Roof pitch calculator: rise and run to degrees, percent and slope factor

Divide the rise by the run and multiply by 12, with both in the same unit and the run measured level. A 6 in rise over 12 in of run is 6 / 12; a 5 ft rise over a 16 ft run is 5 ÷ 16 × 12 = 3.75, printed 3¾ / 12. The angle is arctan(rise ÷ run), 26.57° at 6/12, and the percent slope is rise ÷ run × 100, 50 %. The rise is measured plumb, the run on a level line and never along the slope, and any line parallel to the roof gives the same answer, at the eave, the ridge, a rafter or the rake board. Enter the two measurements in the calculator and it prints every form at once with the slope factor and the hip factor.

Rise divided by run, times 12: 4 in over 12 in is 4 / 12, 7 in over 24 in is 3½ / 12. The measuring methods, with photographs, are in how to calculate roof pitch.

18.43° for 4/12, 26.57° for 6/12 and 45.00° for 12/12, from arctan(rise ÷ 12). Every pitch as an angle, and degrees back to x/12, is on the roof pitch to degrees chart (planned).

On a US job site, nothing: both mean rise in inches per 12 in of level run, and the IRC uses only the word slope, written as units vertical in 12 units horizontal. The older meaning of pitch is rise over the full span of the building rather than over the run, so a roof that rises 6 ft across a 24 ft wide house is a quarter pitch, which is a 6/12 slope; 8/12 is a third pitch and 12/12 a half pitch. Old plans, framing books and some inspectors use that fraction, and it is why a quarter pitch is not a quarter-inch-per-foot roof. The calculator prints the x/12 figure first and the span-fraction pitch beside it, so a plan written either way can be read.

The slope factor, √(rise² + 12²) ÷ 12: 1.1180 at 6/12, 1.0541 at 4/12, 1.4142 at 12/12. It is the sloped length per unit of level length, so a level run times the factor is a rafter length and a footprint times the factor is a roof area; 1,000 sq ft of footprint at 6/12 is 1,118 sq ft of roof. The hip and valley factor, √(rise² + 288) ÷ 12, does the same for a hip or valley rafter, which runs diagonally and rises in 17 rather than 12: 1.5000 at 6/12. The multiplier table on this page lists both for every common pitch, and the calculator prints both for any pitch you enter.

2:12, with two layers of underlayment from 2:12 up to 4:12. IRC R905.2.2 allows asphalt shingles "only on roof slopes of 2 units vertical in 12 units horizontal (17-percent slope) or greater" and requires double underlayment from 2:12 up to 4:12 as Table R905.1.1(2) describes; at 4:12 and steeper one layer is applied shingle fashion. So 3/12 is a code-legal shingle roof with two layers, not a violation, and 4/12 is where the second layer stops being required; the 3/12 and 4/12 minimums repeated on many pages are not the code. The calculator prints the two-layer note from 2/12 up to 4/12 and a not-permitted note below 2/12. Below 2:12 the choices are roll roofing from 1:12, a membrane or a metal system with sealed laps.

It depends on the seam. IRC R905.10.2 sets 3:12 for lapped, nonsoldered-seam panels without applied lap sealant, ½:12 for the same panels with lap sealant applied as the manufacturer specifies, and ¼:12 for standing-seam roof systems. Metal roof shingles are a separate covering with a 3:12 minimum under R905.4.2. A ribbed panel on a patio cover at ½ in per foot is therefore inside the code only with the sealant, and most panel manufacturers publish their own minimum, often 3:12, which governs the warranty. The metal roofing panel calculator (planned) counts panels and screws at any of the three slopes.

Stand square-on to a gable end and count the siding courses between the eave line and the ridge, multiply by the exposure for the rise, and divide by half the width of the house for the run: a 7 ft rise over a 28 ft wide gable is 7 ÷ 14 × 12 = 6/12. A straightedge or a pitch gauge card sighted along the rake board gives the angle directly, and a phone level app held against a straightedge aligned with the rake does the same. Photographs taken from below the eave flatten the roof, so a photo is only usable square-on and with a known width. For a hip roof or a roof you cannot see end-on, the attic method is better.

No. Pitch is a ratio, and every line parallel to the roof plane has the same one, so the overhang, the eave, the ridge, a rafter and the rake board all give the same number. Measuring from the wall or from the eave makes no difference as long as the rise is plumb and the run is level. What the overhang changes is length: it is added to the rafter along the slope, so a 2 ft level overhang at 6/12 is 2.24 ft of rafter. A roof with two pitches, a gambrel, a mansard or a cross gable, is measured one plane at a time; a wedge-shaped building with unequal spans has one pitch and two different runs, so measure the run under the plane you are on.

Rise per 12 in of run = 12 × tan(angle). At 30° that is 6.928 in, which the tool prints as 6⅞ / 12 and reads as nearest the standard 7 / 12; 45° is exactly 12 / 12; 18.43° is 4 / 12; 22.5° is 4.971 in, between 4/12 and 5/12. A 2° fall on a flat extension is 12 × tan 2° = 0.42 in per foot, about 7/16 in. The conversion is not linear: halving 45° gives 22.5° and 4.971 in, not 6/12. Pick the angle setting in the calculator, type the degrees, and read the x/12 headline, the exact rise and the nearest standard pitch; the full table in both directions is on the roof pitch to degrees chart.

