Roof Pitch Calculator

What is roof pitch?

Roof pitch (sometimes called roof slope) describes how steep a roof is. In North America it is almost always written as a ratio of rise : run, where the run is fixed at 12 inches — one foot. So when a roofer says "a six-twelve roof" (written 6:12 or 6/12), they mean the roof goes up 6 inches for every 12 inches it goes across. The first number is the only one that changes; the second is always 12. This convention makes it easy to compare pitches and to communicate them quickly on a jobsite: "this addition is a 4/12 tying into an existing 8/12."

Because pitch is a ratio, it is independent of the actual roof size. A 6:12 pitch on a 20-foot-span garage (10-foot run) has a rise of 60 inches (5 feet). The same 6:12 pitch on a 40-foot-span house (20-foot run) has a rise of 120 inches (10 feet). The angle is identical in both cases — about 26.6° — even though the roofs look very different in scale. The calculator above handles this for you: enter any rise and run and you get the pitch, the angle, the slope percentage, and the rafter length in one pass.

Why roof pitch matters

Pitch drives four big decisions on every roof, and getting it wrong is expensive.

1. Material choice

Low-slope roofs (roughly 1:12 to 4:12) cannot use standard asphalt shingles reliably, because wind-driven rain and melting snow can back up under the tabs. Below 2:12 most shingle manufacturers void their warranty, and building codes require a continuous waterproof membrane (ice-and-water shield), a fully adhered single-ply membrane (EPDM, TPO), or standing-seam metal. Steeper pitches (4:12 and up) shed water fast and are safe for shingles, tile, metal, slate, and wood shakes — you get the full menu.

2. Cost

A steeper roof needs more material (a bigger surface area for the same footprint — see the area multiplier below), longer rafters, more labor to install, and more safety equipment. A 12:12 roof has about 41% more surface area than a flat roof over the same floor plan. Labor also climbs because crews need harnesses, roof jacks, and scaffolding, and they move slower on a steep slope.

3. Walkability and safety

Anything 4:12 or lower can generally be walked like a ramp. From 5:12 to 7:12 you can walk carefully with soft-soled boots. At 8:12 and steeper, a fall is likely without fall protection — harness, anchors, and rope grabs. Above 12:12 (a true 45°), the roof is effectively a wall and should never be walked on without professional rigging. The calculator flags this automatically.

4. Snow and water shedding

In snow country, pitch is your first defense against roof collapse and ice dams. Roofs between 6:12 and 10:12 are the sweet spot: steep enough to shed snow and let rain drain fast, not so steep that snow avalanches dangerously onto people or decks below. Very low pitches hold snow and meltwater, which is why they need extra structural reinforcement and membranes.

Common pitches by region and house style

• 1:12 to 4:12 — Modern, flat, and commercial. Mid-century modern homes, urban townhouses, sheds, porches, and commercial buildings. Requires membrane or low-slope-rated metal.
• 4:12 to 9:12 — The standard residential range. Ranches, split-levels, colonials, most suburban tract housing. The most common US pitches are 4:12, 6:12, and 8:12. Works with any roofing material.
• 9:12 to 12:12 — Steep, traditional, and Victorian. Victorians, Queen Annes, Gothic Revivals, barns, and many European-style homes. Dramatic exterior, more attic or loft space, higher cost.
• 12:12 and steeper — A-frames, cabins, and churches. A 12:12 roof is exactly 45°. A-frame cabins often go 18:12 or steeper, where the roof nearly forms the walls. Beautiful in snow country but expensive to roof and insulate.

The math behind the numbers

Three formulas do all the work. None of them requires anything beyond high-school geometry, but the calculator runs them instantly so you can focus on the design.

Pitch from rise and run

pitch = (rise ÷ run) × 12. Because the convention locks the run at 12 inches, you scale whatever rise and run you have back to a 12-inch baseline. A roof that rises 36 inches over a run of 72 inches gives (36 ÷ 72) × 12 = 6, so it is a 6:12 pitch.

Angle (degrees)

angle = atan(rise ÷ run) × 180 ÷ π. The inverse tangent converts a slope ratio into an angle in degrees. A 6:12 roof has atan(0.5) = 0.4636 rad, which is 26.57°. This is the angle between the rafter and the horizontal run, and it is what you would measure with a digital level or an angle finder.

Rafter length (Pythagoras)

rafter = √(rise² + run²). The rafter is the hypotenuse of the right triangle formed by the rise and the run — the actual board that runs from the ridge down to the top of the wall plate. For a 6:12 roof the rafter per foot of run is √(6² + 12²) = √180 = 13.42 inches. Multiply that by your run in feet (plus your overhang) and you have the rafter length you need to cut. Add a birdsmouth and ridge cut allowance on top.

The roof area multiplier — why it changes your material order

If you know the square footage of your house's footprint, you might assume you need that many square feet of roofing. You do not — you need more, because the roof is sloped. The area multiplier tells you exactly how much more.

multiplier = rafter ÷ run. For a 6:12 pitch that is 13.42 ÷ 12 = 1.118, meaning the sloped roof is 11.8% larger than the flat footprint. A 2,000 sq ft one-story house with a 6:12 gable roof needs about 2,236 sq ft of roofing per roof face (before waste). A 12:12 roof needs 41% more (√2 ≈ 1.414). Get this number wrong on your material order and you will either run short (expensive re-order, delivery delay) or buy far too much (wasted money). The calculator shows the multiplier live so you can size your shingle, underlayment, and metal order with confidence.

How to measure your roof pitch

You do not need to climb onto the roof to measure its pitch. The fastest method uses a 12-inch level and a tape measure, and you can do it from the gable end on a ladder or from inside the attic.

1. The level-and-tape method. Hold a 12-inch level against the roof surface (or against the underside of a rafter in the attic) so the bubble is centered. The level should point horizontally, toward the ridge. Measure the vertical distance from the roof surface straight up to the 12-inch mark on the level. That number in inches is your pitch. If you measure 6 inches, you have a 6:12 roof.
2. From inside the attic. Press the level against the bottom of a rafter, bubble centered, one end touching the underside of the roof deck. Measure up from the deck to the 12-inch mark. Same result — the pitch number — without going outside.
3. From the ground with a photo. Take a square-on photo of the gable end with a known reference dimension (a door, a sheet of plywood). Measure the rise and run in the photo and scale. Less accurate but useful when you cannot get up there.
4. Digital level or smartphone app. Rest a digital level on a straightedge held against the roof. It reads the angle in degrees directly; convert to pitch with the calculator above (switch to "Angle + Run" mode, enter the angle and a 12-inch run).

Once you have the pitch, type the rise and run into the calculator at the top of this page to get the angle, slope percentage, rafter length, and area multiplier. Everything updates instantly as you type.