Roof

Updated August 2026 · Written by Hussnain Mustafa

Every number on this page traces back to two inputs: rise and run. Enter those into the calculator above and everything else — angle, percent slope, rafter length, roofing squares — falls out of the same right triangle. Here’s how each figure gets there, and where the shortcuts start to matter on a real job.

What Is Roof Pitch?

Roof pitch is the ratio of a roof's vertical rise to its horizontal run, expressed as X/12 in US framing — a 6/12 roof rises 6 inches for every 12 inches of run. The same triangle converts to an angle in degrees or a percent slope; they're three views of one geometry.

Rise and run are the inputs. Angle, percent, and rafter factor are just that same relationship read three different ways. Change one, and the calculator's other tab modes reflect it instantly, because they're not separate calculations — they're the same triangle expressed differently.

How to Calculate Roof Pitch

From Rise and Run

This is the default tab, and it's the mode you'll use for 90% of field measurements. Set a level 12 inches out from the rafter, measure straight down to the roof surface, and that vertical distance is your rise. A 6-inch rise over a 12-inch run gives you a 6/12 pitch — the calculator's default state, and a genuinely common pitch on production housing.

At 6/12, the math behind the readouts looks like this:

• Angle: arctan(6 ÷ 12) = 26.57°
• Percent slope: (6 ÷ 12) × 100 = 50%
• Rafter factor: 1 ÷ cos(26.57°) = 1.1180

That rafter factor is the number that matters most once you move past the pitch itself — it's the multiplier every rafter-length and roof-area calculation below runs through.

How to Calculate Roof Pitch in Degrees

If you already have rise and run, the formula is:

Degrees = arctan(Rise ÷ Run) × (180 ÷ π)

For a 6/12 pitch, that's arctan(0.5) × 57.2958 = 26.57°. You don't need to run this by hand — switch to the Angle tab and it does the reverse conversion instantly, which is handy when you're working from a truss shop drawing or engineered plan that specs degrees instead of X/12. Enter the angle, and the calculator hands back the equivalent pitch, percent slope, and rafter factor.

From Percent Slope

Percent slope shows up less on framing prints and more on civil drawings, drainage specs, and solar racking documentation — it's just rise divided by run, times 100. A 6/12 pitch is 50% slope. Don't confuse the two: a "50% grade" on a site plan and a "50% roof slope" describe the same math but very different contexts, and mixing them up on a bid is an easy way to price the wrong roof.

How to Measure Roof Pitch on the Job

Three ways to get a real number, in order of how often you'll actually use them:

On the roof. Set a torpedo or 2-foot level flat against the rafter or sheathing, mark 12 inches from the low end, and measure the vertical gap from that mark down to the roof surface. That's your rise. Works on new construction before decking, or on an existing roof if you can get up safely.

From the attic. Same principle, run in reverse — hold the level against the underside of a rafter and measure up. Faster than getting on the roof, and it's the method most inspectors default to, since you're not exposed to a slope or an edge.

From the ground. Less precise, but useful for a quick sanity check or a bid you're putting together sight-unseen. Measure the horizontal distance from the wall to directly under the ridge, and the vertical distance from the top plate to the ridge, then plug those into the Rise & Run tab as run and rise. A phone level app against a photo taken square-on to the gable end gets you close enough to flag whether you're dealing with a 4/12 or a 10/12 before you ever load the truck.

Whatever method you use, cross-check the result against the reference table above. If your field measurement comes out to 5.8/12, you're looking at a 6/12 roof and some measurement slop — not a nonstandard pitch.

Rafter Length: What the Calculator Is Actually Solving

The Rafter Length section takes your horizontal run (eave to ridge) and overhang, adds them together, and multiplies by the rafter factor from above. At a 9/12 pitch — a clean example, since 9-12-15 is just a scaled 3-4-5 triangle — the rafter factor works out to exactly 1.25.

Say you've got a 14-foot horizontal run plus a 12-inch overhang:

14 ft + 1 ft = 15 ft total run 15 ft × 1.25 = 18.75 ft = 18' 9"

That's the line length — the theoretical measurement along the top edge of the rafter from the outside of the wall to the center of the ridge. It's not your final cut length. You'll still subtract half the ridge board thickness from the top end and account for the birdsmouth depth at the bottom. Skip that step and you'll cut every rafter in the roof long by the same amount, which is a fast way to turn a straightforward reframe into a very long Tuesday.