There is no standard pitch in the IRC; the code sets a minimum slope per roof covering and nothing above it. Most new US houses are framed between 4/12 and 9/12, with 4/12 to 6/12 common in warm, low-snow regions and 6/12 to 9/12 where snow and attic space matter; that is a trade generalisation and not a figure from any code or survey. Sheds and garages are often built at 3/12 to 5/12, porches and lean-tos lower, cape and A-frame roofs at 9/12 to 12/12 and more. What decides it is the covering's minimum, the attic use, the snow and wind design the plans are drawn to, and matching the existing house when adding on.

Walkable is a crew's judgement: on a dry asphalt roof most roofers walk up to about 6/12, use roof jacks and planks from 7/12 or 8/12, and treat anything steeper as a harness job, with wet, metal, tile or mossy surfaces moving every line down. OSHA's rule is the hard one and it applies to employers: 29 CFR 1926.500(b) defines a steep roof as "a roof having a slope greater than 4 in 12" and a low-slope roof as 4 in 12 or less, and 1926.501(b)(11) requires guardrails with toeboards, safety nets or personal fall arrest for each employee on a steep roof 6 ft or more above lower levels. A 4¼/12 roof is a steep roof to OSHA; figures of 7/12, 8/12 or 9/12 quoted as the OSHA line are not in the rule.

15 ft 7 13/16 in to the ridge center line, from 14 ft × the slope factor 1.1180 = 15.652 ft, with a total rise of 7 ft; the tool prints both when you enter 14 as the run of one rafter. That is the line length: add the overhang along the slope, 2.24 ft for a 2 ft level overhang at 6/12, and take half the ridge thickness off at the plumb cut. A 16 ft span with a 2 ft overhang at 4/12 is a 96 in run, 101.19 in of line length and 25.30 in of overhang. Stock lengths and the plumb and seat cuts are on the rafter length calculator (planned), and whether the rafter size spans the run is on the rafter span calculator.

For asphalt shingles, 2:12 is the exact IRC floor and it needs two layers of underlayment; many builders go to 3/12 or 4/12 on a shed so that one layer and ordinary flashing work. For a metal panel roof, 2/12 is above the ½:12 the code allows with lap sealant but below the 3:12 for lapped panels without it, so it depends on the sealant and the manufacturer's minimum. Roll roofing is allowed from 1:12 and a membrane from ¼:12. A fall of 1½ in per foot is 1½/12, below the shingle floor; 5 in over 14 ft is 0.36/12, a membrane slope only. Enter the rise and run and the calculator prints the shingle note for the pitch you have.

It helps snow slide off a smooth roof and it does not change the ground snow load the roof is designed for. The IRC takes the ground snow load from Table R301.2 as your building department fills it in, and the structural design of rafters and trusses for snow follows the code's tables or an engineer; the slope factor in that design is an ASCE 7 matter and not something a pitch calculator decides. Metal sheds snow at pitches where shingles hold it, so a 6/12 metal roof can clear itself while a 6/12 shingle roof carries the load. Steeper roofs also dump snow onto decks, walks and lower roofs, which the plan has to allow for. Choose the pitch for the covering and the look; size the framing for the load.

64 in, or 5 ft 4 in: rise = pitch × run ÷ 12, so 4 × 16 ft = 64 in. The high wall sits that much above the low wall, plus the thickness of whatever the rafters bear on at each end if it differs. Enter 16 as the run of one rafter at 4/12 and the tool prints the total rise and the rafter line length together; at 12 ft it is 4 ft and 12 ft 7 13/16 in. The same arithmetic runs backwards: a wall 16 ft out that must be 5 ft lower is 60 ÷ 192 × 12 = 3.75, printed 3¾ / 12 with 3½ / 12 as the nearest standard pitch. A greenhouse with a 7 ft and a 5 ft 6 in wall 20 ft 6 in apart has an 18 in rise over a 20 ft 6 in run.

Rise over the full span, not over the run: a quarter pitch rises one quarter of the building's width, which is 6/12 in modern notation; a third pitch is 8/12; a half pitch is 12/12; a sixth pitch is 4/12 and an eighth pitch is 3/12. The calculator prints this as the span-fraction pitch, 1/4 at 6/12, and gives a decimal when the pitch is not a tidy fraction, 0.156 at 3¾ / 12. The notation turns up on old plans, in older carpentry books and in some inspection courses, where a 2 ft rise over a 24 ft span works out as 1/12 pitch even though the slope is 2/12. A plan note of "3-4 pitch" is something else again: 3/12 to 4/12.

Yes, with its level or inclinometer app, if the phone is laid on something straight along the slope: a rafter in the attic, a straightedge on the roof surface, or the rake board. Read the angle and enter it in the calculator's angle setting; 26.6° is 6/12, and 30° is 6.928 in, a 7 / 12 for the plans. Laid directly on shingles the phone reads the shingle it sits on, and held in the air sighting along the rake it reads your hand, so the straightedge matters. Phone readings are accurate to about the degree displayed, which is about a quarter pitch; for the eighth-inch figure a framer wants, measure rise and run with a level and a tape.