and working with no signal. The optional unlock in Roof Pitch Gauge & Factor adds exactly three things: stamped photos with the reading burned onto the image, sending that plate straight to Messages or Mail, and exporting a whole job as a PDF. It is a one-time purchase, not a subscription, and nothing renews.
Take the arctangent of the rise over the run, so a 6/12 pitch is arctan(6 / 12), or 26.57 degrees. To go the other way, multiply the tangent of the angle by 12 to get the x in x/12, so 30 degrees works out to 12 x tan(30), about 6.93/12. Four common ones worth memorizing: 4/12 is 18.43 degrees, 6/12 is 26.57 degrees, 9/12 is 36.87 degrees and 12/12 is 45 degrees.
A slope factor is the multiplier that turns a flat footprint into the real sloped roof surface, and it equals 1 divided by the cosine of the roof angle. At 6/12 it is 1.1180, so a 2,400 sq ft footprint is 2,400 x 1.1180 = 2,683 sq ft of actual roof, which is 26.83 squares at 100 square feet to a roofing square. The same number is also the common rafter length per foot of run, so a foot of run at 6/12 is 1.1180 ft of rafter.
The IRC minimum for asphalt shingles is 2/12, and from 2/12 up to but not including 4/12 a double layer of underlayment is required, with 4/12 and above treated as standard application. Below 2/12 you are into low slope, which is a membrane job rather than a shingle job. The local code adopted in your jurisdiction and the manufacturer's own installation instructions both govern, and either one can be stricter than the IRC.