Roof Pitch Calculator: Find Your Roof Pitch in Degrees

How to measure roof pitch

Each method gives a rise and a run; pick the one you can do safely, and enter both in the same unit.

Method 1: From the attic, on a rafter

1. Find an exposed rafter in an unfinished attic. InterNACHI, the home inspectors' association, notes that roof-surface readings can be less accurate because roofing is uneven. While you're up there, you can also check the insulation depth for the attic insulation calculator.
2. Mark 12 in from one end of a level.
3. Hold that end against the underside of the rafter, with the level pointing toward the ridge and the bubble centered.
4. Measure straight up from the 12 in mark to the rafter. That's the rise: 6 in means 6/12. The calculator accepts any run, so 9 in over a 24 in level also works (4.5/12).

Method 2: On the roof with a level

Safety first: don't climb onto a steep, wet, or icy roof to measure pitch. Measure from the attic when you can, or reach the roof edge from a ladder at the eave. If you're not comfortable on a ladder, ask a roofer.

1. Set the ladder up safely. OSHA's ladder safety card says to use it on a stable, level surface, set the base out a quarter of the ladder's working length, extend it at least 3 ft above the roof edge, keep 3 points of contact, and keep metal ladders away from power lines.
2. Lay a straight board on the shingles, running up the slope, so the level rests on a flat surface.
3. Rest one end of the level on the board, far enough up that the 12 in mark is still over it. Point the level out toward the eave and center the bubble.
4. Measure straight down from the 12 in mark to the board. That's the rise per 12 in.

Method 3: From the gable or the plans

This method uses total rise over half the span: the span is the building's width at the walls, and the rise is how far the ridge sits above the walls.

1. Get the span and total rise from the plans; you can check the span by measuring the gable wall. Some plans list the slope directly, such as 6:12.
2. Divide the span by 2 to get the run. A shed roof has one slope, so its run is the whole distance between the low and high walls.
3. Enter the rise and run in feet. A gable 24 ft wide with the ridge 4 ft above the walls has a 12 ft run: 4 ÷ 12 × 12 = 4/12 (18.4°).

Method 4: With a phone level or angle finder

26.6° on a phone level means a 6/12 roof. A level app or a digital angle finder reads the slope in degrees; on an iPhone, the level is in the Measure app and shows how many degrees a surface varies from level. InterNACHI notes that some mobile apps can measure roof slope, including from a photo taken from the ground.

1. Place the phone along the slope: against the underside of a rafter in the attic (method 1), or on the straight board laid on the roof (method 2), lined up with the rafters, not across them.
2. Read the angle once the number settles, and repeat at two or three spots.
3. Convert degrees to pitch: rise per 12 in = 12 × tan(angle), or look the angle up in the degrees to pitch chart below. 30° is 6.93/12; enter 6.93 as the rise and 12 as the run, and the calculator shows "Your roof pitch is 6.93/12 (30.0°)."

Near 6/12, each degree is worth about 1/4 in of rise per foot: 26° works out to 5.85/12 and 27° to 6.11/12. Round a phone reading to the nearest half pitch for estimates, and measure the rise and run directly (method 1) for framing cuts.

Roof pitch chart

Computed from the formulas above. The last column is the rafter length for each foot (12 in) of horizontal run.

The calculator rounds slope percent to a whole number, so 2/12 shows as 17% and 4/12 as 33%, the same figures IRC-based code text puts in parentheses.

Degrees to pitch chart

35° is an 8.4/12 pitch and 40° is about 10/12: multiply the tangent of the angle by 12 to get the rise per foot of run, then round to the nearest half inch. Use it for a phone or angle-finder reading, or for plans and solar tools that give the roof's tilt in degrees:

For an angle between rows, use 12 × tan(angle) on a calculator in degree mode. The roof pitch chart above goes the other way, from x/12 to degrees.

Low-slope, conventional, and steep roofs

The calculator sorts every result into three bands, based on the asphalt shingle rule in section R905.2.2 of the International Residential Code (IRC):

• Very low slope, under 2/12: asphalt shingles aren't allowed. Use a roof covering rated for low slopes.
• Low slope, 2/12 up to 4/12: shingles are allowed with double underlayment.
• Conventional, 4/12 and steeper: shingles go on with standard underlayment.