Roof Area and Roofing Squares

A sloped roof always covers more area than the building's footprint, and the gap gets bigger as the pitch climbs. The calculator multiplies your building length by width to get the footprint, then multiplies that by the rafter factor to get true roof surface area — the same factor doing the work in the rafter calculation above.

Using that same 9/12 pitch on a 36 × 24 building:

36 ft × 24 ft = 864 sq ft footprint 864 × 1.25 = 1,080 sq ft roof surface 1,080 ÷ 100 = 10.8 squares

Add a waste factor before you order material. A simple gable with no penetrations can run as low as 7–10%. Anything with hips, valleys, or dormers — the geometry that actually eats shingles — climbs to 12–15%, and a genuinely cut-up roof can push past that. At 12% waste, those 10.8 squares become 12.1 squares. That's the number you order to, not the raw surface area.

What Counts as a Standard Roof Pitch?

The calculator sorts pitch into five bands, and they're worth knowing outside the tool because they map to real cost and labor decisions:

• Under 2/12 — Flat/Low-Slope: needs membrane roofing, not shingles
• 2/12 to 4/12 — Low-Slope: shingles are possible but usually need extra underlayment
• 4/12 to 8/12 — Standard: the range most residential asphalt-shingle roofs live in
• 8/12 to 12/12 — Steep: harder to walk, slower to install, more waste
• Over 12/12 — Very Steep: approaching a wall in framing terms as much as a roof

Manufacturer specs generally treat 2:12 as the floor for asphalt shingles, and even that usually comes with a requirement for double underlayment or an ice-and-water membrane — check the actual product spec sheet before you bid it that low. Most crews start reaching for fall protection somewhere around 6/12 to 8/12, well before it's technically required, because footing gets unreliable fast once the slope steepens.

Roof Pitch for Sheds and Lean-To Roofs

A shed or lean-to roof is a single slope instead of two meeting at a ridge, but the math doesn't change. Run is the full width of the structure (wall to wall), rise is the height difference between the high and low walls — plug those straight into the Rise & Run tab and read off the pitch, angle, and rafter factor the same way you would for a gable.

The one thing that does change is the practical minimum. A mono-pitch roof relies entirely on one slope to shed water, so most manufacturers won't warranty shingles below roughly 2/12 to 3/12 on a lean-to, even though that same pitch might be fine on one side of a gable. If you're landing under that, plan on a membrane or standing-seam system instead of fighting a shingle spec that isn't rated for it.

FAQ

What's a good roof pitch for a house?

Most US homes fall between 4/12 and 9/12. That range sheds water and snow reliably, keeps material and labor costs reasonable, and stays within the "Standard" band on most shingle warranties.

Can I get an accurate pitch without climbing on the roof?

Yes, within a margin. Measuring from the attic is nearly as accurate as being on the roof itself. A ground-level or photo-based estimate gets you close enough to plan around, but confirm it with an attic or on-roof measurement before you order material.

What's the minimum roof pitch for asphalt shingles?

Roughly 2/12, and even then most manufacturers require double underlayment or an ice-and-water shield membrane below 4/12. Below 2/12, you're generally into low-slope or membrane roofing territory instead of shingles.

What's the difference between roof pitch and roof slope?

They describe the same triangle differently. Pitch is expressed as a ratio, X/12. Slope is usually expressed as a percentage or, less commonly, degrees. A 6/12 pitch and a 50% slope are the same roof.

How do I convert a 4/12 pitch to degrees?

arctan(4 ÷ 12) × (180 ÷ π) = 18.43°. Punch it into the Angle tab above and you'll get the same number back along with the matching rafter factor.

Does roof pitch matter for solar panels?

Less than it used to. Panels perform close to optimal across a fairly wide tilt range, and racking systems can adjust for a roof that isn't at the ideal angle. Pitch still affects mounting complexity and wind loading, so it's worth flagging to your installer, but it's rarely a dealbreaker the way it once was.

Internal linking note: swap in real URLs where this references related tools — a roofing cost/material calculator, a truss calculator, and a rafter span calculator are the natural hub-and-spoke links if those pages exist on the site.