GAF's low-slope bulletin agrees: almost all GAF shingles can go on slopes as low as 2:12 when its instructions are followed, with a double layer of underlayment from 2:12 up to 4:12.

Where does steep begin? OSHA's construction fall-protection standard calls a roof steep when its slope is greater than 4 in 12, so the default 6/12 counts. That's a job-site safety definition, not a shingle rule.

Minimum slopes for other roof coverings

IRC-based code sets a minimum slope for each roof covering in section R905. These are from the 2021 Seattle Residential Code, which follows IRC numbering:

Product instructions may call for a steeper slope, and local codes can differ, so check both.

Minimum slope for flat and low-slope roofs

1/4 in per foot, written 1/4 in 12, is the least slope IRC-based code allows for membrane and other low-slope roofs: about 1.2°, or a 2 percent slope, so a roof that looks flat still drops 5 in over a 20 ft run. The same Seattle code text lists each covering:

Standing-seam metal panels go down to the same 1/4 in 12 (see the table above), while photovoltaic shingles and building-integrated photovoltaic roof panels need 2/12, like asphalt shingles (sections R905.16.2 and R905.17.2). To check a low slope in the calculator, enter the rise per foot over a 12 in run: 0.25 over 12 shows "Your roof pitch is 0.25/12 (1.2°)." and notes that asphalt shingles aren't allowed below 2/12. Where a roof isn't sloped to drain over its edges, the code (section R903.4) calls for a roof drain at each low point.

Walkable roof pitch: OSHA's low-slope and steep roofs

4/12 (18.4°) is where OSHA's construction rules split roofs: 4 in 12 or less is a low-slope roof, and anything steeper is a steep roof. OSHA doesn't define a "walkable" pitch, but the fall protection it requires for workers 6 ft or more above a lower level changes at that line:

These are job-site rules for employers, not a homeowner's checklist, but they show where a roof stops being forgiving: on a steep roof such as the default 6/12, OSHA no longer accepts a warning line or a safety monitor. Measure from the attic, the gable, or a ladder at the eave (methods 1, 3, and 4 above) rather than walking a steep, wet, or icy roof.

Using the slope factor for shingles and rafters

Multiply a roof's footprint, overhangs included, by the slope factor to get its surface: a 42 × 32 ft footprint at 6/12 is 1,344 × 1.118 = about 1,503 sq ft of roof. Use the slope factor to size shingles with our roofing calculator, which turns the footprint and pitch into squares and bundles. The plywood calculator turns the same roof surface into 4 x 8 sheathing sheets.

For rafters, add the overhang the same way: a 1 ft overhang at 6/12 adds 1 × 1.118 = 1.12 ft, so the default rafter grows from 16.77 ft to about 17.89 ft. Cut length also depends on the ridge board and end cuts, and local code sets rafter size and spacing, so have a qualified pro or your building department check framing plans.

Hip and valley rafter factors by pitch

1.5 times the common rafter's horizontal run gives the length of a hip or valley at 6/12: on the calculator's default 15 ft run, the common rafter is 16.77 ft and the hip is 15 × 1.5 = 22.5 ft. A hip runs at 45° to the walls, so it covers 16.97 in of level distance (12 × √2) for every 12 in of common run, and its factor is √(pitch² + 288) ÷ 12, where 288 = 12² + 12²:

The factors hold for a hip or valley between two roof planes of the same pitch that meet at a right angle in plan; with unequal pitches, the hip doesn't run at 45° and each plane needs its own figures. The calculator returns only the common rafter, so multiply the same horizontal run by the hip factor, and allow for overhang, ridge, and end cuts as with any rafter.

Tips and common mistakes

• Mixing units. A 6 in rise over a 1 ft run is 6 over 12 in. Entered as 6 over 1, it reads as 72/12.
• Measuring along the slope. The run is level distance. A tape laid along the rafter gives a longer run and a pitch that's too low.
• Using the full span on a gable roof. Divide the building's width by 2 first, or the pitch comes out half what it is.
• Assuming one pitch for the whole house. Porches, dormers, and additions can have their own pitch; measure and figure each separately.